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Key Tips to Prepare for a Variable Crop Harvest

Wed, 09/11/2019 - 10:42

Harvest is quickly approaching and most are anticipating a highly variable corn and soybean crop. Below are some reminders for regular maintenance, adjustments, and final checks to make sure your combine is ready to hit the fields soon.  

Pre-harvest preparation of combines
  • Sensor checks. Many combines are equipped with capacity, loss, and yield sensors. Prior to harvest, confirm all the sensors are performing properly by performing a computer check for accurate readouts.
  • Inspect for wear and proper mechanical operation. This involves checking the deck plates, cutter bar, and trashing unit (specifically the condition of the rotor and concave), and cleaning shoe sieves. Repair or replace all worn, broken, or rusted parts.
Making adjustments
  • Combine head adjustments. A malfunctioning combine head results in excessive header loss.
    • For corn, set the deck plates so they are tapered from front to back. The top opening should be set 1/8 inch narrower than the bottom gap. With adjustable deck plates, initial settings can be as per the operating manual. However, stalk conditions may require some adjustments. Ideally, the gap between the plates should be narrow enough to avoid shelling kernels, but not so narrow as to cause stalk wedging.   
    • For soybeans, it will likely be necessary to keep the header low in fields with shorter plants. Additionally, shorter plants require narrower clearances between the reel, cutter bar, auger, and the feed conveyor chain, to ensure stems are feeding through the platform and into the feeder house. Reel height should be set so there is at least a two-inch gap between the reel finger tips and the flexible cutter bar when it’s at the highest position.   
  • Threshing and separation unit adjustments. For the threshing and separation unit, refer to your operator’s manual when adjusting the clearance between the rotor and the concave. When adjusting this clearance, pay attention to the condition of the crop being harvested and make adjustments accordingly. Narrow or widen the rotor-concave clearance in increments until it is narrow enough to thresh out the grain without causing damage. Clearance may need to be increased when faced with taller soybean plants or regular to larger-sized corn ears.
  • Fan speed and sieve adjustments. Obtaining good separation between the grain and chaff in the cleaning shoe is a function of the cleaning fan air speed and sieve adjustments. When making pre-harvest adjustments, ensure the cleaning fan is functioning well at different speeds. The intent is to be able to match air speed with the crop throughput coming into the cleaning shoe for separation during harvest. Also, ensure that airflow across the cleaning shoe is as uniform as possible for all fan speeds. Sieve adjustments will depend on the kernel sizes passing through the cleaning shoe during harvest. Smaller soybean or corn kernels will require narrower sieve openings; however this can increase the number of larger kernels going back to the threshing unit as part of the tailings.

While variability in both the corn and soybean crop will be a challenge this fall, the above tips should help farmers better prepare for the conditions and minimize grain loss, damage, and harvest delays.

Crops: CornSoybeanCategory: Grain Handling and StorageTags: harvestadjusting combinecombine harvesting tipsthreshing unitfan speedsieve adjustment

Grain Drying Considerations this Fall

Wed, 09/11/2019 - 10:29

With delayed planting across the state in 2019, it is important to monitor crop development to determine unique grain drying needs this fall.

Potential challenges:

Corn damaged by a freeze before it has reached physiological maturity will create issues of low test-weight, low quality, and high moisture. Even without frost damage, corn that reaches maturity later in the year can still have issues of high moisture with less in-field drying between maturity and harvest. Corn infield drying rate decreases with air temperatures: in September, weekly drying is estimated at 4.5 moisture points per week, and in October, November, and December, this is reduced to 2.5, 1, and 0.5, respectively.

Sell wet corn “as is” or dry it?

Harvesting wet grain leaves you with a decision about drying grain yourself or paying others for drying. Consider your buyer’s moisture discount factor or drying charge and shrink factor, as well as your drying system cost and shrinkage loss when deciding whether to sell wet grain or dry it before selling. Consider an example where a seller has 56,000 pounds (1,000 wet bushels) of 20.5% moisture corn and current corn price is at $3.50 per bushel.

Selling wet grain “as is” with a moisture discount:

If the buyer is assessing a moisture price discount of 2.0% for each moisture point above 15%, the discount would be 5.5 moisture points times 2.0% for a total discount of 11%, making the discount $3.50 times 11%, or $0.39 per bushel. In this case, the seller would see a net revenue of $3,110, or $3.11 times 1,000 wet bushels.

Selling wet grain “as is” with a drying charge and shrink factor:

The buyer may instead use a combination of drying charge and shrink factor. If the buyer is charging a drying fee of $0.048 per wet bushel per point of moisture removed, the drying charge would be $0.048 times 5.5 moisture points times 1,000 wet bushels, or $264. If the buyer uses a shrinkage factor of 1.4% per point above 15%, this would reduce the seller’s bushels by 77 bushels, or 1.4% times 5.5 moisture points times 1,000 wet bushels, leaving 923 bushels of dry grain. The net revenue would be $2,967, or 923 bushels times $3.50 minus $264.

Drying on-farm before selling:

Consider the drying cost per bushel of your system as well as the shrinkage loss from the drying process.  Using the Ag Decision Maker spreadsheet Corn Drying and Shrink Comparison (A2-32) and a propane cost of $1.00 per gallon and electricity cost of $0.14 per kilowatt-hour, we can estimate a high temperature drying system cost of $0.030 per bushel per point of moisture removed. The drying cost would be equal to $165 ($0.030 times 5.5 moisture points times 1,000 bushels).

Drying shrinkage loss is mostly due to water loss, but also includes handling (dry matter) weight loss. A 56,000-pound load of 20.5% moisture corn consists of 11,480 pounds of water and 44,520 pounds of dry matter. After drying 5.5 moisture points there will be 52,376 pounds (44,520 pounds divided by 0.85). Handling loss from on-farm drying has been measured between 0.22% to 1.71% of wet bushel weight. Assuming a common handling loss of 1%, handling shrink is 560 pounds. Dry weight to sell is 51,820 pounds, or 925 bushels (52,376 minus 560). Also assume additional transportation costs of $0.01 per bushel per mile, which would be $40 to haul four miles to the on-farm drying system. The net revenue then becomes $3,033 (925 times $3.50 minus $165 minus $40). Also consider additional drying costs if planning to store more than 6 months at a lower moisture content.

This example is for illustration only. Ask your buyer for moisture discounts or drying charges and shrink factors. Use your actual costs for propane and electricity.

Estimate propane needs for a high temperature dryer by using the following equation: 0.018 gallons times bushels dried times moisture points dried. While 0.018 gallons is an average propane usage estimate, this value may range from 0.010 to 0.025 gallons per bushel per moisture point, depending on the drying system and outdoor temperature.

If harvest is delayed later into the fall, consider that the drying cost of a high temperature dryer increases by around 14% with every 20 degree decrease in average outdoor temperature.


Frost Damage to Corn and Soybeans, PM 1635, Charles R. Hurburgh and Garren O. Benson, 2012.

High Moisture Corn Drying and Storage presentation, Kenneth Hellevang, 2014.

Soybean Drying and Storage, PM 1636, Charles R. Hurburgh, 2008.

Estimating the Cost for Drying Corn, A2-31, William Edwards, 2014.

Corn Drying and Shrink Comparison, A2-32, William Edwards, 2014.

Crops: CornSoybeanCategory: Equipment and MachineryGrain Handling and StorageTags: grain dryinggrain storagegrain harvestharvest challengesmoisture discountdrying chargedrying shrinkagegrain moisture

Plan Ahead for Potential Harvest and Storage Challenges this Fall

Wed, 09/11/2019 - 10:24

With delayed planting across the state, it is important to plan ahead for potential harvest challenges. Scout your fields for crop development to determine whether you might have potential problems with immature, frost-damaged grain, and wet grain.

Potential challenges:

Frost-damaged soybean will have a slower dry-down in the field and may produce green/yellow soybean with above-normal shrink from drying. The green color may subside within two weeks of maturity if allowed to dry in the field or after several weeks of aeration.

Corn damaged by a freeze before it has reached physiological maturity will create issues of low test-weight, low quality, and high moisture. Light corn has a shorter storage life and is more difficult to dry. Even without frost damage, corn in-field drying rate decreases with air temperature: in September, weekly drying is estimated at 4.5 moisture points per week, and in October, November, and December, this is reduced to 2.5, 1, and 0.5, respectively. Use this growing degree day calculator, along with your location and corn hybrid characteristics to estimate date of corn maturity and see how it compares with historical average first freeze dates.

Using a location of Johnson County, Iowa, a planting date of June 1, and corn maturity of 111 days, the calculator estimates black layer on September 29, 20 days before the average first freeze for this location of October 20. Using a later planting date of June 10, the calculator predicts black layer on October 21. For corn planted after May 1, remember to manually adjust the growing degree day requirements to reach maturity by 6.8 fewer growing degree days per day. Remember that there is always a chance of getting an earlier than average first freeze.

Handling and storage recommendations:

Moisture meters are typically inaccurate at high grain moisture levels, so be sure to follow the manufacturer’s procedure for obtaining an accurate measurement. Green, immature beans will read a drier moisture than they actually are, so add 1.5 moisture points when you have these beans mixed in with sound beans.

Harvest and handle grain from low-lying, frost-damaged areas separately, as this grain will have a high storage risk. Frost-damaged corn may not be wanted by ethanol operations; however, it maintains most of its value for animal feed. Test for protein level, amino acid level, and mycotoxins before feeding. Green soybeans are often discounted by processors.

