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Pest resistance management from hospitals to farm fields

Mon, 03/12/2018 - 13:13

Pest resistance is a world-wide challenge from hospitals to farm fields. I recently spent several days in the hospital battling an internal bacterial infection, which required intravenous antibiotics. I noted that the medical staff:

  1. properly identified the bacteria causing the infection.
  2. identified which antibiotic active ingredients were effective against the bacteria identified.
  3. addressed potential resistance issues by rotating between antibiotics, sifting through all of the brand names to be sure they were not sequentially using two antibiotics that were the same thing but sold under different names.
  4. used full rates of the antibiotics.
  5. administered the antibiotics in a timely fashion.
  6. monitored the results to assure success.

One of my nurses, who knows nothing about farming, asked if pest (disease, insect, and weed) resistance management for farmers is as big a challenge as it is in the medical field. My response was an emphatic, “Yes!” And I noted to the nurse that we tell farmers, dealers, and consultants to be sure to:

  1. properly identify the pest.
  2. select products effective against the pest.
  3. use multiple sites of action.
  4. use full rates of active ingredients.
  5. make applications in a timely manner.
  6. scout after the application to determine success.

We each need to make the same effort in pest resistance management on our farms that I observed while I was in the hospital.

Category: Crop ProductionInsects and MitesPlant DiseasesWeedsHerbicide ResistanceTags: WeedsInsectsdiseaseherbicide resistancefungicide resistanceAuthor: Virgil SchmittCrop(s): CornSoybean

Dicamba, monarchs, and milkweeds

Sun, 03/04/2018 - 10:40

The Center for Biological Diversity (CBD) recently released a report (A menace to monarchs) describing the threat to monarch butterflies posed by an increased use of dicamba.  While I have been fairly critical of the Xtend system1 (dicamba-resistant trait allowing postemergence applications of dicamba in cotton and soybean) due to off-target movement and injury, I think CBD has exaggerated the threat to monarchs posed by dicamba.

The CBD reported that within two years 60 million acres of habitat will be sprayed with dicamba.  This is based on Monsanto’s estimates of acres planted with dicamba-resistant cotton and soybean.  In the Cornbelt it is likely a significant percentage of fields planted with dicamba-resistant soybean acres will not be sprayed with dicamba, but it is safe to say there will be a large increase in dicamba use.

Common milkweed is one of the few native prairie plants capable of surviving management practices used in producing agricultural crops.  Prior to the introduction of Roundup Ready crops, about 50% of Iowa’s crop fields were infested with low densities of milkweed.  A decade later both the number of fields infested and the amount of milkweed in fields declined by more than 80% (Hartzler, 2010).  The CBD report suggests that dicamba use will further reduce the amount of milkweed in crop fields.

The loss of common milkweed from crop fields in the Cornbelt is a contributing factor in the monarch’s decline.  However, to expect farmers to intentionally allow milkweed to survive within their fields is unrealistic.  Farmers use the most cost-effective practices available to control weeds.  Decreasing the intensity of management to allow milkweed to survive within fields would increase the likelihood of other weeds surviving, and therefore negatively impact yield and profitability.

The CBD report also states that an additional 9 million acres of habitat will be damaged due to off-target movement of dicamba.  This number is based on extrapolating from the acres of off-target injury in 2017.  I suspect that registration for the new dicamba uses will be further restricted if similar levels of off-target damage are seen in 2018 (the EPA granted the new dicamba products a 2-year probationary label prior to the 2017 growing season).  In Iowa and surrounding states, the majority of off-target injury was limited to non-Xtend soybean varieties.  This is due to the inherent sensitivity of soybean to dicamba.  A dose equivalent to 0.005% of the labeled rate of dicamba can injury soybean; for comparison, it takes 0.1% of the labeled rate of 2,4-D to injure grapes (Hartzler 2016).  The sensitivity of most native plants to dicamba has not been determined, but most likely they are significantly less sensitive to dicamba than soybean.  This does not mean off-target movement to monarch habitat isn’t a concern, but that the risk and extent of injury to other plants outside of crop fields is less than the risk to sensitive soybean in the vicinity of dicamba-treated fields.

Dicamba-damaged common milkweed leaf with monarch egg.

The CBE report cites research indicating 1% of the labeled rate of dicamba reduces common milkweed biomass by 50% (Egan et al. 2014).  It is important to recognize that 4 to 6 leaf seedlings were used in this research.  Under field conditions I suspect the damage to milkweed would be much less since the majority of common milkweed in the field develops from established rootstocks, and milkweed would be exposed to dicamba at later stages of development.  We evaluated the response of common milkweed to low doses of dicamba, and the influence this injury had on ovipositioning by monarchs (Hoey et al. 2016).  Doses simulating drift equivalent to 0.1 and 1.0% of the labeled rate (0.5 lb/acre) caused severe distortion of leaves that emerged following application, but the emergence rate of leaves was not affected.  We did not determine milkweed biomass; while I suspect there was some reduction, it would have been much less than 50%.  Egg laying by monarchs was not affected by dicamba injury (Table 1).

