Although crop condition reports and yield prospects for the 2016 Illinois corn crop continue to be good, there has been some recent discussion about unfilled ear tips and whether or not this might mean lower yields than the appearance of the crop leads us to believe, says a University of Illinois crop scientist.

Corn ears with kernels missing at the outer end of the ear are often said to have “tip-back.”

“The term is a little obscure, but the idea is that something happened to cause the ear to adjust its kernel number downward so it won’t have as many kernels to fill. That exposes the end of the ear,” says Emerson Nafziger.

The missing kernels can be aborted kernels—ones that were fertilized but stopped developing—or can be kernel initials that weren’t fertilized due to problems with the pollination process. Low sugar levels in the plant before, during, and after pollination are often associated with such loss of kernels.

“Because kernel number is closely related to yield, missing kernels on an ear suggests that yield has been lost,” Nafziger explains. “Drought stress, loss of leaf area to hail or disease, or lack of nitrogen all result in stress that lowers photosynthesis which decreases the sugar supply. So we associate low kernel numbers with stress.

“While low kernels numbers per acre and low yields do go together, it’s important in a year like this to consider the overall condition of the crop and to focus on how many kernels are present before worrying about how many kernels seem to be missing. We often see some amount of tip-back even in good years, and this may have no effect on yield if kernel numbers are still high,” he adds.

As an example, under outstanding pollination conditions in 2014, Nafziger says almost no tip-back was seen; ears were filled out to the very end of the cob. “There was much more tip-back in 2015, but kernel counts per acre and yields were as high in many areas in 2015 as in 2014. While we don’t think that having some tip-back is necessary to show that the ear had ‘extra’ room in case it was needed it, it’s much more common to see some tip-back than to see none. We certainly don’t consider tip-back to be a problem if kernels numbers are high,” he says.

Nafziger adds that what matters for yield is the number of kernels per acre that fill, along with the ability of the crop to fill them completely. “So 34,000 ears each with 16 rows of kernels and 35 kernels per row should produce yields in the vicinity of 220 bushels, even if most cobs have ‘room’ on the end for another 50 or 100 kernels. At high yield levels when all of the nutrients the plant produces go to fill kernels, having more kernels may mean that kernels stay smaller, and yield may not change much,” he says.

Nafziger says he is seeing some signs that kernel numbers in some fields may not be as high as expected. In one field on South Farm near Urbana he found tall plants and green leaves, but ears with fewer than 400 kernels per ear, or yield potential of perhaps 160 bushels per acre. In another field with similar soil planted at the same time with slightly lower population, plants were not quite as tall, stalks were larger in diameter, and ears were more uniform in size. Ears show a small amount of tip-back, but with an average of about 600 kernels per ear, this field should yield 225 bushels per acre or more, he says.

“There are no obvious reasons why similar fields planted at about the same time should have such different kernel numbers and yield potential,” Nafziger explains. “The field with lower yield potential has a number of different hybrids and most seem to show some degree of the same problem, so hybrid doesn’t appear to be the main difference. Both fields emerged well and have had good uniformity and dark green leaves from the beginning.

“Variability in ear size and placement suggests that plant-to-plant competition began early and increased during vegetative growth, eventually showing up as non-uniform ear development and lower kernel numbers. Temperatures in May and June were warm and there was a lot of sunshine. Rainfall both months was near normal, but the latter half of June was dry, which could have meant more underground competition,” he says.

The crop appears to have used a lot of resources to grow the plant, including roots as they grew deeper during dry weather in the weeks before pollination. Uniformly warm air and soil temperatures and rapid growth during that period might have meant some diversion of sugars away from ear growth and kernel set, Nafziger says. “It’s also possible that uptake of water was slightly lower in some soils due to texture or root growth and uptake, and that the crop in such soils experienced a little more stress.

“Although we can’t do anything to change kernel numbers now, it is worthwhile to visit each field to note kernel number and other plant characteristics that can help explain what happened in different fields. While the Illinois corn crop condition overall remains good, some fields may have disappointing kernel numbers even on plants that continue to look very good. Note which hybrids show this, but given that this may be a one-time phenomenon, be cautious about discarding hybrids, especially those that have been top-yielding in the past,” he adds.