Pay Attention to Tip-Back
Corn starts to abort kernels as it experiences stress. “Because kernel number is closely related to yield, missing kernels on an ear suggest to many people that yield has been lost,” says Emerson Nafziger, University of Illinois Extension agronomist. “Drought stress, loss of leaf area to hail or disease, or lack of nitrogen all result in stress that lowers photosynthesis and so decreases the sugar supply.”
Corn ears missing kernels at the outer end of the ear are referred to as tip-back, says Nafziger. Aborted kernels are the ones that were fertilized but stopped developing, or they weren’t fertilized due to problems during the pollination process, explains Nafziger. Ears that show a lot of tip-back, and still have silks attached suggest that the aborted kernels were caused from pollination failure.
Why You Should Care
“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,” says Nafziger.
Just because a field looks excellent doesn't mean ears can’t have low kernel numbers. Nafziger advises growers to note which hybrids show this. When hybrids in the same conditions show different levels of tip-back, it’s a sign of stress tolerance.
“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 says.
Focus on Overall Condition
“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,” says Nafziger. “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 points to the 2014 and 2015 growing seasons for examples. 2014 had ideal pollinations and nearly no tip-back was seen. However, 2015 had more tip-back.
“Kernel counts per acre and yields were as high in many areas in 2015 as in 2014,” says Nafziger. “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, and we don’t consider tip-back to be a problem if kernels numbers are high.”
When considering yield, what’s important is the number of kernels per acre that fill, along with the ability of the crop to completely fill them, says Nafziger.
“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,” says Nafziger. “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.”
There are no obvious reasons why similar fields planted at about the same time should have such different kernel numbers and yield potential, he says.
What We Know
“Even when plants emerge well and looked uniform in size and canopy color during vegetative growth, variability in ear size and placement suggests that plant-to-plant competition began early and increased during vegetative growth, eventually showing up as nonuniform ear development and lower kernel numbers,” Nafziger says. “We can only speculate about how this might have happened, and why fields differ as much as they do.”
The crop used a lot of resources to grow the plant, including roots as they grew deeper during dry weather in the weeks before pollination, he says.
“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,” says Nafziger.
Another potential stressor is whether different soil types may have impacted root growth and uptake.
“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,” he says.