People once just surmised that corn planted later when it was warmer grew taller and seemed to shoot up faster than corn planted earlier. Many in the old days used it as a reason not to plant too early while soils were still on the cool side. They wanted to see their corn emerge rapidly and grow quickly.
Experts now say that what they surmised was true. Corn planted later, say May 20 or after, will often come up faster because the soil is warmer. It may grow taller, and it may have slightly different traits than the same hybrid planted earlier. Often that's just one or two fewer leaves. Certain traits, such as the number of spikes on the tassel, don't change.
What the plants are doing is speeding up development so they have better odds of producing kernels. The plant's job is to produce the next generation. Even hybrid plants have that goal. There is no one telling the plant that the hybrid seed won't be planted. A plant does what it is supposed to do- produce as many viable seeds as possible.
The rub is that these plants planted later don't typically yield as much. It remains to be seen whether USDA will factor late planting dates into its estimates when it starts issuing crop size estimates for corn in August this year.
It's fairly well documented that corn planted late, even though it gets a faster start, does what it can to produce as many viable kernels as possible, rather than worrying about producing the maximum number or kernels. That's why fields planted early, even if they take two weeks to emerge, may yield more. They have more growing season to take advantage of, and usually don't receive the signal to speed up the process to finish making viable kernels.
One year ago USDA did not account for extremely high nighttime temperatures in corn. The result was that their estimates were way off, almost to a record level, dropping form August to the final estimate. Others have suggested other reasons for the gaps, but many contend that USDA did not factor in the temperature factor. Corn that must function with extremely hot nights typically expends some of the energy that would normally go into kernels on processes that help plants cool off.