Table 1.  Effect of simulated dicamba drift to common milkweed
on monarch ovipositioning.2

% of labeled rate

Injury rating (1-5)











2Greenhouse grown common milkweed (approximately 8” in height) were
sprayed with dicamba.  Two weeks after application plants were placed
outdoors at ISU Horticulture Farm for three weeks.  Eggs were counted and
removed twice weekly.


Everyone involved in production agriculture recognizes the extent of off-target injury associated with dicamba use in Xtend crops during 2017 was unacceptable.  The labels for the dicamba products registered for use in Xtend soybean include numerous restrictions intended to minimize off-target movement.  The requirement for downwind buffers at field edges is the most important strategy for protecting monarch habitat and other natural areas.  When applicators follow label requirements related to sprayer setup and wind speed, these buffers should protect natural areas that serve as habitat for monarchs, pollinators, and other organisms.  While volatility remains a concern, the quantity that leaves fields due to vapor loss typically is much less than associated with particle drift.

The decline in the monarch population is complex with many contributing factors (loss of habitat in both the overwintering and reproduction areas, climate change, disease and predators, etc.).  Expanding monarch habitat in Iowa is essential for recovery of the monarch since it is estimated that more than 45% of the overwintering monarchs in Mexico originate from Iowa and neighboring states.  I strongly believe herbicide use, including dicamba, and monarchs can co-exist, but it requires appropriate product selection and responsible application to protect resources adjacent to crop fields.  Information related to conservation efforts for the monarch in Iowa is available at the Iowa Monarch Conservation Consortium.


Donley, N.  2018.  A menace to monarchs.  Center for Biological Diversity.  13 pp.  Online.

Egan, J. F., Graham, I. M. and Mortensen, D. A. 2014. A comparison of the herbicide tolerances of rare and common plants in an agricultural landscape. Environ Toxicol Chem, 33: 696–702.

Hartzler, B.  2016.  Not all drift is created equal.  ICM online.

Hartzler.  2010.  Reduction in common milkweed (Asclepias syriaca) occurrence in Iowa cropland from 1999 to 2009.  Crop Protect. 29: 1542-1544.

Hoey, B.L., Lizotte-Hall, S. and Hartzler, B. 2016.  Effect of growth regulator herbicide injury to common milkweed on ovipositioning by monarch butterflies.  Proc. North Central Weed Sci. Soc.  71:75.


1The Xtend system consists of soybean and cotton varieties introduced by Monsanto that are modified to be resistant to dicamba.  Only formulations of dicamba with reduced volatility are registered for use on the the dicamba-resistant crops.  Currently, Monsanto, BASF, and DuPont sell these dicamba formulations.

Category: WeedsTags: dicambaXtendmonarchsdriftAuthor: Bob HartzlerCrop(s): Soybean

New Resource Explains the Revised Worker Protection Standard (WPS)

Thu, 03/01/2018 - 14:58

A collection of educational resources to help people on farms, orchards, forests, and other agricultural establishments comply with EPA’s revised Worker Protection Standard (WPS) regulation is now available online at http://pesticideresources.org/wps. The WPS Compliance Assistance Library provides a decision tree to help people determine their responsibilities, if any, under the WPS regulation. The online library also offers relevant information based on each person’s role within the WPS, including fact sheets, videos, and FAQs. A library of training materials, all available online, includes materials for training trainers, workers, and pesticide handlers.  

The WPS Compliance Assistance Library was produced by the Pesticide Educational Resources Collaborative (PERC) in collaboration with the EPA’s Office of Pesticide Programs.

Category: Pesticide EducationTags: Worker Protection StandardAuthor: Betsy Buffington; Kristine Schaefer

Moth trapping network expands in 2018

Tue, 02/27/2018 - 14:12

The IPM team at Iowa State University has been monitoring for field crop pests for a long time. The target pests have changed over the years, but the goal always remains the same: help inform farmers about pest activity in Iowa. One long-standing project is monitoring for black cutworm, an erratic field crop pest. In 2017, we also included true armyworm as part of the trapping network. We use adult trap captures to estimate feeding injury in field crops. Both of these pests are migratory and therefore the timing of injury is variable between growing seasons.

In 2018, we want to include more Iowa cooperators to help us with black cutworm and true armyworm trapping. Having more traps around the state will help us refine pest movement and scouting/treatment windows. If you are willing to establish a location in 2018, please reach out to us at bugtraps@iastate.edu with your contact information, including mailing address and county where trap will be placed. Cooperators will monitor traps every other day, from early April to late May, and report trap captures to us weekly. We will mail supplies and instructions to you in late March.


