If the appearance of the soybean crop going into late July predicts how it will yield, the 2018 crop in Illinois will be a high-yielding one. As of July 22, the crop in Illinois was rated at a whopping 78 percent good + excellent (G+E), significantly higher than in most recent years on the same date.
“Illinois soybean crop ratings over the past decade have usually remained steady after mid-July, or have risen slowly,” says Emerson Nafziger, emeritus professor in the Department of Crop Sciences at the University of Illinois. “Exceptions included 2008, when the G+E rating rose by about 20 points from late July to mid-August; and 2011, when ratings dropped about 20 points during the same period. In 2017, ratings drifted down after June, from 70 percent G+E in early July to less than 60 percent by late August.”
Nafziger says that late-July soybean crop ratings reflect how the crop looks, but can’t always be counted on to accurately predict yield. In six of the last 10 years, ratings and yields have been average, with ratings around 60 percent G+E and yields between 45 and 50 bushels per acre. In both 2015 and 2017, late-July ratings were only average, but yields were high: 56 in 2015 and 58 in 2017. In 2014 and 2016, ratings were high and so were yields, at 56 and 59, respectively.
“A soybean crop with only average ratings in late July can, with good August weather, sometimes end up producing above-average yields. But high ratings in late July mean high yields, regardless of August weather. With the high ratings we have this year, we have every expectation that soybean yields will be high,” he says.
Another factor favoring soybean crop prospects is the rapid pace of crop development this year. Planting did not start early but it did finish early. With May and June warmer than average, the crop was off to the races, with 44 percent of the crop flowering by the end of June. This continued into July, with 44 percent setting pods by July 15 and 66 percent setting pods by July 22. The five-year average shows 24 percent of the crop setting pods by July 22, and about 50 percent by August 1. So the 2018 crop reached 50 percent podsetting about two weeks earlier than normal.
An early start to podsetting should be favorable, as long as there’s enough water to keep the crop in good shape. The amount of time between podsetting and loss of green leaf color (both recorded by NASS) estimates the duration of the seedfilling period, which is well correlated with yield. On average, Illinois reaches 50 percent leaf color change by about September 15, or about 45 days after 50 percent podsetting. The 2018 crop needs only to stay green through the end of August to have 45 days of seedfilling.
“With somewhat higher temperatures and longer days in August than in September, photosynthetic rates should be high, so having seedfilling begin and end two weeks early should be favorable for seedfilling rate and yield,” Nafziger says.
Temperature and day length influence when the soybean crop matures, and this is also affected by the number of pods that are filling, maturity rating of the variety, and other factors. Because days must shorten to a certain length to signal the end of seedfilling, we should expect a somewhat longer filling period this year, as long as temperatures remain normal and the crop has enough water. Nafziger says that should translate into higher yield. But with the early start to seedfilling, he adds, yields should be good even if the crop starts to lose color in late August.
Other positive signs for the soybean crop this year are the excellent plant stands in most fields, the excellent condition and color of the canopy, and the large number of pods already formed and still forming.
“While canopy color is good now, we expect leaves to become even darker green as seedfilling gets up to full speed over the next week or two,” Nafziger says. “The dark green color means that the plant is fixing plenty of nitrogen and storing it in the leaves, so adding nitrogen fertilizer is unlikely to boost yields. A fair number of fields have received fungicide, and probably insecticide as well, though we can probably ‘call the race’ between canopy development and insect feeding in favor of the crop this year.”
Soybean plants have grown tall due to warm temperatures and adequate water in most fields. Once there are full-sized pods at the top nodes, plants are at or close to their final height, so some are as tall as they will get. Periods of drier weather and warm temperatures have provided enough competition for water to keep the leaf size normal, which means that canopies are good, but not so heavy that we should worry about internal shading that can limit pod formation or seedfilling rates. Nafziger says we can probably expect some lodging, especially in 30-inch rows where plants are already tall. Moderate lodging as pods fill is a signal that pods are heavy, so is not a concern, especially if plants just “lean” without affecting light interception.
“Counting pods and seeds on plants in mid-season is neither a lot of fun nor an overly accurate way to estimate soybean yield potential. But with pods setting early this year, it’s a little easier to see how yield potential is shaping up. Pod numbers and number of seeds per pod appear to be very good in most fields. We’d like to see four or five pods filling at each middle node, and 30 to 50 pods per plant,” he says.
A field with 130,000 plants per acre, 40 pods per plant, 2.8 seeds per pod, and 2,700 seeds per pound at harvest projects a yield of 90 bushels per acre. Nafziger says that while growers shouldn’t expect such yields in most fields, there have been yields that high or higher in some fields in each of the past four years. Based on what we see now, he says, we expect to see this in some fields in 2018 as well.
For more information, please see Nafziger’s related Bulletin article at http://bulletin.ipm.illinois.edu/?p=4318.
Source: University of Illinois
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