Kernel dimensions based on location and pollination
Seed from an ear of corn falls into many classes of size and shape. Usually, large-rounds come from the butt of the ear, flats from the center, and small-rounds and small flats from the tip. Medium rounds can come from either end of the ear. Seed weight increases when going from flat to round, and from small to large seed. We probably still worry too much about seed size and weight -- a factor important to old plate planters, but less important to today's planters -- while almost ignoring test weights.
Why do corn kernel shapes differ? A brief discussion of tasseling and silking processes will help us understand this better. Pollen shed for modern hybrids occurs over a five to eight day period in good conditions. High temperatures hasten this and drought delays it. Pollen shed begins about three-fourths up the tassel and then proceeds upward and then downward from the starting point. Older hybrids had large tassels producing pollen for up to two weeks. Modern hybrids (and inbreds) have smaller tassels than the older hybrids.
Silks of some modern hybrids emerge before pollen shed and some a few days after. With good conditions, most modern hybrids usually shed pollen and silk about the same time. Silks are receptive to pollen for about seven days. Again, pollen tube growth and eventual ovule (kernel) fertilization are temperature and moisture dependent.
Variation in kernel size is due to the timing of ovule fertilization and this is dependent on kernel position on the ear. Kernels an inch or two from the butt end, silk first and thus are fertilized first. This is followed by those just above the butt end kernels, working toward the tip. Tip kernels not fertilized within four to six days of butt kernels either fill poorly or abort. Some have said that kernels fill more like "pigs at a trough" than if they were attached to a manifold. The first ones to the trough get the most. Thus, the kernels at the butt are larger and those at the tip are smaller.
University of Nebraska research related to post-black layer dry matter accumulation measured kernel weights on several hybrids from three sections of ears. The kernel weights varied in this order: Butt > midsection > tip (0.96-0.99, 0.93.-0.95, 0.72.-0.78 oz/100 kernels, respectively). Therefore, the majority of kernels on the ear are fairly similar in weight (0.93 to 0.99 oz/100 kernels), except for the tip kernels which are significantly less.
Portions of this text, written by Roger Elmore, Ken Russell, and Lori Abendroth, originally appeared in Crop Watch (University of Nebraska extension newsletter) written April 8, 2005.