Category: Crop ProductionInsects and MitesTags: pestscoutingcutwormtrapsAuthors: Erin HodgsonAdam SissonCrop(s): CornSoybeanVideo: 

Save on ISU Extension field guides and disease references through bulk deals

Wed, 02/21/2018 - 14:25

The new crop season is right around the corner, and we want you to be prepared for yield-reducing disease, insect and weed issues! ISU Extension publications are great field scouting resources and provide a wealth of information for identifying production issues in your fields.

Four ISU Extension publications are now offered at substantial savings when purchased in boxed quantities. These include the Corn and Soybean Field Guide, the Weed Identification Field Guide, Soybean Diseases and Corn Diseases.


  • Soybean Diseases, recently updated, is available as single copies for $5 each, but can be ordered in boxed quantities of 50 for a reduced price of $3.50 per publication. More details here.
  • Corn Diseases, revised in 2017, also retails for $5 per copy with a reduced price of $3.50 each for ordering in boxes of 50. More details here.
  • The Corn and Soybean Field Guide is normally $15 for a single copy. However, if you purchase a bundle of 25, you only pay $10 per copy. More details here.
  • The Weed Identification Field Guide 2nd Edition, normally $10 per hard copy, is less than $8 per guide if you purchase a box of 44 copies. More details here.

Why buy?  

  • Soybean Diseases is a 40-page, full color compendium with scouting tips, disease descriptions, hi-resolution images and general recommendations for disease management. Also included are illustrated disease cycles for many diseases, a foliar disease estimation chart, and soybean growth and development and staging information.
  • Corn Diseases helps farmers and agriculture professionals identify and scout for corn diseases and provides general recommendations for management. Also included in this 48-page resource are illustrated disease cycles for primary diseases, a foliar disease estimation chart, and corn growth and development and staging information.
  • The Corn and Soybean Field Guide includes updated text and 375 images, illustrations, diagrams and tables to assist farmers with identifying corn and soybean diseases, insects and disorders found throughout the Midwest. This 158-page guide focuses on development stages, pesticide decisions and production-related topics, and for the first time, has information on newer soybean viruses to help you when scouting fields this summer.
  • The pocket-sized Weed Identification Field Guide 2nd Edition, contains 35 illustrations and more than 250 high-quality photographs of weeds found in Iowa. Palmer amaranth information was added to the 108-page field guide, and information on herbicide resistance and management was updated from the first edition. 

Category: Insects and MitesPlant DiseasesWeedsTags: field guidespublicationssoybean diseasesCorn diseasesweed identificationcorn and soybean field guideextension storeAuthor: Adam SissonCrop(s): CornSoybean

29th Staniforth Memorial Lecturer is Dr. Fred Gould

Fri, 02/16/2018 - 11:50

Dr. Fred Gould, University Distinguished Professor and Reynolds Professor of Entomology at North Carolina State University will present the 29th Staniforth Memorial Lecture on April 3rd at 4PM in the Curtiss Hall Auditorium on the Iowa State University Campus.  Dr. Gould was elected to the National Academy of Sciences in 2011 and won the Alexander von Humboldt Award for the most significant agricultural research over a 5-year period in 2004.  He has served the National Research Council of the National Academy of Sciences on several occasions most recently as the Chair of the committee that developed the report “Genetically Engineered Crops: Past Experiences and Future Prospects”.  In 2000, he served as the NRC committee chair that submitted the report “Environmental Effects of Commercialization of Transgenic Plants” and Dr. Gould has served on four Environmental Protection Agency Scientific Advisory Panels on transgenic crops.  Iowa State University is fortunate to have a scholar like Dr. Gould present the 2018 Staniforth Lecture.  The lecture will be titled “Will Genetically Engineered Pests Protect Health, Biodiversity, and Crop Production?” and Dr. Gould will provide a summary of theoretical, experimental, and practical issues being addressed by researchers attempting to genetically suppress or alter characteristics of insect pest populations. 

The Staniforth Memorial Lecture honors Dr. David W. Staniforth who was a weed scientist at Iowa State University from 1947 to 1984. Dr. Staniforth was a pioneer in the field of weed science and his research helped to shape the effective weed control systems used by farmers today.  His experience spanned the development of modern herbicide technology, beginning with work on the mode of action of 2,4-D and continuing through refinements in weed control systems including the development of weed control systems for conservation tillage.  This will be the 29th Staniforth Memorial Lecture.

