Kernel fill problems occur when crop stresses, such as drought, heat, defoliation or nutrient deficiencies lead to problems with photosynthesis, says Bob Nielsen, a Purdue University agronomist. Poor kernel set lowers a field's yield potential.

The most common form of poor kernel fill occurs when the last silks do not receive pollen and leave the tip of the ear without kernels, Nielsen says. "Zipper ears" - so-called because one or more rows of kernels are missing in one section - are another type of poor kernel set.

Problems with grain fill occur because incomplete pollination often leads to kernel abortion. If crop stress occurs in the early grain fill stages and the leaves cannot produce food, Nielsen says kernels will not develop. High rates of kernel abortion on one part of the ear could cause the ear to curve.

While there is no explanation as to why rows of kernels do not develop in one section, Nielsen says the portion of the ear with the kernel set problem is typically the same part over which the silks draped during pollen shed.

"This leads me to speculate that perhaps the draping of the silks resulted in the underlying silks being shaded from initial contact with pollen," he says. "That may have led to those silks never coming into contact with pollen or those silks being pollinated later than the rest."

Regardless of the types of poor kernel fill, weather is the primary trigger, leaving little for farmers to do about the problem.

"The only suggestion I can give is to try to reduce soil compaction so it will take more time for drought conditions to damage the plants," Nielsen says. "Soil compaction limits root growth and keeps the plants from getting water. Plus, reducing compaction is a best management practice that farmers should be working on every year."

More information about kernel set and photos of different grain fill issues are available at http://www.agry.purdue.edu/ext/corn/news/timeless/Zipper.html.