Dr. Dan Talks Agronomy


By Dan Davidson
DTN Contributing Agronomist

The dog days of August are upon us and that typically brings on tip-back worries, especially if conditions have turned hot and dry.

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, according to Emerson Nafziger, University of Illinois agronomist.

"Because kernel number is closely related to yield, missing kernels on an ear suggests that yield has been lost," Nafziger explained in a recent news release. "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 kernel 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 added.

Last week I walked the cornfields on our family farm in Nebraska to look at ear size (big and girthy), maturity (R4 or dough) and tip-back (none evident).

Our fields show no signs of yellowing or nitrogen deficiency. Stalks are green, healthy and viable. Green leaves are still attached from 8 inches above the soil to the tassel. I assume root growth is deep. There have been no signs of stress and few signs of foliar disease. We had plenty of rain this summer and good growing conditions, although we have had a few weeks with above-normal, 90-plus-degree days. At this point, I see nothing that would keep us from a bumper crop in this area at this stage.

As Nafziger noted there are lots of reason for tip-back, but this time of year we generally associate it with moisture or heat stress. In general, I believe we see less tip-back today than five or 10 years ago. I credit intense management practices that promote plant health. There is more sugar available to fill out ears way out to the tip without a sacrifice in stalk strength.

There's always a chance that something happened environmentally to interrupt the pollination nick -- meaning that pollen shed and silking don't totally occur at the same time and the silks for the tips emerge too late and miss pollen shed. This becomes a hybrid performance issue.

After pollination, poor growing conditions, stress or intense plant competition can cause ears to abort kernels at the tip. A lack of nutrients, sugar or water and competition between corn plants or weeds can cause kernel abortion at the ear tip. These kernels will abort in the initial blister stage and within two weeks of pollination, though.

Insects like corn rootworm, western bean cutworm and Japanese beetle feed on emerging corn silks and disrupt pollination. Sometimes, these ears will also have a scatter-grain appearance, and sometimes, symptoms will be limited to the ear tip.

Tip-back can also be used as a barometer of whether population was too high or low. Some agronomists want to see a half-inch tip-back so they know that population isn't the limiting factor. Other agronomists prefer to have the ears fill out to the tip.

Nafziger said 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 kernel numbers are high," he said.

There's no way to improve kernel numbers this late in the season, but understanding what might have happened is worthwhile. To see Nafziger's assessment of what's happening in Illinois, go to: http://bit.ly/…

Dan Davidson can be reached at AskDrDan@dtn.com