Category: WeedsTags: 2018 Staniforth Memorial LectureAuthor: Micheal D.K. Owen

Interactive Crop Scout Training Available Online

Wed, 02/14/2018 - 14:32

Iowa State University Extension and Outreach published the Field Crop Scouting book as an online learning tool for crop scouts and other ag professionals in Iowa. This interactive textbook allows individuals to acquire knowledge about multiple crop scouting topics to help create stronger crop production systems in Iowa. These include:

  • corn and soybean growth stages,
  • insect identification,
  • crop diseases,
  • crop disorders, 
  • weed identification,
  • how to properly scout a field and
  • other topics.

The Field Crop Scouting book was developed for crop scouts, agronomists, commodity groups and others working in the agriculture industry who wish to learn crop scouting basics and to obtain a certificate of successful completion. The certificate will provide verification to employers that they’ve been trained in the basic concepts of crop scouting. Web book tutorials contain guided practice to give the learners experience and confidence. Immediately after the guided tutorials, assessment of student comprehension is conducted to test understanding of concepts. 

The Field Crop Scouting book is $9.99 and can be purchased online at Bleep Blorp Books (bleepblorp.pub/store/1). The online book contains a wealth of information and provides:

  • high-resolution images,
  • videos,
  • slideshows,
  • interactive graphs and
  • animated figures.

Readers will be able to move easily from one topic to another and explore issues based on their interests. The learning tool can be accessed through any online browser or mobile device.  

The online book is sponsored by Iowa State University Extension and Outreach, Integrated Pest Management at Iowa State University and the United States Department for Agriculture.

The Field Crop Scouting interactive book can be accessed through common web browser software on tablets, smartphones and desktop and laptop computers. 

Example pages from the Field Crop Scouting interactive book. The book covers many topics including corn and soybean growth stages, insect identification, crop diseases and weed identification. 

Category: Crop ProductionInsects and MitesPlant DiseasesWeedsTags: web bookentomologyplant pathologyfield cropcrop disordersweed scienceAuthors: Daren MuellerAdam SissonCrop(s): CornSoybean

Southeast Iowa Agricultural Research Association Annual Meeting on March 1

Tue, 02/13/2018 - 09:10

Farmers and agriculture suppliers will want to attend the Southeast Iowa Agricultural Research Association annual meeting at the Johnson County Extension office in Iowa City on Thursday, March 1, 2018 beginning at 10:00 a.m., according to association president Jeff Ellis.

Following a brief business meeting, the day will feature an update on activities at the Iowa State University (ISU) Southeast Research and Demonstration Farm by President Ellis and Superintendent Myron Rees and presentations on “2018 Crop Market Outlook” by Chad Hart, ISU Agricultural Economist, “Dicamba Outlook for 2018” by Kristine Tidgren, Director of the Iowa State University Center for Ag Law and Taxation, “Using ISU Weather, Soil Moisture, and Temperature Station Data for Decision Making” by Elwynn Taylor, ISU Climatologist, and “Crop Rotation and Profitability Research Results” by Matt Liebman, ISU Professor of Agronomy.

The meeting, including lunch, will be provided at no charge and will conclude at 3:00 pm.  Certified Crop Advisor Continuing Education Units will be available.

For more information, contact Meaghan Anderson (mjanders@iastate.edu), Greg Brenneman (gregb@iastate.edu) or Ryan Drollette (drollett@iastate.edu) at (319) 337-2145, Rebecca Vittetoe (rka8@iastate,edu) at (319) 653-4811, or Virgil Schmitt (vschmitt@iastate.edu) at (563) 263-5701.

Category: Crop ProductionTags: research farmsISU Southeast Research and Demonstration FarmSoutheast Research and Demonstration Farmcrop marketdicambaWeathercrop rotationprofitabilityAuthors: Meaghan AndersonGreg BrennemanVirgil SchmittRebecca VittetoeCrop(s): CornSoybean

New Soybean Disease Identification Resource for 2018

Mon, 02/05/2018 - 13:34

Iowa State University Extension and Outreach has published an updated version of the Soybean Diseases publication to help farmers and other professionals in the agriculture industry identify and scout for disease threats to soybean production in Iowa. The publication includes scouting tips, disease descriptions, hi-resolution images and general recommendations for management. Also included are illustrated disease cycles for many diseases, a foliar disease estimation chart, and soybean growth and development and staging information.

Soybean disease issues change over time, and information about diseases advances with new research. Soybean Diseases was updated to reflect these changes and to improve the usefulness of the publication. Several diseases not found in the previous version have been added, such as soybean vein necrosis and tobacco ringspot, along with new images and updates throughout.

The Soybean Diseases publication is available to purchase online at the Extension Store. A hard copy of the publication costs $5, but you also have the option to order it in boxed quantities of 50 for a reduced price of $3.50 per publication. Printable downloads are $2.50 each.