To safely store through the winter, dry good quality corn to 15% moisture and soybean to 13%. To store into the warmer summer months, dry corn to 13% and soybeans to 11%. Dry low test-weight corn and corn with damaged kernels to one percentage point lower in moisture content than normal. High temperature drying should be limited to 160 degrees for frost-damaged corn and to 130 degrees for soybean to limit damage in the dryer.

Low-temperature or natural air grain drying should be limited to 21% moisture corn or dryer. Natural air drying is typically limited after late October. When average daily temperatures cool to below 40 degrees this fall, focus on getting the grain cooled for winter storage, aerate through the winter as needed, and continue drying when temperatures rise in the spring.

Cool grain in bins to 30-40 degrees to store through the winter. Check grain for rising temperatures and moisture levels, odors, and insect activity every two weeks through the winter and every week during spring. Use a grain cleaner or “core” bins with poor grain quality to remove fines accumulated in the center. Core the bin after filling it by removing about half of the peak height to remove fines and improve aeration. If we have wet conditions this fall, look for the development of molds and toxins, such as vomitoxin, on grain left out in the field.  

Grain bin safety:

When removing grain from a bin, visually inspect to be sure that an inverted cone has been created. If no cone is created, there may be bridging of the grain surface and a hollow space beneath. Do not enter the bin until the bridging has been corrected. Do not enter a bin when grain is flowing. Be sure to protect your lungs with proper respiratory protection when working with dusty or moldy grain.

Crops: CornSoybeanCategory: Crop ProductionEquipment and MachineryGrain Handling and StorageTags: harvestgrain storageharvest challengescorn damagegrain bin safetygrain handling

Plan Ahead to Minimize Soil Compaction During Harvest

Tue, 09/10/2019 - 12:31

While there is significant uncertainty about this year’s harvest weather, the struggles with previous year soil compaction may still be lurking in corn and soybean fields across Iowa. This article will highlight challenges with wet conditions at harvest and opportunities to minimize the long-term consequences of harvesting fields with wet soils.

One of the challenges harvest time brings is the impact of harvest on soil health, such as increased soil compaction and bulk density as well as reduction in water infiltration. Wet soils are susceptible to compaction during harvest operations. When soils are near saturated conditions, heavy equipment loads weaken soil structure where water works as lubricant, leading to the collapse of soil aggregates. This will cause significant surface compaction, rutting, and deep subsoil compaction.

Damage from soil compaction can significantly impact water infiltration, root development, and ultimately crop yields the following season. It is estimated that yield loss due to soil compaction caused by wheel traffic ranges between 10-20% depending on the extent of soil compaction. An axle load from a 12-row combine with full a grain tank is estimated at 26 tons/axle and a single-axle fully loaded grain cart is estimated at 22 tons/axle. The combination of such heavy equipment and wet soil can create significant soil compaction.

Wet field conditions make soil prone to compaction that is occasionally unavoidable in the fall, as farmers make an effort to get grain harvested without pre-harvest yield losses and worsening grain quality. Avoid wet and near saturated soil conditions. Ideally, field operations would be delayed until soil water is below field capacity, where soil retains maximum amount of water. Tillage fuel and time cost could increase significantly to remediate excessively compacted soils during harvest operations.

Figure 1. Ruts made when harvesting soybean under saturated soil conditions. (Mark Licht)

Tips to minimize soil compaction during and after harvest

Prior to entering the field with equipment, check the soil moisture status with a simple in-field test. Most of Iowa’s soils have medium textures that the “Feel-Method” can be used to estimate moisture status. Push a ribbon of soil between the thumb and index finger. If it breaks off within one or two inches, the potential for creating compaction is low. However, if the ribbon stretches out to four or five inches, it is too wet. The chances are good that being in the field under these conditions may cause more problems than it will solve. Probing the top 12-18 inches with a hand soil probe to assess the field's soil moisture conditions is time well spent.

Another soil consistency test is to roll soil against the palm of your hand to determine if it will clump and roll or if it is fragile and falls apart. Soil that clumps together is more prone to compaction. After checking your soil moisture conditions, the following tips are valuable to reduce or control soil compaction in the field:

  1. Dedicated travel lanes. Many combine operators use “on-the-go” unloading into a grain cart to speed up harvest. In areas that have received excessive rainfall, farmers may want to consider having dedicated travel lanes for the grain cart. It has been documented that 60-80% of soil compaction occurs from the first wheel passes, subsequent field operations account for a much smaller amount of compaction.
  2. Don’t run at full capacity. Reduce the axle loads of both the combine and grain carts by not loading them to full capacity. This may not be an attractive option in high-yielding cornfields and where harvest has already been delayed. This is much easier to implement in soybean where the grain volume is much less than corn. A compromise may be to try and keep axle loads lower in the far reaches of fields and achieve the highest axle loads (full capacity) near the end rows where grain will be transported out of the field.
  3. Tire size and inflation pressure. Use appropriate tire sizes for the conditions and adjust tire air pressure to match the axle load being carried. Larger tires with lower air pressure provide more surface area, allow for better flotation, and reduce pressure on the soil surface.
  4. Concentrate non-harvest field activates near the field exit. While it is tempting to move semi-trailers and tractors with wagons along the field edge as harvest continues, this practice increases compaction along the end rows. Also, moving semi-trailers along the combine will spread soil compaction throughout the field. Try to limit compaction to the smallest area possible.
  5. Harvest around the wettest areas. The wettest areas are the most at risk for soil compaction and are also an accident risk that could lead to longer harvest delays. Additionally, buried equipment may come with large financial penalties. Weigh risk versus benefits and come back to these areas later in the fall once the soil is drier or frozen.
  6. Avoid or minimize tillage. Tillage is not always the solution to soil compaction. The reason we create compaction in the field is due to weak soil structure that is caused by intensive tillage systems. Remember to hold off on tillage operations and if it is needed for correcting deep cuts or rutting use minimum tillage (i.e., field cultivation, light disking, etc.) when soil conditions are drier than field capacity. It is important to consider the soil moisture at the depth of tillage. Tillage in wet conditions results in further compaction and smearing of soil instead of the intended fracturing of the soil. If it is absolutely necessary to cosmetically fill in ruts, use a disk unless soil conditions are dry enough for fracturing of the soil through use of more aggressive implements.

In wet conditions, the best choice farmers can make is to stay out of the field to reduce soil compaction, but the steps above can help minimize the damage from necessary fieldwork in wet soil conditions. Remember that how you approach fieldwork after a heavy rain event can impact your soil for future growing seasons. More consequences of soil compaction can be found in Top 10 Reasons to Avoid Soil Compaction.

Figure 2. Testing soil moisture condition in the field.

Additional resources:

Top 10 reason to avoid soil compaction

Understanding and Managing Soil Compaction


Crops: CornSoybeanCategory: Crop ProductionEquipment and MachinerySoil ManagementTags: soil compactionharvesttillagefield compactionEquipmentwet soilstire inflation

Nutrient Removal When Harvesting Corn Stover

Tue, 09/10/2019 - 08:53

Increasing demand to use corn plant biomass for producing energy and various products has spurred interest in harvesting corn stover and specific plant components in addition to grain. Harvesting more biomass means increased carbon (C) and other nutrient removal from fields. What is the nutrient removal when different corn plant components are harvested?

Corn Plant Nutrient Content

Most producers in Iowa are familiar with harvesting corn for grain, but less familiar with other plant biomass removal. Those harvesting corn silage are aware of the increase in phosphorus (P) and especially potassium (K) removal, and fertilization guidelines consider this increase. For example, with corn silage based on a bushel grain equivalent, the pounds of P as P2O5 goes from 0.32 to 0.44 and the pounds of K as K2O goes from 0.22 to 1.10, respectively, for corn grain and corn silage (15% grain moisture content, ISU Extension publication PM 1688).

For corn stover the pounds per ton are 4.8 lb P2O5 and 18.0 lb K2O (15% stover moisture content, publication PM 1688). That data indicate there is considerable K in the non-grain part of the corn plant. Calculating the P and K removal in harvested stover is somewhat complicated as nutrients can be leached (especially K) from leaves and corn stalks with rainfall after grain harvest. The effect of rainfall on K is much more important than for P because all plant K is in a soluble inorganic form (K+) while most P is in organic forms of low solubility. Therefore, the concentration of nutrients at plant maturity will typically be higher than found for baled stover because it often rains after grain harvest. Also, the frequency and amount of rainfall will affect the concentration remaining. Costs for replacing removed nutrients will vary depending upon prevailing prices and stover removal amounts.

Corn biomass also contains other plant nutrients. Examples for nitrogen (N), P, K, calcium (Ca), magnesium (Mg), and sulfur (S) are shown in Table 1. There is variation in concentration due to differing data sources, hybrids, fertilization levels, etc. While there may typically be adequate supply of nutrients like K, Ca, and Mg from Iowa soils, stover harvest can lead to removal of large amounts of “basic cations” (K+, Ca+2, and Mg+2) and result in accelerated decrease in soil pH. Of course fertilization, liming, or manuring soils aids in replacement of these nutrients.

