Like corn, soybean planting has had an unusually early start in Illinois this year, with some of the crop planted before April 1 and 5% planted by April 22. Was this early planting advisable? University of Illinois crop sciences professor Emerson Nafziger is not sure about planting in March and early April, given that some earlier work showed yield losses with very early planting. However, research conducted in the past two years showed that yields were highest for the earliest (mid-April) planting dates. Yields declined slightly by early May, and by mid- to late May, losses were up to 0.3 to 0.4 bu./day of delay. By early June, each day of delay meant a loss of about half a bushel of yield per acre.
Such high yields from mid-April planting and the early start to yield losses with planting delays are not consistent with results of earlier research on soybean planting date. “We, and researchers in other states, had generally found that soybeans tended to suffer little yield loss from planting delays until after the middle of May, after which yield loss accelerated,” says Nafziger.
Why the difference between earlier results and the more recent work?
According to Nafziger, much of the earlier work took place under lower-yielding conditions, not the 70-80-bu. yields seen in the research of the past two years. This raises the interesting possibility that early planting is most likely to help yields when conditions for high yields exist throughout the season.
Whatever the reason, “We now have evidence that waiting until some date in May to plant soybeans may not be necessary or even helpful to yields,” says Nafziger. Finishing corn planting and then starting soybean planting may be a good option, at least after mid-April.
“Soybeans planted in late March and early April this year are being affected by cool weather now, and while we don’t think they’re in danger, we’ll have to keep watching them to see if they grow well once it warms up,” Nafziger adds.
One caution that applies this year is that surface soil moisture levels may be too low for soybeans to germinate in some areas. “We have heard similar reports regarding corn planting, and some have been holding off on corn until rainfall moistens the surface,” says Nafziger.
It is not a very good idea with either crop to try to “plant to moisture” this early. “Deep-planted seeds take longer to emerge, and that emergence can be reduced substantially if we get rainfall accompanied by lower temperatures,” Nafziger explains.
Finally, because there has been so little rain in April, many fields do not have good uniformity of soil moisture at planting depth. Using tillage to improve soil moisture uniformity often does not work well when moisture is in short supply. Planting into uneven moisture can result in uneven emergence and plant size, which can reduce yield.
For soybeans, if there is enough water to start the germination process but not enough water to get plants to emerge, seeds and seedlings often die. Thus, if soils have limited moisture, it may be better to wait until after rainfall to plant. Corn seed needs less water than soybean to germinate, and corn usually emerges better than soybean, so planting corn a little deeper and into drier soils is less risky.
This work was made possible by funding from the Illinois Soybean Association.