"If there is no green color left on either corn or soybean at this point, then there is little or no chance to add yield," said Emerson Nafziger, University of Illinois Extension agronomist. "No green leaf area means no more photosynthesis; the 'factory is closed' for this crop."
In earlier-planted fields or fields with early-maturing hybrids or varieties, the shutdown of the leaves may have been normal. In Urbana, 2,700 growing degree days have accumulated since May 15, so mid-season hybrids planted in mid-May should be close to "natural" maturity.
"In very dry areas, plants without green leaf area may have died early, without completely filling the seeds," he said. "In corn, if kernels still have a milkline, with fluid at the base of kernels, they might still be able to receive some sugars from the stalk. But stalks in such fields have probably been depleted of sugars by now, and it's unlikely that yields will increase further."
If husks are dry and stalks seem to be soft, then Nafziger recommends watching for standability problems and harvesting as soon as possible.
In the Urbana planting date study, March 31-planted corn has reached physiological maturity, and grain has dried to the lower 20s, he said.
"The late May planting still has most of the upper half of its leaves still green and is at about half milkline," he said. "Kernels of the late-planted crop seem to be of average size, and although the grainfilling process is slow, the crop appears to have enough resources to fill kernels out normally, even though it might be a slow process. Some rainfall would still help such crops add some more yield."
Soybeans are starting to turn yellow in some fields, possibly triggered in part by the cooler night temperatures experienced 8 to 10 days ago. The "senescence signal" in soybean — the signal to leaves to move nitrogen to seeds and to turn yellow and drop — is not very well understood, but most fields that have yellowed so far have seeds that appear to be relatively well filled, he said.
In very dry areas, some fields may be losing their leaf color as a result of simply running out of water, he added. In such cases, seeds will be small and leaves will tend to dry out before they turn yellow.
Most of the soybean crop remains green or at least has some green leaves. These are still capable of producing sugars to help fill seeds, though day/night temperatures of low 70s/upper 40s are not favorable for seedfilling.
"The return to cool weather likely has helped the crop cope with water shortage by lowering water use rates," Nafziger said. "But lower water use rates and low temperatures mean lower photosynthetic rates, and so we expect that yield is being added slowly in most soybean fields. This will speed up as it warms up again, but senescence may not be far behind."
In the Urbana planting date study with Maturity Group 3.6 soybeans, the mid-April and early May plantings are both close to maturity, with little green color left. In the earliest planting, upper pods are starting to dry. In contrast, the early June planting is dark green, with seeds perhaps half-filled. Very early-maturing soybeans planted in early June around that study are yellow, and seeds appear to be well filled, he said.
For more information, read the Sept. 8 edition of The Bulletin at bulletin.ipm.illinois.edu.