As the crop headed into September still without rain in many areas, soil moisture has become more of a concern.
By Emerson Nafzinger, University of Illinois Extension
Soybean planting was very late in Illinois in 2013; it was early June before 50 percent of the crop was planted, and well into July by the time planting was completed. Even so, the crop condition ratings were good by mid-season, with some 70 percent of the crop rated good or excellent in mid-July.
With the late planting and some cool temperatures in July, soybean flowering and pod-setting started late, with half the crop flowering by July 21, and half the crop setting pods by the end of the first week of August. The 5-year averages against which these numbers are compared include 2008 and 2009, when crop development was very late, and 2012, when the crop developed very quickly. So while the 2013 crop was not that far behind the 5-year average, it was still well behind what we would consider to be normal.
The periods of low temperatures in late July and mid- August were not kind to the soybean crop. Night temperature fell to the upper 40s and low 50s during both of these periods, and as I mentioned in my August 8 article, such low night temperatures interfere with soybean plant physiology, and set the crop up for lower photosynthetic rates the following day. The effect of this over several days tends to be cumulative, and during the mid-August round of low temperatures, which coincided with the ongoing (and already-delayed) pod-setting process, low temperatures very likely lowered pod numbers.
Soybean plants form racemes – flower-bearing branches – on 15 to 20 or so stem nodes. On each raceme, many flowers form, and most initiate small pods. The number of pods that make it past the initial "tiny" stage to get to full-sized pods with beans depends on the supply of sugars from photosynthesis. Flowers appear on a raceme over several days, and it’s common for the first one, two, or three flowers to develop productive pods; subsequent flowers result in pods that often abort, depending on how much sugar is available at that raceme at the time they develop. Thus cool temperatures that reduce photosynthesis during the flowering process can reduce pod numbers.
Anything that reduces the amount of photosynthesis, including drought, leaf disease, or insect injury to leaves can lower pod numbers. While most pods abort when they’re very small, it is possible for even full-sized pods to abort, up to about the time that seeds start to grow in the pods. The most common reason for abortion of larger pods is shortage of water; without enough water available to keep stomata open, photosynthesis shuts down, cutting the sugar supply. Some have invoked things like ethylene production to explain such "stress effects." While ethylene might be an intermediary of some sort, plants maintain pod and seed development only if they can maintain photosynthetic rates. We can sometimes help relieve the causes of low photosynthetic rates by irrigating or by helping keep leaves stay healthy and intact, but there’s often little we can do.
The soybean crop canopy generally looked good throughout most of August, and crop ratings were still at 65 percent good-excellent on August 18. To some extent the appearance of the crop disguised what might have been low numbers of pods developing, though the August 1 yield estimate for Illinois, which is based partly on pod counts, was 47 bushels per acre, which suggests that pods numbers were not too low. The return of warm weather (including warmer nights) the last week of August provided a boost to photosynthetic rates, and should have helped the crop to retain more pods. This effect was limited by dry soils in some areas.
As the crop headed into September still without rain in many areas, soil moisture became more of a concern. In most fields, leaves tended to stay reasonably well "hydrated" even in the afternoon heat. In dryer fields or parts of fields, though, leaves at some point during the day took on the grey-green appearance that signals inadequate water to maintain photosynthesis. If this condition holds for only an hour or two in a day, it may not have much effect. But if the leaves remain inactive for half of the day, that represents lost productivity that often cannot be recovered.
Why would a few hours of decreased photosynthetic activity in a day, which would usually lower corn yields, have less effect in soybean? Soybean plants tend to have a little more leaf area than they normally need for maximum photosynthesis, and because they can’t move sugars away from the leaves fast enough, they typically form starch during the day to tie up the extra sugar. While much of that starch is converted back to sugar and goes to good use, small reductions in photosynthesis over the course of a day may not decrease the available supply of sugars enough to hurt yields. This is especially true if pod numbers are less than normal, which means less demand for sugars.
The amount of seedfilling that the soybean crop is able to do when it’s dry during the second half of the season is directly related to two things: 1) how many seeds are present to fill; and 2) how long the crop is able to maintain enough active canopy to fill seeds. Most reports in late August showed a lot of concern about pod (seed) number under the stress conditions. That remained a concern into September, especially in the driest areas, and especially in the latest-planted fields, some of which were still setting (or trying to set) pods at the end of August.
Counting pods is a messy and tedious exercise, with large variations in pod counts between plants, and so the need to count a lot of pods to get a reasonable estimate. Pod counts are part of the USDA-NASS yield estimating procedure for soybeans. In the September report just released, the average pod count in Illinois was 93 pods per square foot. Assuming 2.5 seeds per pod, this would calculate to 10.13 million seeds per acre, and using the September 1 yield estimate of 46 bushels per acre, that would mean 220,000 seeds per bushel, or about 3,700 seeds per lb. That is a very small seed size, so reflects either an expectation that seeds will not fill very well or (more likely) an estimate of only 2 seeds per pod, which would mean about 2,900 seeds per lb.