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Corn and Soybean Crops at Mid-Season

Emerson Nafziger
July 20, 2013
Recommended citation format: Nafziger, E.. "Corn and Soybean Crops at Mid-Season." Department of Crop Sciences, University of Illinois at Urbana-Champaign, July 20, 2013. Permalink

The third week of July brought above-normal temperatures to much of Illinois. This is expected to be temporary, with temperatures the fourth week expected to return to normal. Because soils in most areas had enough water to carry the crop through the week, we don’t think the high daytime temperatures were much cause for concern.

By July 14, 21 percent of the Illinois corn crop was pollinating. This included most fields planted before May 10 to 15. Because planting was so concentrated in the third week of May, we expect as much as half the crop to pollinate by July 21. That’s a week or so later than normal. Higher than normal night temperatures this past week might have hurt pollination success some in areas where soils are starting to dry out. By the heavy silking that preceded the full emergence of tassels that we have noted in recent years is very much in evidence again this year. This indicates that silks should be present when pollen is being shed.

In some fields that were planted in mid-May or later, especially those planted at high populations, stalk diameter is noticeably smaller than we often see in earlier-planted corn. Later-planted corn that has plenty of soil moisture often grows taller than early-planted corn because of higher temperatures during internode elongation. But photosynthetic rates (dry weight increases) are not higher in late-planted corn, and so taller plants usually mean less weight per foot of stalk length. Plants with smaller stalks often have less leaf area, and so 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 leads to lower yields.

If we have a return to better soil moisture conditions along with lower night temperatures over the next two weeks, we can expect kernel set to be good. But after a wet June, 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. Fields that were planted by mid-May have had a chance to produce a good crop canopy, which in turn has enabled deeper root growth. It’s doubtful that roots are 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 major issue with water supply now is in the crop that was planted in late May or early June, and that won’t pollinate until late July or early August. These plants are in mid-vegetative stages, and are often showing mid-afternoon drought stress symptoms (leaf curling) 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 earlier or if soils had not been so wet earlier.

Another concern with the wet weather earlier in the spring is nitrogen supply for the crop. Leaf color remains good in areas with adequate soil water where the crop has continued to grow well. But in areas where soils have dried and some drought effects are starting to appear, leaf color had lightened some. In most cases this is because nitrogen reaches the roots with water, and when water uptake slows, so does N uptake. Rainfall in such places should return leaf color to normal green. If leaves have been pale during the pollination period, however, success of kernel set can be lowered.

It is possible that enough nitrogen has been lost in some lower-lying parts of fields to mean shortages for the crop, even after root systems recover. Supplemental applications of N have been made in some such areas, usually as urea, 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 falls to move them into the soil.

As we move through pollination and into the grainfilling stages, the major concern will be how well the leaf canopy holds up to provide maximum amounts of photosynthate (sugars) to fill kernels. Soil water supply will be the major factor, followed by leaf health and nutrient supply. 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.

The soybean crop has had a slow start, but warmer temperatures have finally produced a surge of growth in fields planted by early June. According to NASS, 32 percent of the soybean crop was blooming by July 14. That’s a little behind average, and less than half of what it was in 2012, but for a crop planted as late as this year’s, it’s reasonable. As is typical with late-planted soybeans, plant height at the beginning of flowering is fairly short this year.

The soybean crop canopy is starting to develop and close, but some of the very late-planted crop will flower for some weeks before the canopy is effectively complete. As we saw last year and in some previous work, as long as flowering and early podsetting take place under good conditions, having the canopy close early might not be that critical. The long-held idea that failure to close the canopy by first flower means lowered yield potential does not seem to hold true, at least with indeterminate soybeans that continue vegetative growth for weeks after first flower.

While we want to have good canopy color and cover in mid-July, yield potential for the 2013 soybean crop is far from being set at this point. We might consider it as we would the corn crop a month before pollination. There’s a long ways to go, and we won’t have a good idea about yield potential for at least a month.

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