Even though much corn throughout Pennsylvania is in dent and soybeans are at R6, it still means that there is some time for the “grain fill” period to continue in both corn and soybean crops. With continued grain fill comes the need for additional moisture.
Grain fill in corn will begin with pollination and ends at “black layer” when the kernels are physiologically mature. This process takes approximately 60 days to complete. We are usually concerned with keeping the top half of the plant healthy (from the ear leaf up) to maximize grain filling capacity. If the plant is stressed, it will neglect the lower stalk health to allow for adequate photosynthesis in the upper canopy.
As plants approach R5 (dent stage) many farmers will think that the crop is “made” by then, and don’t worry about weather conditions following dent. The reality is that there is an additional 30 days where corn is continuing the grain fill process in the kernels. Stress in the plant leading up to black layer can have a drastic effect on kernel dry weight, and in turn, yield. Severe stress that would lead to total plant death at full dent is about 40%, while total plant death at half-milkline would decrease yield by about 12%.
About 60 days after silking, kernel dry weight is usually at its maximum and the crop is safe from frost. If the black layer is visible at the tip of the kernel, the crop has reached maturity. Any stress after this point will have little effect on yield unless the stalk or ear has been compromised in some manner. Examples would be stalk rot, lodging caused by excessive wind, etc.
According to the University of Wisconsin, corn yield is most sensitive to water stress during flowering and pollination, followed by grain filling, and finally vegetative growth stages. Corn in the R5 (dent) stage utilizes 0.26” of water per day. If the corn plant is stressed during this time, the yield loss can range between 2.5-4.0% per day of stress.