Are corn and soybeans maturing earlier this season? A general rule of thumb is that the corn crop takes about 62 days after pollination begins for it to reach physiological maturity (black layer). Of course for soybeans, maturity group has the greatest influence on when the crop reaches the R7 (physiological maturity) stage. The question many growers are asking this season is whether the precipitation pattern we saw throughout the Midwest and related factors may have hastened corn and soybean maturity.
We do know that a nutrient deficiency resulting from either inadequate fertilization or loss through leaching and runoff generally does not accelerate crop maturity. In fact, we would expect inadequate levels of phosphorus, potassium, or sulfur to result in the opposite effect, delaying maturity slightly when in short supply. And while there is some evidence that a nitrogen deficiency in some crops may hasten maturity, this is not expected for either corn or soybeans. Research indicates a nitrogen deficiency in either corn or soybeans will most often slow crop maturity. And yes, soybeans can experience a nitrogen deficiency if plants are unable to obtain a nominal amount of N from the soil, which can be about 20 to 40% of its total needs.
So why might we be seeing sections of a field, or in some rare cases, an entire field maturing earlier than normal this year? I think we can attribute this to the weather stresses the crop has encountered. You will recall that with the persistent rains experienced throughout much of the Midwest in May and June there was little need for the crop to establish a deep root system. This is particularly evident in low spots where corn appears yellow and soybeans take on a gray cast.
So when the crop encounters the hot and drier weather of July and August, its inability to cope with the stresses of heat do become a factor in accelerating maturity. (Think how old you would feel sitting in the hot sun all day with little to drink?) Although a deficiency in nitrogen resulting from losses via leaching or via anaerobic conversion and volatilization may have resulted in stunted plants, where accelerated maturity is evident, the cause is likely to be stresses resulting from heat or water stress. One other factor, crop diseases, can also accelerate crop maturity. If you want to think of it in these terms, anything that might accelerate the breakdown of cellular material is likely to hasten crop maturity.