Related links:

ISU Extension Store

Crop Protection Network

Related publications:

A Farmer's Guide to Soybean Diseases

Corn and Soybean Field Guide

Corn Diseases

Category: Plant DiseasesTags: Soybeandiseasecrop scoutingplant pathologyidentificationmanagementfoliarseedlingstemrootAuthors: Adam SissonDaren MuellerCrop(s): Soybean

2018 Soil Fertility and Nutrient Management Short Course

Fri, 01/26/2018 - 11:46

If you are interested in an in-depth course covering soil fertility and nutrient management, then consider attending the Iowa State University Extension and Outreach 2018 Soil Fertility and Nutrient Management Short Course. This is a two-day event that provides an opportunity to dig deeper into the fundaments of soil fertility, nutrient management, and relationships to profitable crop production and water quality. The course is February 20-21, 2018 in Ames, Iowa.

Course Topics Include:

  • Iowa soils, soil properties, crop productivity
  • Soil sampling and testing
  • Cation exchange
  • Soil pH and liming
  • Essential plant nutrients
  • Secondary and micronutrients
  • Nitrogen management
  • Soil nitrogen dynamics, crop uptake, and diagnostic tools
  • Phosphorus and potassium management
  • Environmental phosphorus and the Iowa P-Index
  • Site-specific management
  • Manure nutrient management
  • Nutrient recommendation development

Class size is limited to 40 individuals and pre-registration is required to attend. More information about the course, course registration, and contacts for registration information are located at the following web site.


If you have questions about the course, you can also contact John Sawyer at jsawyer@iastate.edu.

Category: Soil FertilityTags: soil fertilitynutrient managementAuthors: John SawyerAntonio MallarinoCrop(s): CornSoybean

New Site Gives Midwest Farmers Easy Access to Crop Management Resources

Thu, 01/25/2018 - 10:40

Farmers and agribusiness have a new tool to help them tackle crop management challenges. The Crop Protection Network, a multi-state and international collaboration of university and provincial extension specialists, has redesigned its website at www.cropprotectionnetwork.org.

In addition to the corn and soybean publications it is known for, the site now offers videos, newsletter and blog articles, featured articles, and Twitter updates from CPN partners on important crop management issues. The website also features an encyclopedia of field crop diseases designed to help farmers identify diseases using extensive image galleries and keywords to filter results.

“The new website still has all of the great CPN content that users are familiar with, but also adds new resources, and will be updated frequently,” said Kiersten Wise, University of Kentucky Extension plant pathologist, and co-director of CPN. "Our goal is to help farmers make crop management decisions with relevant and timely information.”

Over 45 extension specialists from land-grant universities and CPN partner institutions help develop content, which means that stakeholders can trust that the information they see on the website and in the publications is research-based.

“Farmers and agricultural personnel will be provided with information to help with decisions to protect field crops," said Daren Mueller, Iowa State University Extension and Outreach plant pathologist and co-director of CPN. "Information on wheat management and other crops will be added in 2018, expanding resources for farmers.”

Visit the new Crop Protection Network website at www.cropprotectionnetwork.org .

Category: Crop ProductionTags: crop production and managementAuthor: Daren MuellerCrop(s): CornSoybeanBiomass and ForageCover Crop

High yields despite drought conditions

Fri, 01/05/2018 - 09:50

Most of Iowa was wet and cool during planting time (end April to mid-May), warm and dry during vegetative growth (June to July), and cool and wet during reproductive development (August to September). In some locations, the June-July drought was severe with precipitation deficits exceeding eight inches. Despite this drought, yields were high and above the long-term trend for a third straight year.

In-season rainfall is not the only source of water for crops in Iowa. Stored soil water and shallow groundwater were significant contributors to total water uptake. For this reason, crops survived the June-July drought. Field measurements revealed that the depth to water table was approximately five to six feet below the soil surface in mid-July (corn silking time) and crop roots had reached that depth. Deep roots and shallow water tables compensated for much of the precipitation deficit. In fact, below normal June precipitation is favorable in Iowa for two reasons: i) rapid and unconstrained root growth (up to 1.3 inches per day); ii) given generally high soil water content in the early spring, additional precipitation will stimulate nitrogen loss.

In terms of grain yield equivalent, model analyses suggest that root access to shallow groundwater accounted for 1% to 46% of total grain yield. The contribution was lower in sites with sufficient rain and higher in sites with drought. In addition to water, the same analyses indicated that the subsoil had more than enough nutrients for sustaining high crop yields.

Cool temperatures in August and September reduced transpiration rates, slowed development, and extended the grain-filling period. Additionally, the growing season was extended by a later than normal first killing frost in the fall. This resulted in higher than normal seed weight in corn.

Together, these factors contributed to surprisingly high corn yields in 2017. Quantifying temperature, precipitation, and water table dynamics across the state has been an important component of moving the science of digital agriculture forward to enable better predictions of yield and will greatly assist management decisions and predictability of crop yields and nitrogen losses.