Dry Matter and Nutrient Composition in Corn Plant Components

Silage harvest results in almost complete removal of aboveground plant biomass. Baling corn stover typically does not remove as much plant biomass and amounts removed vary greatly across fields, years, and desired removal level. Also, there may be interest in specific plant component removal, such as targeting corn cobs. Table 1 lists the various corn plant components, the associated dry matter, and the nutrient composition from N rate trials at 14 site-years in Iowa. While not measured in those trials, cob composition for P would be approximately 1.1 lb P2O5/ton cobs and for K 14 lb K2O/ton cobs (both dry matter based). The corn grain harvest index averaged 51% (percent of the total aboveground biomass as grain on a dry matter basis at maturity). The grain harvest index can be used to estimate non-grain biomass from grain yield. For example, at a grain yield of 200 bu/acre (at 15% moisture standard, the per bu grain dry matter is 47.6 lb) the grain dry matter is 9,520 lb/acre and the non-grain (vegetation plus cob) dry matter is 9,150 lb/acre.


Harvesting corn plant components in addition to grain does result in greater removal of plant nutrients. Effects of increased P and K removal on nutrient application needs are immediate and should to be accounted for in fertilization plans. Effects on needs of other nutrients such as N and S, liming requirements to maintain desirable soil pH levels, soil organic C, and several physical, chemical, and biological soil properties are less apparent in the short-term but have consequence in the long term. Therefore, consideration of the impact on nutrient cycling, nutrient removal, and soil resources should be a part of the decision process regarding harvesting corn biomass. Additional information on nutrient management related to stover harvest can be found in publication PM 3052C, available from the ISU Extension and Outreach Extension Store.

Table 1. Corn nutrient composition at plant maturity by plant part.













P (P2O5)




K (K2O)




































Values are lb/ton (DM)

From 14 site years in 2006-2007.

J.E. Sawyer and D.W. Barker.


Crop: CornCategory: Crop ProductionTags: corn stoverharvestcrop nutrients

Can Weed Seed Production be Reduced with Late-season Herbicide Applications?

Fri, 08/30/2019 - 14:20

Problems caused by unfavorable conditions this season have resulted in greater than normal weed escapes. These weeds may reduce crop yields and definitely will contribute to future weed problems via new seed. While it is too late to protect crop yields, a common question is whether herbicides can be used to reduce the quantity of viable weed seed produced by weeds. While there is no simple answer due to the many different scenarios across the state, in most situations late-season applications are not warranted.  

The potential to limit seed production is affected by two main factors: 1) susceptibility of the weed to the herbicide, and 2) stage of seed development at the time of application. It is important to recognize that herbicides that are effective early in the season will be much less, if at all, effective on the mature weeds in fields now. Waterhemp is unlikely to be killed by any labeled herbicides at this time of the year. While other species may be killed with herbicides, the impact on seed production will be highly variable.

Research in the early 1980’s looked at the effect of late-season 2,4-D applications in corn on seed production by velvetleaf and cocklebur (the 1980’s equivalent of today’s waterhemp). While brown-silk 2,4-D applications were able to kill both species, the treatments were much more effective at preventing seed production by cocklebur than velvetleaf.  Much of the velvetleaf seed had filled by the time of the 2,4-D application, whereas cocklebur was still in early flowering stage. Seed that had filled prior to 2,4-D retained their viability even if the parent plant was killed prematurely. Before committing to any late-season treatments, examine weeds to determine the stage of seed development. Both waterhemp and giant ragweed present in fields in central Iowa had fully developed seed during the week of August 26 (Fig 1 and 2).  The 2,4-D label for late-season applications was changed in the 1980's from after brown-silk to after the dent stage, this change will reduce the effectiveness of the treatments since it provides more time for weed seeds to mature.

Figure 1.  Waterhemp seed, Aug 29.

Figure 2.  Giant ragweed seed, Aug 29.

Reducing weed seed production is essential in order to minimize the seed bank. By reducing the size of the seed bank, future weed management will be simplified and the risk of new herbicide resistant biotypes will be reduced. However, late-season herbicide applications are unlikely to provide significant benefits for most fields. Hand pulling weeds is an alternative, but at this time of year the plants would need to be removed from the field. In situations where a patch of a weed is present that is suspected to possess a new resistant trait for that field, removal of the weeds probably would be worth the effort. Rather than spending money on a questionable treatment, spend time determining why this year’s program failed and develop an effective weed management plan for 2020.

Category: WeedsTags: waterhempseed production24-D

Upper Soybean Leaves Began Showing Potassium Deficiency Symptoms Since Early August in Some Iowa Fields

Fri, 08/23/2019 - 11:14

Since early August, soybean in several fields began showing typical potassium (K) deficiency symptoms on leaves located in the middle to upper canopy. This is not surprising in fields or portions of fields with soil-test values in the very low or low K soil-test interpretation categories that did not receive adequate preplant K fertilization. Potassium deficiency symptoms are well-known and very common in older leaves during early growth stages. Due to poorly understood reasons, during the last couple of decades K deficiency symptoms in upper soybean leaves also have become common at middle to late reproductive stages. Moreover, K deficiency symptoms can develop in upper leaves in well-fertilized soybean when no deficiency was observed at early stages, mainly when drought conditions develop during late spring or summer.

In low-testing or draughty soils, K deficiency symptoms may develop from the V3 stage to more advanced vegetative stages mainly in the older leaves, but with severe deficiency, symptoms may progress to the upper leaves. Figure 1 shows typical soybean K deficiency symptoms at early growth stages. The symptom is yellowing of the leaflet margins with mild deficiency that becomes brown or necrotic with extreme deficiency. The symptoms of these leaves often remain until the reproductive stages, but may not be seen because the leaves have been shed or partially decomposed. The reason symptoms are observed mainly on older leaves at early vegetative growth stages is because K is very mobile within the plant and K is translocated from older leaves to new leaves.

Figure 1. Soybean potassium deficiency symptoms at early vegetative growth stages.

The K deficiency symptoms at early vegetative stages should not be confounded with soybean iron deficiency chlorosis (IDC), which often occurs in high-pH (calcareous) soils. In contrast to chlorosis or necrosis of leaf margins associated with K deficiency, IDC symptoms are yellowing of the interveinal area of mainly entire young leaflets. With extreme iron deficiency, browning and necrosis may also occur in leaf margins. The ICM News article “Is It Iron or Potassium Deficiency?” describes IDC symptoms in soybean.

The K deficiency symptoms in soybean during middle to late reproductive stages are similar to those observed earlier in the season on older leaves. Figure 2 shows typical examples. The physiological reasons for late-season development of deficiency symptoms during the last couple of decades are not entirely clear. Reasons might be that with increasing soybean yield potential there is more K translocation from the middle or upper leaves to developing pods and grain.

Figure 2. Soybean potassium deficiency symptoms during the reproductive growth stages.

Observations over many years have shown that severe K deficiency can advance soybean maturation. Therefore, it is not surprising to see senescing soybean, with most leaves yellow or brown, in low-testing field areas a few days before plants in other parts of a field. Figure 3 shows an example of this in research plots. Keep in mind that deficiency of other nutrients or conditions such as excessively wet or dry soil can also advance soybean senescence.

Figure 3. Potassium deficiency advances soybean senescence.

Several soybean diseases can also produce yellowing of upper leaves, which also may advance senescence. Sometimes, the disease symptoms and K deficiency symptoms occur at the same time. This is not surprising because Iowa research has demonstrated that K deficiency aggravates the incidence or severity of several soybean leaf diseases. Part of this research was summarized in the proceedings article for the 2016 Integrated Crop Management Conference “Watch potassium management - It also affects corn response to nitrogen and soybean diseases ”. Additional field observations suggest possible interactions with soybean cyst nematode (SCN) and aphid infestation levels. Potassium deficiency symptoms in soybean can develop or be worse in field areas associated with SCN or aphids.

Sometimes it is difficult to distinguish between K deficiency and disease symptoms in upper soybean leaves during reproductive stages unless the plants or leaves are submitted to a plant pathology lab for analysis. Soil and leaf K testing of apparently normal and affected field areas also may help identify the cause for the symptoms. Recently published interpretations for K tissue testing are useful for soybean plants at the V5 to V6 vegetative growth stages or for upper leaves at the R2 to R3 reproductive growth stages, but not for later growth stages because leaf K concentrations decline. Tissue test interpretations are available in publication CROP 3153 “Phosphorus and Potassium Tissue Testing in Corn and Soybean”.

Crop: SoybeanCategory: Crop ProductionSoilsSoil FertilitySoil ManagementTags: soil fertilitypotassiumpotassium deficiencySoybeannutrient deficiencysymptoms

Scout Now for Palmer Amaranth

Thu, 08/01/2019 - 07:04

Now is a great time to scout for Palmer amaranth (Amaranthus palmeri) in Iowa crop fields. As of late 2018, this species had been identified in over half of Iowa’s 99 counties. While new identifications have waned since the widespread introductions in 2016, Palmer amaranth is a species to watch out for in virtually any Iowa crop field. A native of the American southwest, Palmer amaranth is more competitive than common waterhemp (Amaranthus tuberculatus), a pigweed native to Iowa. Both species are known for fast development of herbicide resistance, prolific seed production (>500,000 seeds/plant possible), and prolonged emergence.

