Its that time of year when corn producers are asking replant questions. Deciding whether or not to replant is one of the most difficult decisions corn producers make. Each field has its own set of circumstances; however, issues related to the remaining stand and likely replant date are key. Ultimately, the decision hinges on trying to decide which case will maximize net income.
Table 1 (not available electronically) is based on the most recent research on stands and planting dates. Although results will vary somewhat due to location, season, hybrid, and other factors, Table 1 serves as a good starting point in decision-making.
The optimum stand level (established stand) is set at 100 percent and includes stands from 26,000 to 30,000 plants per acre. From this base, relative yields for lower stands are suggested. Yields are based on stands that are normal in terms of uniformity of plant size and distribution. Research indicates that with full stands, uneven plant size and distribution can reduce yield. This certainly is also a factor in a reduced standbut determining an exact value for this loss, in addition to the loss due to a low stand, is very difficult. Older University of Illinois studies indicated yields are reduced about 2 percent if the stand includes several small gaps (112 to 3 feet) and by 5 or 6 percent for numerous larger gaps (4 to 6 feet).
Once you have evaluated stand level and uniformity, compare that yield percentage with a full stand at the expected replant date. Convert the difference to bushels per acre. If the replant yield is lower, do not replant. If it is greater, consider the cost of replanting (tillage, planting, seed, and other factors) to see if the extra yield is enough to make replanting profitable.
For example, lets say corn is planted on April 25, and the stand goal is 26,000 plants per acre. If the actual stand turns out to be only 18,000 plants per acre, and there are a few stand gaps, the table indicates that the expected yield is 91 percent minus an estimated 2 percent for gaps, or 89 percent. On the possible replant date, May 20, the expected yield is 90 percent. In this example one percent of yield would not cover replant costs. However, if the original stand was only 14,000, the decision may have been the reverseassuming the value covers replant costs. There is, of course, some risk associated with how good the replant stand will be.
There is no exact prescription for each case, but this information provides a starting point for making your decision. Where stands and/or dates fall between the values listed, you must interpolate. Also, if the original planting date and/or stand level doesnt fit the optimum date or stand definition, use something less than 100 percent as a basis of comparison. Other factors that can influence a replant decision include health of the remaining stand, hybrid, soil conditions, tillage, weeds, and herbicide program.
As planting is delayed, also consider the maturity of the replant hybrid. The suggested last planting dates for adapted full season maturity hybrids for an area are about May 20 to 25, while for very full season hybrids the date may be as early as May 10 to 15. After these dates, somewhat earlier hybrids often are a better choice because of maturity and grain moisture concerns in the fall. If corn planting is delayed into June, consider even earlier corns, especially in northern Iowa.
Unfortunately, you cant calculate the growing degree days (GDDs) between, say, a June 1 date and one week before the average frost date and just pick a hybrid that equals the number of GDDs. Hybrid GDD ratings are based on normal planting dates. As planting is delayed, a hybrid requires fewer GDDs to reach physiological maturity. By June 10 or 15, economics often indicate that soybean is the replant crop of choice.