September 9, 2014
Forecast for Cold Weather Later this Week Could Lead to Frost
The last major hurdle for both the corn and soybeans is to reach maturity before a killing frost ends the growing season. Meteorologists are now forecasting the potential for frost later this week or over the weekend. If it turns out to be just a light frost, then it would have a minimal impact on the crops. If it turned out to be a killing frost, then that would be a significant event. In order to be a killing frost that ends the growing season, the temperatures need to be 27 or 28 degrees for approximately three hours.
Of the two crops, soybeans would be more vulnerable to yields losses from the potential cold weather than the corn. The extent of losses for soybeans depends on the maturity of the crop, how low the temperatures go, and the weather immediately following the frost.
If the soybean seeds are still completely green when a killing frost occurs then what you may end up with is smaller than normal green beans or even tiny green beans. Losses at this stage would be in the range of 20% to 30%. If the seeds have turned light green, then the losses would be 10% to 15%. If the seeds have turned light yellow, then the losses would be 0% to 5%. If the seeds have matured, then the losses would be 0%.
Without looking inside the pods, you can judge the maturity of the crop by looking at the field. If the soybean leaves have already started to turn yellow, then the seeds inside the pods have started to turn yellow as well. If the leaves are starting to drop from the plant, then the seeds inside the pods will be yellow and the only thing left to occur is to loose water. Therefore, losses would be in the range of 10% if a freeze occurred when the leaves are already turning yellow and losses would be less than 5% if a freeze occurred when the leaves have already started to drop from the plant.
It is usually difficult to assess the extent of crop damage immediately after a frost occurs and it usually takes at least several days before an accurate assessment can be made. If it is just a light frost, then the upper leaves in the canopy will be killed, but the plant in general will survive. If it is a killing frost though, you will know immediately the very next day. The soybean leaves will start dropping the next morning and the entire field will smell like newly cut hay.
The weather immediately following the frost is also important in determining how much damage will occur. If temperatures quickly return to normal levels, then the damage will be minimalized. If the temperature remain below normal immediately after a frost, then the damage could be greater than expected.
If the corn crop has not yet matured when a killing frost occurs, then what you end up with is lighter kernels and lighter ear weights and the amount of loss will depend on the maturity of the corn. This of course would reduce the corn yield, but generally the amount of losses in corn is less than the amount of losses in soybeans.
Finally, my general rule is that the forecast of yield losses from a frost is generally worse than the actual event. Most of the dire predictions of losses from frost generally turn out to be exaggerated. When the frost actually occurs, the losses are generally less than what had been predicted.