The addition of Palmer amaranth to Iowa’s noxious weed law as of July 1, 2017 highlights the importance of this weed to Iowans and its potential impact on Iowa agriculture. Early identification is key to eradicating this weed from Iowa fields. Eradication cannot happen without vigilance, early detection, and appropriate response soon after it invades an area. Palmer amaranth is reaching the growth stage where distinguishing it from waterhemp is easier due to the presence of flowers. In addition to fields where Palmer amaranth has been found previously, other priority areas to scout include farms that utilize feed and bedding from southern states, fields receiving manure from those farms, and farms where out-of-state equipment has been used. 

Palmer amaranth and waterhemp lack pubescence (hair) on plant parts like stems, petioles, and leaves, while other common amaranth (pigweed) species have hair on stems and/or leaves. Early in the growing season, Palmer amaranth is difficult to differentiate from waterhemp due to the high variability in both species. Leaves on Palmer amaranth often have a petiole that is longer than the leaf blade, this is the most reliable vegetative trait to differentiate the two species (Figure 1). Leaves on Palmer amaranth are often clustered tightly at the top of the plant. People often observe Palmer amaranth as a denser-canopied weed as well (Figure 2).

Figure 1. Palmer amaranth leaf with a petiole longer than the leaf blade. Folding the leaf over at the base is the fastest way to check for this trait.

Figure 2. Waterhemp's open canopy (left) compared to Palmer amaranth's denser, leafy canopy (right).

Once they flower, Palmer amaranth and waterhemp produce male and female flowers on separate plants. Identifying males from females should be relatively simple due to the small, black seed produced by female flowers or the presence of pollen on male plants. Female Palmer amaranth are easy to distinguish from waterhemp due to their long, sharp bracts (Figure 3) surrounding the flowers on tall terminal inflorescences (Figure 4). If you discover this weed, steps should be taken to remove all plants to prevent seed production.

Figure 3. Comparison of a female Palmer amaranth flower and a female waterhemp flower.

Figure 4. Female Palmer amaranth with long terminal inflorescences.

Continued vigilance is imperative to slow the speed with which Palmer amaranth invades our state. If you observe a plant that you think may be Palmer amaranth, please don’t hesitate to contact Bob Hartzler at 515-294-1164 or hartzler@iastate.edu or Meaghan Anderson at 319-331-0058 or mjanders@iastate.edu.

Crops: CornSoybeanCategory: WeedsTags: palmer amaranthcharacteristics of palmer amaranthPalmer Amaranth and WaterhempPalmer amaranth identificationweed identification

Assessing Corn Rootworm Activity Using Sticky Traps

Tue, 07/23/2019 - 07:15

Western and northern corn rootworm are major corn pests in Iowa and surrounding states (Photos 1 and 2). Farmers have seen several management changes, including the release of four Bt-rootworm traits to suppress corn rootworm larvae since 2003. Although both species are persistent pests, western corn rootworm is particularly adaptable. The Gassmann Lab at Iowa State University (ISU) has confirmed western corn rootworm resistance to all Bt rootworm traits in Iowa.

Photo 1. Color and size variation of western corn rootworm. Photo by Adam Varenhorst.

Photo 1. Color and size variation of northern corn rootworm. Photo by Adam Varenhorst.

In order to prolong the effectiveness of Bt, farmers should monitor for corn rootworm and make management decisions based on larval injury to roots or adult activity. We recommend using the 0 to 3 node-injury scale developed at ISU to evaluate larval injury. Corn rootworm egg hatch peaked mid-June this year, and adults are beginning to emerge in many fields.

Adult emergence coincided with silking in corn this year throughout much of Iowa. This can be problematic as adults can clip silks and interfere with pollination. To monitor for adults, follow the guidelines outlined below. Use Pherocon AM yellow sticky traps (unbaited). Set up sticky traps at corn silking and continue through the dent stage (i.e., mid-July through August). For areas in eastern counties with suspected rotation-resistant western corn rootworm, set up sticky traps at soybean flowering and continue through full seed set (i.e., mid-July through August).

Guidelines for sampling adult corn rootworm
  1. For every 10-50 acres of corn or soybean, create two transects of six sticky traps/transect (Figure 1). Transects should be at least 330 feet apart. Use a flag or stake to mark transects along the field edge.
  2. Place the first trap at least 165 feet in from the field edge; place remaining traps within the same row and at least 165 feet apart.
  3. For corn, attach the sticky trap directly to the developing ear. For soybean, attach the sticky trap to a post approximately 18 inches above the canopy.
  4. Check traps weekly and count the number of western and northern corn rootworms. Replace sticky traps weekly and adjust trap height if needed.
  5. For each sample, estimate the average number of western and northern corn rootworms per trap per day. For example, 123 total corn rootworms/12 traps/7 days = 1.46 rootworms/trap/day.
  6. Action thresholds in corn are two corn rootworms/trap/day, regardless of species. Consider crop rotation if action thresholds are met. If planting corn again the following growing season, use a pyramided Bt trait with Cry34/35Ab1, or a soil-applied insecticide on non-rootworm Bt corn. Action thresholds in soybean are 1.5 western corn rootworms/trap/day. If action thresholds are met in soybean, consider using a pyramided Bt trait or a soil-applied insecticide on non-rootworm Bt corn the following season.

Figure 1. Schematic drawing of adult corn rootworm sticky traps in corn or soybean fields.


Crop: CornCategory: Crop ProductionInsects and MitesTags: insectsamplingrootwormsticky cards

Potato Leafhopper Management in Alfalfa

Tue, 07/23/2019 - 07:05

There have been some reports of potato leafhopper activity and plant injury in Iowa alfalfa this season. Some fields experienced winter injury and the cooler spring provided a slow start to plant growth in 2019. It is time to think about assessing alfalfa stands. Potato leafhoppers (Photo 1) do not overwinter in Iowa, but they are persistent alfalfa pests every growing season. Storms along the Gulf of Mexico bring adult potato leafhoppers north and drop them into fields every spring. Heat or drought stress can make alfalfa more susceptible to injury, and plants are more likely to experience injury after the first cutting. Current climate conditions and harvest activities in Iowa align with these factors, making scouting critical to ensure yield protection.

Photo 1. Potato leafhopper adult and nymph. Photo by Penn State College.

Mated females begin to deposit two to three eggs per day in alfalfa stems as soon as they land. Pale, green nymphs emerge in 7-10 days depending on the temperature; the fastest development occurs at 86°F. They go through five instars in about two weeks. Therefore, a large population could develop three weeks after the northern migration. The extended egg-laying period can result in at least two overlapping generations in Iowa every year.

Plant Injury

Potato leafhopper nymph and adults have piercing-sucking stylets. They cause physical injury when probing to feed and also inject saliva that plugs vascular tissue. Initially, alfalfa leaf tips will turn yellow, which is commonly referred to as "hopperburn" (Photo 2). Heavily infested plants will be stunted, and new stands and regrowth after cutting are particularly affected. In some cases large leafhopper populations can significantly reduce tonnage of the current crop, as well as the following crop.

Photo 2. Typical hopperburn caused by potato leafhopper feeding. Note the v-shaped yellowing pattern characteristic of this injury. Photo by Rebecca Vittetoe.


Potato leafhoppers do not typically build up to damaging levels during the first crop in Iowa. Fields should be monitored weekly after the first cutting until the end of the season. A sweep net is the most effective way to sample for potato leafhoppers because adults and nymphs are very active and easily disturbed. Adults will jump or fly away while nymphs quickly move sideways and backwards. A detailed description on how to make and use a sweep net is available here.

Fields should be sampled during calm conditions and when dry. Sweep vigorously through foliage, using a 180-degree motion for one sweep. For each field, stop at four to five locations and take 25 sweeps per location. Count the number of nymphs and adults at each location and estimate the number of potato leafhoppers per sweep for each field. Typically, nymphs will be near the sweep net ring and adults will be at the bottom of the net.

Photo 3. Alfalfa trichomes. Photo by Purdue Extension. 


Remember, healthy and vigorous stands are able to tolerate some potato leafhopper (and other insect) feeding. Protecting alfalfa from potato leafhopper usually involves a three-pronged approach:

  1. The use of glandular-haired alfalfa varieties can significantly reduce yield losses (Photo 3). More than 70% of alfalfa is now resistant to potato leafhopper. Adults are repelled by plant hairs, and nymphs get caught in the sticky hairs and starve. Newly planted resistant fields may not show resistance immediately, but should develop sticky hairs after becoming established. Glandular-haired alfalfa is not the same as non-yellowing varieties. These tolerant plants only hide leafhopper feeding and do not prevent yield loss.
  2. The cultural control tactic of cutting stands can disrupt potato leafhopper populations as they develop in alfalfa. Delaying harvest will allow nymphs enough time to become adults and start reproducing. Timely cutting will destroy or starve young nymphs before regrowth occurs and force adults to move to nearby crops, but they often move back into a field as it regrows. It is important to start scouting 7-10 days after each cutting to monitor for possible reinfestations.
  3. Insecticide applications can protect alfalfa yield from potato leafhoppers and are economically justified with regular scouting and the use of economic thresholds. The fluctuating values of hay and control costs are important considerations for making a treatment decision. Table 1 offers a dynamic threshold for potato leafhopper. There are several products registered in Iowa for potato leafhopper control in alfalfa. Follow label directions and pay attention to preharvest interval guidelines.

Table 1. Economic threshold of potato leafhopper, based on the average number of leafhoppers per sweep (originally published by John Tooker, Penn State Extension).