Figure 1: Cumulative difference between 2017 precipitation and 35-yr average for five locations in Iowa. SW = southwest, NE = Northeast, NW = Northwest, SE = Southeast. Grain yields ranged from 190 to 235 bu/ac. 

Figure 2: Cumulative difference between 2017 cummulative GDD and 35-yr average for five locations in Iowa. SW = southwest, NE = Northeast, NW = Northwest, SE = Southeast. Grain yields ranged from 190 to 235 bu/ac. 

Figure 3. Leaf number, node number, and grain yield (top panels) and water table and root depth (bottom panels) for corn in central Iowa (left panels) and soybean in southeast Iowa (right panels) in 2016. Ordonez et al., 2018.

Category: Crop ProductionTags: groundwaterrootsWeatherdroughtyieldAuthors: Sotirios ArchontoulisMark LichtMike CastellanoCrop(s): CornSoybean

Dicamba: Past, Present, and Future

Wed, 12/27/2017 - 15:37

Dicamba has been an important component of Iowa weed management systems for more than 40 years.  The history of its use is somewhat unique in that its popularity has ebbed and flowed over time.  The increase in herbicide resistant weeds combined with the introduction of dicamba-resistant soybean (Xtend) promises a large increase in dicamba use in both corn and soybean.  This article will review the characteristics of dicamba that differentiate it from other herbicides, provide an overview of problems observed in 2017, and describe how risks can be minimized in 2018.


The discovery of 2,4-D and other phenoxy herbicides in the 1940’s started the era of chemical weed management.  Dicamba was first described in 1958, and registered for use in 1962.  These herbicides mimic the action of auxin (indoleacetic acid), and are frequently referred to as growth regulator herbicides, synthetic auxins, or Group 4 herbicides (Table 1).  They bind to the receptor for auxin and initiate transcription of genes involved in cell growth.  While plants can closely regulate concentrations of auxin within cells, they lack this ability with the Group 4 herbicides.   Presence of Group 4 herbicides in cells results in deregulation of numerous important processes, resulting in abnormal growth and/or plant death.  

Table 1.  Chemical families that interfere with auxin activity (Group 4 Herbicides).

Chemical family Active ingredient Tradename Phenoxy 2,4-D Weedone, many others   2,4-DB Butyrac   MCPA Mecocrop       Benzoic acids dicamba Banvel, Clarity, Engenia, Xtendimax
with Vapor Grip Technology, many others   chloramben Amiben       Carboxylic acids, Pyridines triclopyr Garlon, Remedy Ultra   clopyralid Stinger, Transline   aminopyralid Milestone   picloram Tordon   aminocyclopyrachlor Streamline

Herbicide activity

Nearly all Group 4 herbicides selectively control broadleaves in grass crops.  The exception is quinclorac which is used to control certain weedy grasses in rice and turf.  There is a wide range in selectivity among the products, and they are commonly used in combination to provide a broader spectrum of weed control.   A combination of 2,4-D and dicamba was the most popular postemergence program in Iowa corn production in the 1970’s and early 1980’s.  Dicamba was more active on smartweed than 2,4-D, whereas 2,4-D provided better control of velvetleaf.

Group 4 herbicides vary widely in soil persistence, and hence, length of residual weed control.   Generally, the phenoxy herbicides have the shortest half-lives of the Group 4 herbicides, whereas the pyridines are most persistent.  An advantage of dicamba over 2,4-D for use in resistant soybean is dicamba’s longer half-life (14 days) compared to 2,4-D (6 days); however, the half-life of dicamba is less than half of most preemergence herbicides.  Thus, the value of dicamba as a preemergence herbicide is limited for managing  weeds with prolonged emergence patterns, such as waterhemp.

Plant sensitivity
Group 4 herbicides induce plant responses at lower fractions of use rates than most other herbicides.  For example, it takes 1% of the standard glyphosate use rate (0.75 lb/A) to injure corn, whereas 0.005% of the dicamba use rate (0.5 lb/A) can injure soybean (Figure 1).  Due to this high activity, injury to sensitive plants outside of treated areas has been a problem since the introduction of Group 4 herbicides.  In a 1971 bulletin, Dr. Ellery Knake, extension weed scientist at the University of Illinois, discouraged the use of dicamba in Illinois due to the sensitivity of soybean to the herbicide.   Improvements in application technology have reduced, but not eliminated, problems with off-target movement of the Group 4 herbicides.

Figure 1.  Fraction of labeled rate required to cause visible injury on susceptible species.  Adapted from Bhatti et al. (1996), Everitt and Keeling (2009) and Solomon and Bradley (2014).

Another distinguishing characteristic of dicamba and certain other Group 4 products is their relatively high vapor pressure.  Herbicides with high vapor pressures may evaporate following application, resulting in off-target movement even when the applicator uses appropriate application practices.  The combination of vapor loss and the high sensitivity of certain plant species to dicamba results in a higher risk of off-target injury than with most other herbicides.  The following factors influence the potential for dicamba volatilization following application.

