Seed corn cultivation involves a sequence of production steps that differs from conventional methods of raising field corn for grain. In the first phase of seed corn production, inbred lines possessing certain traits are raised as parental stock in corn plot nurseries and are usually cultivated under the supervision of crop breeders from seed corn companies. In the second phase, an inbred line is crossed with another inbred line to produce a single-cross hybrid. Most hybrid seed corn sold is made from single-cross hybrids and is produced by growers under contract with seed companies. In some instances, a single-cross hybrid is crossed with another single-cross hybrid to produce double-cross hybrid seed. Each phase requires a separate corn growing cycle and some form of pollination control in the female parent corn plants. This usually involves removal of the tassels to prevent self-pollination. Seed corn is marketed in units consisting of a standard quantity of 80,000 kernels per unit. The retail value of seed corn can be more than $100 per bushel.