Crop: Biomass and ForageCategory: Crop ProductionInsects and MitesTags: insectpestIPMscouting

Have Our SCN Sorrows Been Drowned?

Sun, 06/23/2019 - 13:32

Those looking for any bit of good news in all of the rain-soaked suffering we have endured this spring have asked if the extreme overabundance of moisture has drowned soybean cyst nematode (SCN). Unfortunately, the answer is no.

Nematodes are worms (animals) that require oxygen. And they absorb oxygen through their body wall or cuticle, which is made almost exclusively of proteins (and no chitin). Waterlogged soils may have greatly reduced levels of oxygen. But many plant-parasitic nematodes, including SCN, can survive long periods of time with little oxygen.

In the early 1970s, scientists in Arkansas conducted experiments to determine whether SCN could survive in flooded conditions. They found that SCN juveniles (see figure below) survived in water up to 630 days (probably longer, but the experiment ended after 630 days!). They also tested survival of SCN in flooded soils, and the juveniles survived 7 to 19 months depending on soil texture. The research paper is available online here.

SCN eggs (see figure) can survive in a dormant state for many years in the absence of soybeans, particularly the eggs that occur within the body of the dead female or cyst (see figure). Typically, eggs are more tolerant of environmental stresses than hatched juveniles. So it’s likely that SCN eggs in infested fields are not adversely affected by waterlogged soils either.


Figure: Egg-filled cyst (left) and egg and hatched juvenile (right) of the soybean cyst nematode - both images are of the same magnification.


More bad news...

A bit of additional bad news is that soil moved by erosion due to heavy rains and flood waters may spread SCN to new places. It is not possible to quantify the magnitude or frequency of this movement. And considering how widespread SCN already is in Iowa and the Midwest, perhaps the spread of SCN in soil moved by rainfall and flood waters will not have a great impact. Nonetheless, it is quite possible that some fields may have had SCN introduced in soil from other fields this spring. Consequently, soil samples should be collected this fall to test for SCN in fields where soybeans will be grown in 2020. Guidelines for collecting SCN soil samples can be found online here.

A possible silver lining to the storm clouds?

Multiple SCN generations occur (likely 4 to 6 or more) throughout a normal growing season in Iowa. And it takes about 30 days for SCN to complete a single generation once soils warm up in late spring and summer. If soybean planting is delayed by several weeks, as in 2019, there likely will be one or two fewer generations of SCN during the season. This means less of an increase in SCN numbers simply because there are fewer weeks for SCN to reproduce on soybeans in 2019.

But beware! The potential for large increases in numbers and for severe damage always exists with SCN, especially if the weather turns hot and dry - ideal conditions for SCN reproduction. The numbers of SCN eggs in soil can build up quickly over multiple generations. A few hundred eggs can increase to nearly 40,000 in just three generations, as shown in the infographic that is available online here.

Manage SCN for the long term

Successful, long-term management of SCN requires an active, integrated approach of growing nonhost crops such as corn in rotation with SCN-resistant soybean varieties. Farmers should seek out and grow soybean varieties with different sources of resistance in different years. And nematode-protectant seed treatments are available to bolster the performance of SCN-resistant soybean varieties. For more information about the biology and management of SCN, visit soybeancyst.info, soybeanresearchinfo.com and thescncoalition.com.

Crop: SoybeanCategory: Plant DiseasesTags: SCN

Tissue Testing for Field Crops Requires Cautious Use and Interpretation

Fri, 06/21/2019 - 09:08

In-season plant tissue testing can be useful in diagnosing nutrient deficiencies in field crops, but it must be used with caution. Extra care is needed this year given the unusual crop planting and growing conditions.

Iowa State University (ISU) Extension and Outreach has research-based interpretations for in-season tissue testing only for phosphorus (P) and potassium (K) in corn and soybean, and for sulfur (S) in alfalfa. Interpretations and guidelines for using the end-of-season cornstalk nitrate test are in ISU Extension and Outreach publication CROP 3154. There are no interpretations for other nutrients or crops due to lack of research, infrequent deficiency that precludes meaningful test calibration, or results that show tissue testing is an unreliable diagnostic tool.

As is the case for soil testing, use of tissue testing as a reliable diagnostic tool requires field research to correlate nutrient concentrations with crop yield response. Establishing reliable tissue test interpretations is even more difficult than for soil testing, however, because tissue nutrient concentrations vary greatly with the crop growth stage and the plant part sampled, and may also vary across hybrids or varieties and growing conditions. For example, effects of drought or plant diseases on plant growth and nutrient uptake often result in tissue nutrient concentration (increase) or dilution (decrease) in tested plant material.

Tissue Testing for Phosphorus and Potassium in Corn and Soybean

Last year the new ISU Extension and Outreach publication CROP 3153 “Phosphorus and Potassium Tissue Testing in Corn and Soybean”provided the first-ever ISU sampling and interpretation guidelines for using tissue testing for P and K in corn and soybean. As both crop yields and interest in tissue testing have increased in recent years, extensive field research was conducted during the last decade to determine the value of tissue testing for these nutrients. Publication CROP 3153 provides sampling guidelines and interpretations for an early-season test and a mid-season test, as well as research results used to establish the guidelines.

For the early season test, sample the entire aboveground corn or soybean plant by cutting plants one inch from ground level at the V5-V6 growth stage. For the mid-season test in corn, sample the blade portion of the leaf opposite and below the primary ear at the R1 (silking) growth stage. For the mid-season test in soybean, sample the three top trifoliate leaves with leaflet borders not touching (including the trifoliate leaf petioles) at the R2-R3 stage growth stage. To ensure the tests results represent the collection area, each sample should be a composite from at least ten corn or soybean plants. That is, ten plants at the V5-V6 stage, ten corn ear-leaf blades at the R1 stage, or three trifoliate soybean leaves from ten plants at the R2-R3 stage.

Tissue test interpretations in Table 1 are from publication CROP 3153. Test results in the Low category indicate likely P or K deficiency, whereas test results in the High category indicate a high probability of P or K supply beyond amounts needed to maximize yield. A test result in the High category does not indicate nutrient supply that reduces yield, since fertilization did not cause yield decreases even for the highest observed concentrations.

Table 1. Interpretation categories of P and K tissue tests for corn and soybean based on two growth stages and plant parts.

Tissue Testing for Sulfur in Alfalfa

Publication CROP 3072 “Sulfur Management for Iowa Crop Production”provides guidelines for S management in corn, soybean, and alfalfa as well as interpretations for using S tissue testing in alfalfa. Extensive Iowa research during the last decade showed that tissue testing for S is not a reliable diagnostic tool in corn and soybean, but it is a useful tool in alfalfa. In fact, S tissue testing for alfalfa is recommended whereas S soil testing is not.

As with other nutrients or crops, the S tissue test for alfalfa was calibrated for a specific growth stage and plant part. Sample the top six inches of alfalfa plants at the bud stage before harvest including stem, leaves, and any buds or flowers. To represent an area reliably, each sample should be a composite of at least 15 plants. An S concentration of 0.22-0.25 percent indicates adequate S levels and unlikely alfalfa response to applied S. Lower S concentrations indicate a high probability of response to S application. Higher test results indicate an S supply higher than needed to maximize alfalfa dry matter yield, but the research has not shown yield reductions for these higher levels.

Tissue Testing for Micronutrients in Corn and Soybean

In spite of extensive field research in Iowa for decades, no tissue test interpretations for micronutrients in corn or soybean has been possible due to usually adequate soil supply and very infrequent or lack of yield response to fertilization in trials across the state. This was also the case in numerous trials with both crops conducted as recently as 2012 to 2015. In corn, there was no yield increase at any of 47 trials from boron, manganese, zinc or their mixture when applied to the soil or foliage (copper was applied in ten trials). Only very few and isolated corn zinc deficiencies have been reliably documented in Iowa and neighboring areas of surrounding states. In soybean, there was one yield increase and one yield decrease in 63 trials from boron, copper, manganese, or zinc or their mixture when applied to the soil or foliage (copper was applied in 46 trials). Soybean response to iron was not evaluated because although deficiency chlorosis is common in calcareous (high pH) soils, reported yield responses to iron fertilization in Iowa and the region have been infrequent and small.

The lack of yield response and the observed tissue test results strongly suggest that that “sufficiency ranges” for tissue tests published elsewhere are too high for most micronutrients and would encourage unneeded fertilization in many fields. Therefore, the only ISU Extension and Outreach guidelines for micronutrients are for zinc in corn and sorghum, and include only soil-test interpretation (see publication PM 1688, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa).