Temperature.  The potential for dicamba to volatilize increases as temperature increases.  A threshold of 85° F is frequently cited as the temperature where caution should be used when applying dicamba in the vicinity of sensitive vegetation.  Minnesota and North Dakota recently prohibited applications of dicamba if air temperature is forecast to exceed 85° F the day of application due to increasing risk of volatility.

Application surface.  The amount of dicamba that volatilizes varies depending on the characteristic of the surface it lands upon.  Behrens and Leuschen (1979) reported that approximately 35% more dicamba volatilized off corn and soybean leaves than from a silt loam soil.  Thus, there is greater risk of volatilization with postemergence applications when significant herbicide is intercepted by the crop rather than the soil surface. 

Formulation.  Almost all postemergence herbicides are weak acids, compounds capable of donating a proton (hydrogen ion).  These herbicides are often formulated as a salt of the parent acid, replacing the hydrogen with some other positively charged ion (e.g. dimethylamine, potassium, etc.).  There are a variety of reasons why salts of the parent acid are used rather than the acid itself, but improving compatibility with hard water and tank-mix products is a primary reason. The volatility of dicamba and certain other herbicides is also influenced by formulation. 

Several formulations of dicamba have been introduced with the intention of reducing the risk of volatilization.  The parent acid of dicamba is the form of the molecule that volatilizes following application.  Low-volatile formulations such as Clarity, Engenia, and Xtendimax with Vapor Grip Technology are intended to reduce the amount of dicamba disassociating to the parent acid. Independent research has verified these formulations reduce volatilization compared to the original dimethylamine salt used in Banvel, but they do not eliminate these losses.

The 2017 Iowa experience

In December the Iowa Department of Agriculture and Land Stewardship (IDALS) reported there were 253 pesticide misuse complaints in 2017, a record number. This increase was largely due to 157 off-target injury complaints associated with growth regulator herbicides, the majority involving dicamba.  It is important to recognize the number of formal complaints to IDALS is a small fraction of total problems associated with pesticide applications. At the time this article was written IDALS had not released the breakdown on the percentage of complaints associated with contaminated spray equipment, particle drift, and volatilization. Most people involved in investigating dicamba complaints acknowledge that multiple avenues of dicamba exposure were involved with off-target injury.  Problems associated with contaminated spray equipment and particle drift can be minimized through better training and improved decision making; however, risks associated with volatilization are not easily managed since vapor movement is determined by the environment following application rather than actions of the applicator.

Moving forward in 2018

There has been considerable debate on how to reduce off-target movement associated with dicamba use in soybean. The United States Environmental Protection Agency (EPA) introduced several important label changes for the new products registered for use on dicamba-resistant soybean. These products are now classified as Restricted Use Products (RUPs).  This classification requires users of the products to be certified applicators, and also requires recordkeeping above-and-beyond those necessary for other RUPs.  In addition, applicators of the products will be required to complete dicamba-specific training prior to use.  The maximum wind speed allowed for applications was reduced from 15 MPH to 10 MPH, and applications are limited to hours between sunrise and sunset.  Label language regarding sprayer cleanout and avoiding applications near susceptible crops has been expanded.  These label changes are appropriate, and should reduce problems associated with particle drift and sprayer contamination.  However, they do not address the issue of off-target movement associated with dicamba volatilization. 

Due to concerns regarding volatilization of dicamba, ISU Weed Science recommends that dicamba only be used preplant or preemergence in dicamba-resistant soybean. Preemergence applications of dicamba reduce the value of dicamba in managing waterhemp, but in our opinion, the risks associated with postemergence applications exceed the weed management benefits.  While early postemergence applications made in May would reduce the volatility risk compared to June applications, label restrictions regarding wind and rain would frequently delay applications into high-risk scenarios (i.e. temperatures above 85° F, nearby soybean reaching sensitive stages).

In summary, dicamba has been a popular herbicide in Iowa corn production for nearly 40 years.  Farmers have learned how to manage dicamba in corn while minimizing risks associated with off-target injury.  Postemergence use in dicamba-resistant soybean presents a much greater challenge due to higher temperatures and more advanced development of adjacent sensitive crops, particularly soybean.   Failure to significantly reduce complaints associated with off-target injury may result in further restrictions on not only dicamba products, but also on other pesticides used in crop production.

Behrens, R. and W. E. Lueschen. 1979. Dicamba volatility. Weed Sci. 27:486-493.

Bhatti, M.A. et al.  1996.  Wine grape response to repeated exposure of selected sulfonylurea herbicides and 2,4-D.  Weed Technol. 10:951-956.

