No. 21 Article 8/September 9, 2011

Can Corn and Soybean Still Add Yield?

With dry weather continuing over much of Illinois and crop condition ratings continuing to fall, many are wondering if the corn and soybean crops are done for the year or if adding yield is still possible. The large drop in temperatures, from highs in the 90s at the end of last week to highs in the low 70s early this week, also raises hopes that the crop might hang on a little longer to add some more yield.

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 to what is there now. 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; that is, enough time and temperature have accumulated for the leaves to senesce naturally. Here at Urbana, we have accumulated about 2,700 growing degree-days since May 15, so midseason 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. 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 it will be important to watch for standability problems and to harvest as soon as practicable.

In the planting date study here at Urbana, the corn planted March 31 has reached physiological maturity, and grain has dried to the lower 20s. In the late-May planting, most of the upper half of leaves are still green and kernels are at about half milkline. Kernels of the late-planted crop seem to be of average size, and although the grain-filling process is slow, the crop appears to have enough resources to fill kernels normally, though the process might be slow. 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 we 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. In very dry areas, some fields may be losing their leaf color as a result of simply running out of water. 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 plants are still capable of producing sugars to help fill seeds, though day/night temperatures of low 70s/upper 40s such as we have had the past few days are not very favorable for seed-filling. The return to cool weather likely has helped the crop cope with water shortage by lowering water use rates. But lower water use rates and low temperatures mean lower photosynthetic rates, 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 planting date study here at Urbana with Maturity Group 3.6 soybeans, the mid-April and early-May plantings are both close to maturity, with little green color left, and 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.

We're not in the driest area of the state here in Urbana, but with less than 3.5 inches of rain total in July and August, soybeans still appear able to produce fair to good yields this year. Once leaves drop, it becomes much easier to get a sense of pod numbers and how well seeds have filled. My observation of farm fields with dropped leaves in the Champaign area gives me cause for limited optimism.

Yield prospects for corn are more mixed, and while some early reports of yields have been modestly positive, much of the crop being harvested now was planted early and may have higher yields because of that. Fields that have deteriorated over the past month may not have enough kernels for high yields, but the cooler weather may help plants that still have green leaf area to push a little more sugar into the kernels that are there.--Emerson Nafziger

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