Use Tissue Testing Wisely
  • Tissue testing for P and K in corn and soybean can be useful but does not substitute for recommended soil testing and interpretations in making fertilization decisions.
  • Tissue testing for S is useful in alfalfa managed for hay but is not reliable to diagnose S status in corn and soybean.
  • Sample the plant parts at the growth stage based on research that developed interpretations.
  • No reliable tissue test interpretations for micronutrients could be developed due to infrequent deficiency and yield response. Research suggests that most interpretations used elsewhere recommend unnecessary fertilization in many Iowa fields.
  •  A potentially useful approach for tissue testing is when there are areas within a field that look normal and areas with poor growth or symptoms that could be related to nutrient supply. In such situations, collect and analyze both soil and plant tissue from normal and poor areas and compare results to previous information. Use of tissue testing alone can be misleading because stress caused by drought, excess moisture, pests or diseases, or severe deficiency of other nutrients can influence plant growth and nutrient uptake, nutrient concentrations, and thus result incorrect interpretations.
Additional Online Resources Crops: CornSoybeanBiomass and ForageCategory: Crop ProductionSoil FertilityTags: plant analysistissue testingnutrient deficiencysoil fertilityphosphoruspotassiumsulfurmicronutrients

Corn Rootworm Egg Hatch Peaking in Southern Iowa

Tue, 06/18/2019 - 08:55

Corn rootworm egg hatch in Iowa typically occurs from late May to the middle of June, with an average peak hatching date of June 6 in central Iowa. In 2019, the average hatching date will be behind the average, due to cool spring temperatures. Development is driven by soil temperature and measured by growing degree days. Research suggests about 50% of egg hatch occurs between 684-767 accumulated degree days (base 52°F, soil). Most areas in Iowa have reached peak corn rootworm egg hatch (Fig. 1).

Figure 1. Accumulated soil degree days in Iowa as of June 17, 2019. Expect 50 percent corn rootworm egg hatch between 684-767 degree days. Map data courtesy of Iowa Environmental Mesonet, Iowa State University Department of Agronomy.

To generate degree day accumulation on corn rootworm egg hatch for your area, use the ISU Agronomy Mesonet website. To create an accurate map, make sure to set the start date to January 1 of the current year and the end date to today, and set the plot parameter to “soil growing degree days (base = 52).” Be aware that some locations are having some technical difficulties with the soil temperature probes this year.

A severe corn rootworm larval infestation can destroy nodes 4-6; each node has approximately 10 nodal roots. Root pruning can interfere with water and nutrient uptake and make the plant unstable (Photo 1). A recent meta-analysis showed a 15% yield loss for every node pruned.

Photo 1. Severe root pruning by corn rootworm larvae can dramatically impact yield. Photo by Aaron Gassmann, Iowa State University.

Regardless of agronomic practices to suppress corn rootworm (e.g., crop rotation, Bt rootworm corn, or soil-applied insecticides), every field should be scouted for corn rootworm root injury. Continuous cornfields and areas with Bt performance issues are the highest priority for inspection. Looking at corn roots 10-14 days after peak egg hatch is encouraged because the feeding injury will be fresh. Assess corn rootworm feeding and adjust management strategies if the average injury is above 0.5 on a 0-3 rating scale. Also consider monitoring for adult corn rootworm to supplement root injury assessments. Aaron Gassmann, Iowa State University corn entomologist, has a webpage for additional corn rootworm management information including an interactive node-injury scale demonstration and efficacy evaluations.

Regardless of agronomic practices to suppress corn rootworm (e.g., crop rotation, Bt rootworm corn, or soil-applied insecticides), every field should be scouted for corn rootworm root injury. Continuous cornfields and areas with Bt performance issues are the highest priority for inspection. Looking at corn roots 10-14 days after peak egg hatch is encouraged because the feeding injury will be fresh. Assess corn rootworm feeding and adjust management strategies if the average injury is above 0.5 on a 0-3 rating scale. Also consider monitoring for adult corn rootworm to supplement root injury assessments. Aaron Gassmann, Iowa State University corn entomologist, has a webpage for additional corn rootworm management information including an interactive node-injury scale demonstration and efficacy evaluations.

Crop: CornCategory: Crop ProductionTags: rootwormCornscoutingpests

Stalk Borers Worth Scouting in Iowa This Summer

Tue, 06/18/2019 - 07:54

Within the last week, I have heard about higher-than-normal stalk borer infestations along field margins compared to previous years. According to degree-day tracking of 2019, the caterpillars should be moving from overwintering hosts to corn throughout Iowa this week.

Tracking degree days is a useful tool to estimate when common stalk borer larvae begin moving into cornfields from their overwintering hosts. Foliar insecticide applications, if needed, are only effective when larvae are migrating and exposed. Start scouting corn for larvae when 1,300-1,400 degree days (base 41°F) have accumulated. Much of Iowa has reached this important benchmark (Fig. 1), and therefore scouting for migrating larvae should begin now to make timely treatment decisions.

Figure 1. Degree days accumulated (base 41°F) for stalk borer in Iowa (January 1 – June 16, 2019). Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.


Female moths prefer to lay eggs in weedy areas in August and September, so managing weeds (especially giant ragweed) in and around corn during that time will make those fields less attractive. Long-term management requires mowing grassy edges around field edges so that females will not lay eggs in that area during the fall. To prevent stand loss, scout and determine the percent of infested plants. Consider applications at peak larval movement, or 1,400-1,700 degree days (base 41°F). The use of an economic threshold (Table 1), first developed by Iowa State University entomologist Larry Pedigo, will help determine justifiable insecticide treatments based on market value and plant stage. Young plants have a lower threshold because they are more easily killed by stalk borer larvae.

Table 1. Economic thresholds (expressed as percent of infested plants with larvae in the whorl) for stalk borer in corn, based on market value, expected yield, and leaf stage.

Stalk borers tend to re-infest the same fields, so prioritize fields with a history of stalk borers for scouting first with extra attention to the field edges. Applying insecticides after larvae have entered the stalk is not effective. Instead, target foliar applications to larvae as they migrate from grasses to corn. Look for larvae inside the whorls to determine the number of plants infested. The larvae are not highly mobile and typically only move into the first 4-6 rows of corn. Look for new leaves with irregular feeding holes or for small larvae resting inside the corn whorls. Larvae will excrete a considerable amount of frass pellets in the whorl or at the entry hole in the stalk. Young corn is particularly vulnerable to severe injury, but plants are unlikely to be killed once reaching V7.

Using burndown herbicides before corn planting can force stalk borers to move and infest emerging corn. If an insecticide is warranted based on stalk borer densities, the application must be well-timed to reach exposed larvae before they burrow into the stalk. Border treatments should be considered since the infestations are usually localized. Make sure to read the label and follow directions, especially if tank-mixing with herbicides, for optimal stalk borer control.


Stalk borer larvae have three pairs of true legs and four pairs of fleshy prolegs. The body is creamy white and dark purple with brown stripes. Often there is a creamy white stripe running down the back of the thorax and abdomen. A distinctive feature of stalk borer larvae is an orange head with two dark lateral stripes (Photo 1). The adults are dark grey and brown colored moths, with jagged white lines and two to three clusters of white spots (Photo 2).

Photo 1. Common stalk borer larva. Photo credit Adam Varenhorst.

Photo 2. Common stalk borer adult. Photo credit Adam Sisson.


Stalk borers have one generation annually in Iowa. Stalk borer eggs are laid on grasses and weeds in the fall and overwinter in this cold-hardy stage. Egg hatch typically occurs around April 19 – June 5, and about 50% of egg hatch happens at 494 degree days. Young larvae will feed on grasses and weeds until they outgrow the stem of the host plant. The number of larval molts is variable, depending on food quality, and ranges from seven to nine instars. Migration to larger hosts begins around 1,300-1,400 degree days. Fully developed larvae drop to the soil to pupate. Approximately 50% of pupation happens at 2,746 degree days, with 50% adult emergence at 3,537 degree days. Peak adult flight occurs during the first two weeks of September.

Corn adjacent to grassy and weedy areas becomes a suitable food source for migrating larvae. The most susceptible corn growth stages for infestation are V1-V5 or about 2-24 inches in plant height. Larvae can defoliate leaves and cause non-economic injury. More often, larvae kill corn plants by entering the stalk and destroying the growing point (i.e., flagging or dead heart). A dead heart plant will have outer leaves that appear healthy, but the newest whorl leaves die and can cause barren plants.

Photo 3. Stalk borer larvae can shred corn leaves and destroy the growing point.

For more information on stalk borer biology and management, read a Journal of Integrated Pest Management article by Rice and Davis (2010), called “Stalk borer ecology and IPM in corn.”


Crop: CornCategory: Crop ProductionInsects and MitesTags: caterpillarscoutingCornpestIPM

Japanese Beetles Get a Slow Start to 2019

Mon, 06/17/2019 - 12:19

Several reports from ISU Field Agronomists have indicated Japanese beetles are emerging in southern Iowa. The emergence is about 7-10 days behind the last few years, due to slowly accumulating degree days in 2019. Literature shows Japanese beetle adults need about 1,030 growing degree days (base 50°F) to complete development and will continue emergence until around 2,150 degree days. Based on accumulating degree-day temperatures in 2019, Japanese beetle adults should be active in some areas of southern Iowa this week (Figure 1).

Figure 1. Growing degree days accumulated (base 50°F) for Japanese beetle adults in Iowa (as of June 16, 2019). Adults begin emergence around 1,030 degree days. Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.

The false Japanese beetle (Photo 1) often emerges just before Japanese beetle (Photo 2). It is important to distinguish the two species. The former is not considered a field crop pest, but the later can be a pest on a number of crops, ornamentals, and garden plants. The insects resemble each other in the general size and shape, and are in the same subfamilies of beetles, called shiny leaf chafers (Rutelinae). True Japanese beetles are more iridescent with a metallic green head and thorax with copper-colored forewings. The false Japanese beetle is not quite as shiny (sorry, that is up for your interpretation!) and the white tufts of “hair” along the sides and tip of the abdomen are not as obvious.

