Normally we talk in terms of the haves and have not's when summer thunderstorm patterns fire up and some areas get rain and some don't. That will be the case for sure even this year in areas that got those minimal-coverage but intense thunderstorms early enough to help the corn before pollination occurred. However, rain within the last few days in a spotty pattern may simple draw the line between bad corn, for those who got some rain, and terrible corn, for those who didn't.
The bottom line is that for corn, the rain may be too little too late, except for the latest planted fields. Since many of those had already started to pollinate, the jury is out on how much rain can help them at this point. For fields that did pollinate, if the drought breaks, the plant will sense better conditions and perhaps make deeper kernels or carry a better test weight than if it didn't get rain.
Dave Nanda, director of genetics and technology for Seed Consultants, Inc., says that corn makes important decisions at various stages. For example, by waist high it already decided how many rows of kernels to form. It's somewhat set by genetics, but it can definitely be influenced by environment, Nanda says. A hybrid that typically produces 18 rows on an ear could drop back to 16 or even 12 if it was badly stressed when the decision was made. On the other hand when things look good to a plant that is bred for 18 rows, it might throw 20 or even 22 instead.
Rain and cooler weather is too late for that decision. For many fields, it will also be too late to affect pollination patterns. Once pollination occurs, which is usually limited to about a week to 10 days, that portion of the yield component is set as well.
What's left are possible tip abortion and depth of kernel, plus test weight. Late rains, if they come in enough quantity, might help make some improvements there, but most experts agree that as far as corn is concerned, in areas which didn't receive rain until the past few days, it will likely turn out to be too little too late to make much difference in yield of the corn.