As June 1 looms in the not-too-distant future, large areas of land intended for small grains or corn remain unplanted in Northwestern Minnesota. Likewise, there are localized areas in East-central and South-central Minnesota where the corn crop has not yet been planted. With recent rainfall, and a May 31 crop insurance cut-off date around the corner, some producers are considering switching to soybeans.
Soybean is a natural fallback crop as it still has a yield potential of 80-85% of an early planted crop when planted as late as June 1. There are just a couple of common questions that such a switch often raises. 1) will fall applied fertilizers affect the soybean crop? And 2) what soybean maturity should be selected for late planted soybeans?
In general, the additional fertilizer intended for a corn or small grains crop will not have a negative effect on soybean growth, development, or yield. It is possible that the additional N added to the soybean crop will increase vegetative growth and result in a taller soybean; however, the potential negative effects of such a change are likely to be greatly reduced simply by the late planting date. Tall soybeans are not usually a problem with June planted soybeans. The soybean crop will utilize the available N from the fertilizer and that from N mineralization first. As the soybean draws down these available pools of N, it will begin fixing N naturally. Extra available N early in the season will not cause an N deficiency later due to inhibition of N fixation; it only causes it to be delayed. It appears that the only negative effect of this altered N utilization pattern by the soybean crop is an economic one. While the soybean plant sees the soil N as 'free', it came at a significant cost to the producer.
Historical recommendations for soybean maturities have indicated that full-season soybeans can be planted until June 10, at which time soybeans with maturity rating of 0.5 shorter than is normally adapted should be chosen. So, there is still time to choose soybeans with a similar maturity to those planted earlier. However, carefully evaluate the varieties that are offered. In many cases, these are the leftovers. Don't get stuck with a dog. Shop around to find seed with high yield potential. Returned seed from a more northern producer might be the best option in a year like this.
A third issue that cannot be ignored is that of pre-plant herbicides. While it is unlikely that many fields destined for corn crops received an herbicide application, it is critical to carefully examine any of these before a switch to soybean is considered. Many pre-plant corn herbicides will cause injury to a soybean crop. Carefully read the label(s).
Good luck getting this spring season wrapped up.
For a recap of this issue from 2011, see http://blog.lib.umn.edu/efans/cropnews/2011/05/guidelines-for-late-planted-co.html