Photo 1. False Japanese beetle. Photo by Erin Hodgson.

Photo 2. Adults Japanese beetles are metallic bronze and green with white tufts along the side of the abdomen. Photo by Teresa Cira. 

Plant Injury and Management

Japanese beetles have a wide host range that includes many species of fruit and vegetable crops, ornamentals, and field crops. On soybean, adults prefer to feed between the leaf veins and can ultimately consume most of the leaf. The treatment threshold for Japanese beetle in soybean is 30 percent defoliation before bloom and 20 percent defoliation after bloom. It is important to note most people overestimate plant defoliation.

Visit my recent ICM Blog post for a sampling plan to estimate defoliation; use Photo 3 to help calibrate defoliation estimates. I also recently published a review article for Japanese beetle if you want to learn more about this corn and soybean pest.

Crops: CornSoybeanCategory: Crop ProductionInsects and MitesTags: pestscoutingbeetledefoliation

Soybean Gall Midge Confirmed in Iowa and Nebraska

Sat, 06/15/2019 - 18:53

Last year, the widespread outbreak of soybean gall midge took many farmers and entomologists by surprise. There was significant field edge injury and economic loss in at least 65 counties in Iowa, Nebraska, Minnesota, and South Dakota. A small team organized a concerted effort to learn more about the life cycle, biology and management of soybean gall midge in 2019. The first step was to establish emergence cages in various habitat types to better understand where they overwinter. We used the “corn rootworm” style traps to collect adults emerging from the ground. A series of traps in Iowa, Nebraska and South Dakota have been monitored for several weeks this spring (Figure 1).

Figure 1. Soybean gall midge emergence trap locations for 2019. Map by Justin McMechan, University of Nebraska-Lincoln.

Mitchell Helton is a new ISU entomology graduate student and has been checking traps frequently. On Friday, 14 June, he had the first positive detection from emergence cages (Figure 2). The first adult collected was at the ISU Northwest Research Farm near Sutherland, IA. The trap was located in a midge-infested soybean field in 2018. Just a few hours later, Nebraska also had their first positive detection near Eagle in the east-central part of the state (Figure 2). They collected a few more adults from Cass County over the weekend.

Figure 2. First soybean gall midge adult collected in 2019 from northwest Iowa. Photo by Mitchell Helton, ISU.

Figure 3. First soybean gall midge adult collected in 2019 from east-central Nebraska. Photo by Justin McMechan, UNL.

We plan to continue adult emergence trapping this spring to help us understand peak activity for mating and laying eggs in soybean. At this point, just a few individuals in traps does not warrant a foliar insecticide. But our plans are to make treatments when adult captures increase. We will be sure to keep you updated on subsequent detections and application recommendations in the future.

Growers spraying too early may not have enough residual insecticide activity when adults emerge in the area and may not be able to spray the field again in that period depending on label restrictions limiting efficacy and increasing the likelihood for plant injury from gall midge.

Crop: SoybeanCategory: Crop ProductionInsects and MitesTags: midgeSoybeanscoutingpestIPM

Submitting Samples to the Plant and Insect Diagnostic Clinic

Wed, 06/12/2019 - 12:55

With the crops of Iowa in the ground, it is time to start thinking about seedling diseases. The Plant and Insect Diagnostic Clinic is a resource for corn and soybean growers assessing their field  throughout the season.

Making a diagnosis

The first step in managing a plant problem is to know what is causing the symptoms observed. Accurate pathogen or insect pest identification is one of the most important integrated pest management (IPM) tactics leading to a successful management strategy.

So how do you determine what is causing a particular set of symptoms? Sometimes different pests and disorders cause similar symptoms. How can these be differentiated? One option that many farmers and agribusiness affiliates take advantage of is the Iowa State University Plant and Insect Diagnostic Clinic (PIDC). At the PIDC, we diagnose plant problems, identify insects and provide management advice. Our team in the lab includes an entomologist (Dr. Laura Jesse) and two plant pathologists (Dr. Lina Rodriguez Salamanca and Ed Zaworski). As diagnosticians, it is our job to help solve the issues for you. Send us a sample, we are here to help.

Sending in a good sample

Once you have decided to send a sample, it is very important to submit a sample that is of good quality.

If the sample we receive is in poor condition, we may not be able to make an accurate diagnosis (Figure 1). Also, if the sample is allowed to degrade (left in a warm vehicle for example), secondary fungi and bacteria can colonize the plant tissue. After these organisms colonize a sample, it is often difficult to detect the pathogen or insect pest that was harming your crops.

Figure 1a. Good sample submission

Figure 1b. Poor sample submission

In many situations, it is important to submit the whole plant, especially during the early stages of plant development. Sending the whole plant gives us the opportunity to examine both the roots and the foliage of the plant. When you send the entire plant, it is a good idea to dig up the roots rather than to pull the plant from the ground. Pulling a plant from the ground can rip away the infected tissue, reducing the likelihood of us finding the pathogen. An important tip: when digging up a root ball, wrap the soil in something like newspaper or a plastic bag. Wrapping up the roots will keep the soil off the plant foliage.

Other times you may not need to submit the whole plant. If you are concerned only about a foliar problem, just the leaves are needed.

We need plenty of plant tissue to work with as it allows us to see a range of symptoms and run multiple types of diagnostic tests. When submitting foliage samples, make sure to gather lots of plant tissue. Send at least 6-8 plants when sending a whole plant sample.

As a reminder, we do not test for herbicide or other residues in plant tissue, but we can examine the symptoms to determine if it appears to match symptoms associated with exposure to herbicides or other chemicals. 

General tips
  • DO NOT add water to the sample!
  • Submit sample in plastic bags rather than paper (paper bags allow the sample to dry out)
  • Wrap root balls in newspaper or plastic to keep the soil separate from the foliage
  • Take pictures in the field, especially of symptom patterns, and close-ups of symptom details (see our guide on how to send us samples)
  • Provide lots of written information - the more, the better: seed treatment, crop variety or cultivar, chemical application history, the pattern observed or distribution in the field, crop history (or rotation), etc.
  • More general tips are available.
Crops: CornMinor cropsSoybeanBiomass and ForageCover CropCategory: Plant DiseasesTags: Corn diseasessoybean diseases

Staying on Top of Weeds in 2019

Tue, 06/11/2019 - 10:05

While good progress has been made toward getting crops in the ground, the adverse early spring conditions are likely to complicate weed management throughout the 2019 growing season. The most important step in minimizing problems is to scout fields regularly to identify problems quickly and allow timely adjustments to management.

Timely scouting of fields is critical to determine appropriate actions in time to determine effective action and protect crop yield potential.

Performance of preemergence (PRE) programs  

Products used in fields planted in a ‘timely fashion’ are likely to have shortened residual activity due to abundant rain, therefore increasing the importance of timely postemergence (POST) applications.  A different issue will be encountered in fields that were planted in the last week or two. These fields were planted during peak periods of weed emergence, and many areas have gone more than a week without rain to activate PREs.  Many of these fields will require a POST treatment several weeks earlier than normal.

Crop development  

A full crop canopy is a critical component of successful weed management.  While last week’s growing degree days helped move corn forward, the slow development throughout May, along with early degradation of the PRE products, will make achieving full season control more difficult this year than most.  

Delayed soybean planting is also likely to complicate weed management.  Many fields were planted under less than ideal conditions that may result in uneven stands, creating gaps in the canopy.  Soybean planted in 30-inch rows may not form a full canopy, further favoring the survival of late-emerging weeds. The delayed or hindered crop canopy development will make timely application of an effective POST program more important than in most years.

The application window for many products will be reduced due to the crop advancing through growth stages more quickly than with normal planting dates.  Flower development in soybean will still occur in the latter part of June for most fields, even many late-planted ones.  Dicamba products registered for Xtend soybean varieties must be applied prior to R1 stage soybean (before first flowers).  Liberty (Liberty Link) has the same restriction (up to R1), whereas glyphosate (Roundup Ready) or 2,4-D (Enlist) can be applied through the R2 stage.

Performance of POST programs  

While we have reduced our reliance on POST products due to widespread herbicide resistance, POST products will be more important this year due to the environmental factors influencing PRE herbicides.  The best way to reduce risks is to scout fields earlier than would be typical, spray when weeds are smaller than label limits, and include a residual herbicide with the POST program to extend control later into the season.  Waiting until weeds reach the maximum controllable size to begin applications and failing to include a residual herbicide will result in a greater risk of control failures, more weed survival and seed production, and greater opportunity for new resistance issues.

Herbicide carryover 

The potential for herbicides to affect rotation crops or cover crops is affected by many factors.  While herbicide half-life and rainfall throughout the growing season are the most important factors, application date also plays a role.  Delayed application results in the herbicide being applied closer to the establishment of the following crop, and in some cases might lead to problems.  Check rotational restrictions on products to ensure they won’t affect plans for rotational crops this fall or next spring.  Herbicides that late applications can increase carryover risk include fomesafen, imazethapyr, and chlorimuron.


The impact of this spring’s cool and wet weather on weed control is not over.  Weed management programs in many fields will need to be adjusted from what was planned in order to avoid costly control failures.  Early scouting of fields is essential to determine how well PRE programs are working and allow timely adjustments to the POST program.  Please reach out to your local ISU Extension and Outreach Field Agronomist if you have questions about managing weeds this year.

