It wont be long until corn producers ask: Should I tear up my corn stand and replant or stay with a less than optimum stand? Each year this decision must be made for hundreds of Iowa corn fields. Each field has its own set of circumstances that influence the decision. The most important are the remaining stand level and likely replant date. Table 1 (next page), based on ISU research, was developed to compare yields for less than desired stand levels with the yield expectations of possible replant dates.
Factors such as replant costs, stand uniformity, and other related concerns must also be considered. However, at this time I want to consider whether more recent research results agree with the values listed in the table.
Recent four-year tests by Emerson Nafziger in northern and west central Illinois and myself in southeast Iowa make appropriate planting date and stand level comparisons. I wont attempt to present the data in the newsletter, but here are general conclusions that apply:
1. The ISU test corn planted April 19 and May 6 yielded about the same, while corn planted May 19 yielded 87 percent of the earlier dates. The relationship varied little with yield level and within a population. This is in contrast to the 92 percent figure listed in Table 1 for May 20.
2. In the Illinois tests, the yield trend slightly favored an April 25 average planting date rather than April 14 or May 8. For the May 19 date, yield reductions were nearly identical to the Iowa results.
3. Even if 1989 data are not included because of a severe second generation corn borer infestation in late planted plots, yields for June 1 planting in southeast Iowa were only 75 to 80 percent of maximum. This is below the 84 percent suggested in the table, and certainly below the 88 percent once suggested for southern Iowa. Again, the Illinois results are very similar to the more recent Iowa test for this date. However, recent results from our northeast research farm dont show as sharp a decline in yield with delayed planting.
4. None of the recent studies included a June 10 date. My guess is that the percent of maximum yields for that date may be slightly lower than listed, but I may be biased by what happened in 1993.
5. Recent tests indicate that yields top out somewhere in the 25,000 to 35,000 plant per acre range. With modern hybrids, yields no longer increase up to a specific level, then drop sharply above that level. Using the 26,000 to 30,000 plant per acre range as the basis of comparison for lower stands in replant decision making appears valid.
6. If the maximum is a bit higher, it is likely that the yield percentages for 22,000, 18,000, etc., may be slightly lower. This is supported by these tests, although year-to-year yield variability complicates results somewhat. Only the Illinois tests had stands of 10,000 and 15,000 plants per acre, and their results were very much in agreement with similar stands given in the Iowa chart.
In summary, the results suggest yield reductions for planting delays andmoderate stands may be slightly greater than earlier research indicated.