The spike of above-average daytime temperatures throughout much of Illinois during the third week of July should not raise concerns for the corn crop as most areas had enough water in the soil to carry crops through that week, said University of Illinois professor of crop sciences Emerson Nafziger.
However, higher than normal night temperatures during the past week might have hurt pollination success in areas where soils are starting to dry out, Nafziger said.
“The heavy silking that preceded the full emergence of tassels, as noted in recent years, is very much evidenced again this year. This means there should be little concern about having silks present when pollen is being shed,” he said.
Nafziger said that 21 percent of the Illinois corn crop was pollinating on July 14, and this moved to 64 percent by July 21. “Planting was concentrated in the third week of May this year, so pollination is also occurring relatively quickly. That’s a week or so later than normal,” he added.
In some fields planted in mid-May or later, especially those planted at high populations, stalk diameter is noticeably smaller than often seen in earlier-planted corn. Later-planted corn with adequate soil moisture often grows taller than early-planted corn because of higher temperatures during internode elongation. Because late planting doesn’t increase plant weight, stalks end up tall and on the thin side, Nafziger said.
“Plants with smaller stalks often have less leaf area, and thus less ability to set and fill a large ear. It’s too early to know if this will decrease yield potential, but it is one of the ways in which late planting can lead to lower yields,” he said.
A return to better soil moisture conditions along with lower night temperatures over the next two weeks should allow good kernel set. After a wet June, Nafziger said soil moisture is becoming a concern in some places. July rainfall has been less than normal over much of Illinois, with less than an inch so far in parts of western and northern Illinois.
“The soil water supply was good coming into July, but how well the crop is tapping into this supply will make a difference as we move into the second half of the season,” Nafziger said. “Fields that were planted have had a chance to produce a good crop canopy, which in turn enabled deeper root growth. Roots may not be as extensive as they were in July 2012, but as we saw last year, good root systems don’t help when there’s no available water in the soil.”
The available water supply is not as certain for later-planted crop which won’t pollinate until late July or early August. These plants are in mid-vegetative stages and are showing mid-afternoon drought stress symptoms (leaf curling) in areas where recent rainfall has been limited. “This crop has not grown enough to have used up the available soil water to 3 or 4 feet deep. Rather, the root system is simply not as deep or as well connected as it would be had planting been done earlier or if soils had not been so wet earlier,” Nafziger explained.
Another concern with wet weather earlier in the spring is nitrogen supply for the crop. While leaf color remains good in areas with adequate soil water and where the crop has continued to grow well, in areas where soils have dried and some drought effects are starting to appear, leaf color has lightened some. Nafziger said rainfall in these areas should return leaf color to normal green. However if plants have been under stress from lack of nitrogen during the pollination period, he cautioned that success of kernel set can be lowered.
“It is possible that nitrogen loss in some lower-lying parts of fields may mean shortages for the crop, even after root systems recover. Supplemental applications of nitrogen have been made in some such areas, usually as urea, and sometimes with urease inhibitor. Such applications might produce enough added yield to provide a positive return if they are made before or even during pollination, but are likely to be effective only when rain moves the nitrogen into the soil,” he said.
As crops move through pollination and into the grain filling stages, the major concern will be how well the leaf canopy holds up in providing maximum amounts of photosynthate (sugars) to fill kernels.
Soil water supply will be the major factor, followed by leaf health and nutrient supply, Nafziger said. “We expect the crop to take about 8 weeks to reach maturity after pollination is complete, and maintaining green, healthy leaves through this period is the only way to maximize kernel fill and yield. If the crop takes less time from pollination to maturity, it will have stopped filling prematurely and will produce lower yields,” he said.