Ellis, J.M. et al.  2003.  Rice and corn response to simulated drift of glyphosate and glufosinate.  Weed Technol. 17:452-460.

Everitt, J.D. and J.W. Keeling.  2009.  Cotton growth and yield response to simulated 2,4-D and dicamba drift.  Weed Technol. 23:503-506.

Solomon, C.B. and K.W. Bradley.  2014.  Influence of application timings and sublethal rates of synthetic auxin herbicides on soybean.  Weed Technol. 454-464.

Category: WeedsTags: dicambaXtend soybeandicamba resistant soybeanAuthor: Bob Hartzler

A historical perspective on dicamba

Tue, 12/19/2017 - 08:30

At the recent North Central Weed Science Society annual meeting I was asked to provide the opening presentation (A historical perspective on dicamba) in a symposium focusing on issues with dicamba.  Following are the slides and the abstract of my presentation.

Powerpoint presentation pdf

The auxin-like activity of the phenoxyacetic and benzoic acids was discovered in the early-1940’s. The herbicide dicamba was first described in 1958, Velsicol acquired the patent for the molecule, and dicamba was first approved for use in the US in 1962. In subsequent years, the label was expanded for use on a wide range of grass crops and for non-crop areas. Dicamba has been described as either a benzoic acid or carboxylic acid compound, and mimics the activity of indole-3-acetic acid (Group 4 herbicide). According to USDA/ERS data, dicamba was used on less than 10% of US corn acres in 1979. Use increased to 15% of corn hectares by 1990, then as herbicide-resistant weeds spread, dicamba use on corn increased to 28% of hectares in 1995. Prior to the introduction of herbicide-resistant crops and Group 27 herbicides (HPPD inhibitors), dicamba primarily competed with atrazine and 2,4-D for broadleaf weed control in corn. Atrazine was preferred over dicamba and 2,4-D by most farmers due to its preemergence use, greater margin of crop safety, and lower risk of off-target injury. Dicamba use was much higher in northern states with high pH soils due to the carryover risk associated with atrazine. Dicamba was used on more that 70% of the 1985 corn hectares in North-Central and Northwest Iowa, compared to 12% of US corn hectares. High pH soils in this region prevented use of atrazine rates greater than 1 kg ha-1 when rotating to soybean or other sensitive crops. The high sensitivity of soybean to dicamba has been an issue since its introduction. In a 1971 University of Illinois Extension bulletin, Dr. Ellery Knake discouraged the use of dicamba due to the risk it posed to adjacent soybean. Behrens and Leuschen published a seminal paper in 1979 reporting on factors that influence volatility of dicamba, including temperature, rainfall following application, application surface (soil vs foliar interception), and formulation. A wide range in volatility was found among the salts of dicamba evaluated. The first dicamba product (Banvel) contained the dimethylamine salt of the parent acid. Over the years, several different salts of dicamba have been introduced, often with the intent of reducing dicamba volatility. Low volatility formulations include Banvel II (sodium) in 1981, Clarity (diglycolamine) in 1990, and most recently Xtendimax/Fexapan with Vaporgrip Technology (diglycolamine) and Engenia (BAPMA). Current research will determine the reductions in volatility achieved with these formulations. Increasing problems with herbicide-resistant weeds have led to an increase in dicamba use, and the introduction of dicamba-tolerant crops will continue this trend. The International Survey of Herbicide Resistant Weeds lists 36 weed species with evolved resistance to Group 4 herbicides, seven of these species are reported to be resistant to dicamba.

Category: WeedsTags: dicambaAuthor: Bob Hartzler

Soil Fertility Web Site Update

Mon, 12/18/2017 - 08:06

The Iowa State University Extension and Outreach Soil Fertility web site has undergone a recent redesign and update. The site was updated, but the URL has remained the same (http://www.agronext.iastate.edu/soilfertility/). While the site has been a resource for several years, it was in need of an overhaul. Part of the update was to improve flow and access to the different areas of the site. Other updates were needed to meet university requirements.

The original Nutrient Topics areas are still there, such as lime and soil pH, nitrogen, phosphorus, potassium, manure nutrients, secondary and micronutrients, soil/plant sampling and testing, and nutrients and water quality. In each nutrient topic section, information is available that is pertinent to that topic; including Extension and Outreach publications, newsletter articles, conference proceedings and reports, presentations, and links to related web sites. The Photo Gallery still contains a large number of nutrient related pictures. As before, the Current Topic articles are still located on the main page. New features include a Quick Links list and a featured web site or publication on each page.

We hope you will use the ISU Agronomy Extension Soil Fertility Web Site as your source of soil fertility information and your initial stop when accessing the Web.

Category: Soil FertilityTags: soil fertility websiteAuthors: John SawyerAntonio MallarinoCrop(s): CornSoybeanBiomass and ForageCover Crop