For the past six years we have conducted a corn planting date study at the Bradford Research and Extension Center near Columbia in central Missouri. In each year we select four to six hybrids and plant them on five dates. We attempt to include a late March planting date but spring weather does not always cooperate. The last planting date is in mid-June.
Two planting rate treatments were used. For one treatment (fixed), 26,000 kernels/acre were planted on all planting dates. In the other treatment (thinned), plots were over-planted and thinned to 24,000 plants per acre at the 3-leaf stage. The “fixed” treatment allows us to compare planting dates for emergence. Soil conditions including temperature vary among planting dates and these conditions often affect emergence. The thinned treatment holds stand densities of all plots constant across hybrids and planting dates. Yield data from these plots allow for a better comparison of planting date effects on corn yield.
Table 1 presents the effects of planting dates on corn yield for each year. Each data point is the average of all hybrids. Years differ greatly for yield potential primarily because of precipitation amounts, especially during the critical R1 (silking) stage of development. In general, yields decrease with delayed planting date. Table 2 presents the data averaged over all years, except 2005. The 2005 data were eliminated because extreme drought resulted in yields that are atypical. Our data indicate, on average, little difference for yield potential of planting dates in April. From May 1 to June 1 corn yields decrease about 25%. Yield loss is much greater if planting date is delayed until mid-June.
However, large differences occur among years for the response of corn yield to planting date. One result of delaying planting is that the silking occurs when weather is warmer and drier. Thus, weather conditions in July and early August can greatly affect yield response to planting date. Yields from the May planting dates in 2008 were relatively high partly because timely precipitation occurred in July and August. Unfortunately, the 2008 weather pattern was not normal.
Spring weather can affect emergence percentage and early seedling vigor. Emergence percentages for the first three planting dates in 2008 were 84, 82, and 82%. Weather during emergence from these planting dates was cool and wet. Table 3 presents yield data from 2008 for both the fixed and the thinned treatments. Because emergence was reduced for early planting dates, the stand densities for the thinned plots were higher than for the fixed plots. This resulted in higher yield. For the last two planting dates, soil conditions improved so that stand densities for the fixed treatment were equal to or slightly higher than the thinned treatment. So, yield was also equal or slightly greater for the fixed treatment.
Unfortunately, spring weather conditions in 2009 are similar to last year. Our data provides some optimism that reasonably high yield can be obtained when corn is planted in mid to late May. However, the yield potential is very strongly dependent on weather conditions in summer. The 2008 data strongly suggest that care must be taken to adjust planting rates to fit soil and weather conditions during stand establishment. Stressful soil conditions means increasing seeding rates, perhaps more than expected, to obtain target stands.
REVISED: October 2, 2015