Soybean Planting Date: Early or Late, Does It Matter?

The follow summary of research has been extracted from a report written by Dr. Emerson Nafziger on a study that was funded in part by the Pioneer Crop Management Research Awards Program. While the study was conducted at 5 locations across Illinois, this summary includes the data from the more northern U of I research Centers: DeKalb, Monmouth and Urbana. The study was conducted 2001-2003. The adapted (mid) variety for each location was: DeKalb Pioneer 92B84, Monmouth P93B26, and Urbana P93B67. For additional information the optimum population for split-row no-till soybeans was also addressed in the November 2008 edition of this newsletter.

1. What is the best time to plant soybean, and is it possible to plant too early?

In Northern Illinois, late April and the first half of May produced the highest yields (Table 1). Planting in early April reduced yield by about 10 percent, and planting in late May reduced yield by about 5 percent; these results suggest that planting “too early” tends to reduce yield more than planting late. In Southern Illinois, planting in late May produced the highest yield, and mid-April planting yielded more than 10 percent less than the highest yield (data not shown). It makes sense to wait to plant until late April in Northern Illinois, and early May in Southern Illinois. The “planting window” extends for three weeks or so over most of the state.

2. Does the best seeding rate change with planting date?

Yes, in Northern Illinois, where early planting responded more to planting rate than later planting (Table 1). Optimum planting rates for planting in the optimum window were about 150,000 to 160,000 viable seeds per acre, but this rose to above 200,000 per acre if planting was earlier than the optimum time. It pays to wait to plant, but if planting is earlier than ideal, it pays to add extra seed. In Southern Illinois, there was less response to planting rate, and planting date had little effect on the seeding rate response. Still it appears that planting rate should be between 150,000 and 175,000 viable seeds per acre in Southern Illinois.

3. Did seed treatment fungicide increase yields or reduce optimum seeding rates?

No. There was no significant effect of seed treatment fungicide on yield at any of the 15 site-years; averaged over all data, yields with fungicide were only 0.2 bu/acre higher, and this was not consistent enough to be statistically significant (that is, it could have been due to random chance.) At DeKalb in 2001, with relatively low yields, fungicide increased yield of the early-planted crop, though the effect was somewhat erratic. All together, we saw no reason not to use seed treatment fungicide as an “insurance” input, but under most conditions we do not expect to see much yield response from using it.