Learn how recent rains have affected Midwestern corn, soybean and wheat crops.
By Susan Jongeneel, University of Illinois
Last week, the remnants of Hurricane Isaac dropped several inches of rain in several parts of the Midwest.
University of Illinois crop sciences professor Emerson Nafziger said that, "While the rain came too late to help corn yields and did little to help soybean yields, it fell very nicely over 3 days. Most of it disappeared without a trace, with creeks still dry, and no tile flow for the moment. Moistened surface soil is the only sign that there has been rain."
In the driest parts of the state, the recent rainfall has helped to restore soil water to more normal levels. At the U of I South Farms, depending on rotation, soil moisture in the top 40 inches is now close to the levels found in late May. Corn following corn appears to have taken in more water, possibly due to larger, deeper cracks. This return to favorable soil moisture lessens concerns about the drought carrying over to next year, though as happened in 2012, rainfall during the season will still be the critical factor.
According to the latest NASS report, as of Sept. 2, 63% of the corn crop was "mature" and 12% harvested while 41% of the soybean crop was listed as "turning yellow" and 7% as "dropping leaves." These numbers will continue to rise in the coming weeks, and both crops have nearly finished adding yield, with corn a little ahead of soybeans.
Growing degree day (GDD) accumulations since May 1, when planting was nearly complete, range from approximately 2,700 in northern Illinois to 3,000 in the southern part of the state. GDD requirements for hybrids grown in Illinois range from approximately 2,500 for very early hybrids to 2,800 for late-maturing hybrids. Thus, GDDs in nearly every field have been sufficient to mature the corn. Most producers are waiting for the corn to dry before harvesting.
The drought caused some cornfields to die, but those that still have some green should be close to maturity (black layer). Nafziger said that, as corn-grain moisture approaches 20%, the advantages to getting the crop harvested tend to outweigh the disadvantages.
"Crop-insurance considerations have to be taken into account, but low-cost drying offered by some elevators, weak stalks that may fall in the next moderate wind, grain quality that can get worse as the crop stands in the field, and the increase in harvest loss as grain dries below 18 to 20% moisture, all weigh on the side of early harvest," he explained.
Reasons for delaying the harvest include the fact that elevator shrink (typically 1.4% per point of moisture loss) exceeds actual shrink (about 1.2% per point), meaning fewer bushels are sold than are actually harvested. This loss, about 1% at 20% moisture, diminishes as grain dries. Last, with soils near field capacity and hence highly compactible in areas where there has been recent rain, it may pay to wait for soils to dry a little before driving a heavy combine on them.
In most areas, soybean harvest has not begun. Some fields are showing a lot of green color, in some cases because pods are still filling, but in other cases the plants may be staying green because they have few pods to fill. "If a soybean field was under a lot of stress early in the season, check to see if it is filling a good number of pods," Nafziger advised.
Drought during the growing season can cause early senescence even when there are not many pods, but the factors that determine when a crop with a green canopy will end the seed-filling process are not well understood. The cool nights in mid-August, the shortening days, and the the slowing of sugar movement into the pods when seeds reach a certain size could also send a signal.
Nafziger noted that the beginning of soybean yellowing was fairly uniform across the state this year. As of Sept. 2, 40 to 50% of the crop in the north and 50 to 60% of the crop in the south had turned yellow. The fact that these amounts had no correlation with the percentage of corn rated as mature implies that soybean senescence and maturity is affected more by weather or day length than by stress levels.
While the official estimates and observations during crop tours in recent weeks indicate that soybean yields will be relatively low, it is difficult to make accurate projections. Samples are usually taken in one or a few places in the field because soybeans tend to look uniform.
"In fact, yield monitors show that yields are often not as uniform as the crop appearance would suggest," Nafziger noted. "Samples tend to be small – only a few feet of row for plant counts, then a few plants of those for pod counts – and may not be accurate, but taking enough samples to improve accuracy may often not be worth the time and effort."
Another reason that it is difficult to make accurate yield estimates for soybeans is that pod counts, and hence seeds per plant, vary considerably across plants. "My only advice is that, if your counts and calculations indicate the field will yield 70 bushels and you think from experience that it will yield only 50, trust your experience," said Nafziger. "It may make more sense to guess than to try to do pod counts and calculations."
Estimating pod numbers once leaves drop is usually a better method than a lot of pod-counting when leaves are still green. "There’s an art to this, and experience counts a lot, but in general, stems that look as if they are bristling with pods -- three to five pods per node on six to eight nodes with pods sticking out away from stems -- and with pods filled at the top of the stem indicate good yields, while two or three pods per node that do not stand out away from the stem (hence aren’t as well filled) and on fewer nodes will mean lower yields," Nafziger said.
He added that a good place to practice on this is in strip trials where information about actual yields after harvest is collected.
Questions remain about wheat planting following failed corn fields and the need for fertilizer, especially nitrogen, in such fields. Rainfall over southern Illinois has certainly moved much of the soil nitrate to below the zone where the wheat roots would reach it early. Many producers are chopping, tilling in, or rolling down corn residue, putting it in contact with the surface soil. With warm soils and a month still before planting is expected to begin in southern Illinois, it is likely that the levels of nitrogen tied up in the surface soil will increase.
"Our earlier suggestion that wheat following corn shouldn’t need nitrogen at planting in dry areas this year is probably no longer a good one now that most areas are no longer dry," said Nafziger. Hence, the advantage that wheat following corn can retrieve some of the leftover soil nitrogen is not an important consideration.
In some cases, planting wheat following soybean may be more advantageous than wheat following corn. If wheat follows corn, Nafziger advises using normal monoammonium phosphate (MAP) or diammonium phosphate (DAP) applications to supply some nitrogen and phosphorus, especially in fields that show low or medium phosphorus levels.
"If the winter is dry, we might be able to adjust spring nitrogen applications, perhaps based on soil-test nitrate in March," Nafziger said. "Chances of that would appear to be small at this point, and with more rain will get even smaller."
In short, because of the rain, normal procedures should be used to manage wheat: approximately 1.4 to 1.6 million seeds per acre, planted after October 3 to 5 in south-central and southwestern Illinois and after October 7-8 in the southernmost parts, with serious attention to uniformity of seed placement regardless of tillage or previous crop.
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