Crop: CornCategory: WeedsTags: preemergence herbicidespostemergence herbicides

Nitrogen Fertilization Options with Delayed or Rescue Applications

Fri, 06/07/2019 - 10:57

It’s been a wait-go-stop (repeat) corn planting season this spring. Whether you planted early or are just now getting corn planted, it seems planting windows were short and rushed. In some cases this meant planting (corn/soybean) and worrying later about getting nitrogen (N) applied. And in some areas of Iowa, wetter than normal conditions are raising questions about supplemental N application. What are the options for sidedress N?

Typical Sidedress Application Timing

If decisions were made to plant corn and then apply N sidedress, be certain to check that fertilizer products and application equipment being considered will be available. Sidedressing options are listed below in order from generally most to least preferable; however, product/equipment availability and personal preference often constrains fertilizer/application choice:

  • Injected anhydrous ammonia, urea-ammonium nitrate (UAN) solution, or urea.
  • Broadcast dry ammonium nitrate or ammonium sulfate.
  • Surface banding UAN.
  • Broadcast urease inhibitor treated urea.
  • Broadcast urea.
  • Broadcast UAN (depends on plant size, see below).

Sidedress injection can begin immediately after planting if rows are visible or GPS guidance positioning equipment is used. Be careful so soil moved during injection does not cover seeded rows or small corn plants. It is easiest to inject in the row middle and generally there is no season-long advantage to place an N band close to the row. Corn roots will reach the row middle at early growth stages. In cases where no N was applied preplant, no starter N applied, or preplant N injected deep into the soil, a dribble application close to the corn row may help with early N availability and corn early growth. We may see streaking in fields with preplant anhydrous ammonia application; due to wet soils restricting early corn growth and low soil nitrate this spring (especially in corn following corn). Sidedress injected or dribbled N can also be applied between every-other-row.

Broadcasting dry urea, ammonium sulfate, or ammonium nitrate across growing corn might cause some leaf spotting or edge browning where fertilizer granules fall into the corn whorl. The chances of this happening increases with larger corn and high application rates. As long as the fertilizer distribution is good and not concentrated over plants, leaf damage should only be cosmetic. Ammonium nitrate and ammonium sulfate will cause more leaf tissue damage than urea, however, supply is typically limited for those fertilizer products.

Since UAN solution is comprised of one-half urea and one-half ammonium nitrate, it has less volatile ammonia loss concern that dry urea. Surface banding UAN will reduce loss potential. A urease inhibitor with surface applied and non-incorporated urea and UAN will help reduce volatile loss (product labels often state up to 10 days, with loss increasing over time). The rate of N applied, i.e. the amount of potential N loss, has to be large enough to offset the urease inhibitor cost. Conditions increasing chance of volatile loss include: no precipitation for many days after application, high surface residue, warm temperature, high surface soil pH, moist-to-drying soils, and low soil cation exchange capacity. Precipitation within 2-3 days of application (about 0.25-0.50 inch) will stop volatile loss as urea is moved into the soil. Also, precipitation is needed to move surface applied N into the crop root zone.

Broadcast application of UAN solution across growing corn has the potential to cause leaf burn and reduced early growth. Research conducted in Minnesota indicated that when corn plants were at the V3 growth stage (three leaf collars visible), phytotoxic effects were worse at rates greater than 60 lb N/acre (rates applied were 0, 60, 90, and 120 lb N/acre), but damage was not permanent and did not adversely affect stand or yield. When plants were larger than the V3 stage, plant damage was worse and some yield depression occurred with the 120 lb N/acre rate. Broadcast UAN applications beyond the V7 stage are not recommended, and the risk of injury increases during hot, dry conditions. Many preemergence herbicides are applied using UAN as the carrier to minimize trips across fields. However, this strategy is only recommended prior to crop emergence. Most herbicides prohibit application in N solutions after corn has emerged. Check herbicide labels closely.

Mid-to-Late Corn Vegetative Stage Applications

If corn becomes too tall for normal sidedress equipment, it is possible to use high clearance equipment to apply N. The N sources typically will be UAN solution, with equipment available to either dribble the solution onto the soil surface with drop or drag tubes or shallow inject with coulter-shank bars (coulter-disk injected), and dry urea which can be broadcast spread across the top of corn. Leaf tissue symptoms from broadcast dry urea can be more than with smaller corn, especially with full N rates.

Research in Iowa has shown corn can respond to mid-to-late vegetative corn growth stage N application when there is deficient N supply, but there can be loss in yield potential. Reduced yield occurs more frequently and to a greater extent when soils are dry at and after application (applied N not getting into the root zone) and with severe N stress. Best responses occur with sufficient precipitation shortly after application to move N into the active root zone. If late N application is needed, it should be applied than the tassel stage. Having some non-N limiting (approximately 25% more than normal rate) reference strips or small areas are helpful for comparisons. These areas can be used to visually (or with sensing tools) determine if corn would respond to additional N, as a check to see if earlier N applications are not sufficient, and to determine that a crop color or growth issue is due to N deficiency and not some other stress. These reference areas should be planned and N applied early in the season if possible, small areas with N applied now to check plant visual response or field areas that are known to be non-N deficient.


The most important thing is to get N applied to corn. While all product/method/timing options may not be ideal, not getting N applied is a much greater concern. Across the Corn Nitrogen Rate Calculator database in Iowa, corn yield increase from N application (yield at optimum N minus yield with no N) averages 100 bu/acre in continuous corn and 70 bu/acre in corn following soybean. Fine-tuning applications certainly helps insure best yields possible for the year, but sometimes risk of not getting N applied should also be considered. At this time there is still a wide window for getting successful sidedress N applications completed.

Crop: CornCategory: Soil FertilityTags: sidedress Nlate sidedress N

It’s Time to Think About Sampling for Nematodes that Feed on Corn

Thu, 06/06/2019 - 11:43

As Iowa’s corn crop gets slowly planted and established in 2019, it’s time to turn thoughts towards pests. Plant-parasitic nematodes that feed on corn can cause damage and yield loss. Some nematode species are damaging to corn at very low population densities (numbers). But most species are not harmful until population densities reach many hundreds or more per 100 cm3 (a little less than a half-cup) of soil. And then there are some nematode species that are not thought to be harmful to corn at all. It is very common for Iowa fields to have several different species of plant-parasitic nematodes present at low numbers. A summary of results of testing for nematodes that feed on corn in Iowa from 2000 to 2010 is available online.

Sampling to check for damaging levels of nematodes must be done during the growing season - ideally when symptoms of damage are seen. Following are guidelines on how to collect samples for assessing the potential for damage and yield loss caused by nematodes that feed on corn.

What type of sample should be collected?

Up until corn growth stage V6: collect soil and root samples.

  • Use a soil probe and collect cores that are at least 12 inches long.
  • Collect 20 or more soil cores to represent an area.
  • Collect soil cores from within the root zones of plants with symptoms of damage (see figure).
  • Combine (but do not mix) the soil cores and place them in a sealed plastic bag labeled with a permanent marker.
  • Also collect, with a shovel, the root mass from 4 to 6 plants with symptoms of damage. Take care not to strip off the smaller, seminal roots (see figure). The tops of the plants can be cut off and discarded. Place the roots in a sealed plastic bag labeled with a permanent marker.
  • Protect the samples from physical jarring and keep the samples cool (room temperature or below).

Figure: collecting a soil core (left) and a young corn plant for nematode testing (right)

From corn growth stage V6 through R3 (milk): collect only soil samples.

  • Use a soil probe and collect cores that are at least 12 inches long.
  • Collect 20 or more soil cores to represent an area.
  • Collect soil cores from within the root zones of plants showing symptoms of damage. Combine (but do not mix) the soil cores and place them in a sealed plastic bag labeled with a permanent marker.
  • Protect the samples from physical jarring and keep the samples cool (room temperature or below).

From corn growth stage R4 (dough) to harvest: sampling is not recommended.

There is not a reliable relationship between damage or yield loss and the number of nematodes present in soil and roots once the corn crop reaches the R4 growth stage. Therefore, sampling is not recommended after this point in the growing season.

Where to send samples?

Several private laboratories and most land-grant university plant diagnostic laboratories or plant disease clinics process samples and identify and count the numbers of plant-parasitic nematodes present. A list of the university laboratories and their contact information can be found online here.

The Iowa State University facility's location and address are: Plant and Insect Diagnostic Clinic, Room 2445 Advanced Teaching and Research Building, 2213 Pammel Drive, Iowa State University, Ames, IA 50011-1101.

The test for nematodes that feed on corn from the ISU Plant and Insect Diagnostic Clinic is called the complete nematode count. Samples sent to the ISU Clinic should be accompanied by a Nematode Sample Submission Form (ISU Extension Publication "PIDC 32") and a check for the $35 per sample processing fee ($45 per sample for out-of-state samples).

Management options if damaging levels of nematodes are found

If damaging population densities of nematodes are found, there is nothing that can be done during the season to limit the build-up of nematode numbers and lessen the yield loss. Management options for future corn crops include use of soil-applied Counter® 20G nematicide and/or seed treatments such as Aveo®, Avicta®, Escalate®, Nemastrike™, Trunemco™, and Votivo®. Use of these management options must be decided upon before the corn crop is planted.

Crop: CornCategory: Plant DiseasesTags: nematodessampling for nematodes