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Issue No. 21, Article 7/August 15, 2008

Soybean Crop: Progress and Prospects

The latest report from NASS indicates that as of August 10, only 55% of the Illinois soybean crop was setting pods, compared to the 5-year average of 79%. Due to late planting, only about a third of the crop in the southern part of Illinois is listed as setting pods, while about two-thirds of the crop in northern Illinois has reached this stage.

Only 84% of the crop is listed as "blooming" by August 8, the lowest for any year in the past decade. This is a surprisingly low number, given that the photoperiod is short enough by mid-July to trigger flowering by the time plants have about three leaves. It may be that many of these fields have some flowers but are not yet in full bloom (stage R2). The lack of warm night temperatures over an extended number of days might be the cause. In any case, plants with few or no flowers as we near the middle of August are a sign of a crop that is likely to have problems reaching maturity before cool weather starts to limit the crop's ability to fill seeds.

The shape of the curve of soybean crop progress has been more or less normal up to now, just two weeks or so late (Figure 1). It is likely that the crop that is just flowering now will also pod late, meaning a long "tail" on the pod-setting process this year.

Figure 1. Pod-setting progress in the Illinois soybean crop, 2008. Source: Illinois Weather & Crops, Vol. 29. No. 24, USDA, NASS.

Is this delayed development a problem, and what will it mean? In 2004, when Illinois producers harvested their highest soybean yield ever, 89% of the crop was setting pods by August 15. The percentage of the crop setting pods by August 15 has been about 90% in seven of the last 10 years, and in the 70% to 75% range in the other three years; there have not been any years with intermediate values. Statewide yields above 45 bushels per acre have always been in years with higher pod-setting percentage by mid-August, while those with slow pod-setting include 2003, with the lowest yield (37) of the last decade. The other two slow-podding years had yields of 43 (2002) and 44 (1999). The just-released projection for Illinois yields in 2008 is 42 bushels per acre. That is typical of years when development is late.

With the cool temperatures we do not expect pod-setting numbers to move up as quickly as they normally would during this week, and the weather forecast does not indicate a rapid return to warmer temperatures. This puts 2008 among the slowest of the "slow-podding" group of years, and with low temperatures in the 50s and low 60s this week, we are fast losing any chance that this problem will be reversed. In a real sense, the soybean crop in Illinois by the end of August will look much like the crop in a normal year looks at about the end of the first week of August.

We normally think that one of the advantages that soybeans have over corn is that the flowering process takes perhaps three times as long in soybean, providing it a way to overcome poor conditions during part of that period by making more flowers when conditions improve. At the same time, we know that flowers do not automatically produce pods with seeds that fill to produce yield. It is commonly thought that fewer than half of the flowers that form on a soybean plant result in productive pods, and in poor conditions this percentage can be very low. Pods can also fail to develop and can abort and drop off the plant any time during their growth, up through the start of seed filling. Pods usually stay on the plant after seed filling gets underway.

Flowers do not abort simply because of poor conditions, but also because of competition from other pods that are developing at the same node. There is often a range of pod sizes within a raceme (flower cluster) at an individual node, and these pods compete for sugars from the same leaf. The smaller pods at the outer end of the raceme do not compete very well, and they often abort and drop off the plant (W.G. Spollen, W.J. Wiebold, and G. Scott Glenn. 1986. Intraraceme Competition in Field-Grown Soybean. Agronomy Journal 78:280-283). We do not know very much about factors that start and stop the flowering and pod-forming processes at individual nodes, but extended periods of below-normal temperatures most likely lengthen this process.

Pods of different sizes forming at a single node of a soybean plant.

The plant seems to be able to sense how many pods it can support and then abort those it can't fill. This may happen simply due to lack of sugars to maintain the smallest pods, or there might be hormones involved. Low night temperatures tend to reduce overall photosynthetic rate of soybean plants, and so might contribute to this lack of pod retention. Some have related extended cool temperatures (nights in the upper 50s) to increased abortion of pods, resulting in lower pod numbers even though flower numbers were increased (H. Kurosaki, S. Yumoto, and I. Matsukawa. 2003. Pod Setting Pattern During and After Low Temperature and the Mechanism of Cold-Weather Tolerance at the Flowering Stage in Soybeans. Plant Production Science 6:247-254).

The number of pods and seeds that form on soybean plants is usually a strong determinant of the yield potential of the crop. This is one of the reasons why Kip Cullers of southwestern Missouri, who has set the yield record for irrigated soybeans, focuses on ways to retain pods on the plant. Preventing drought stress through irrigation is certainly the most consistent way to do this. Mr. Cullers also applies fungicides and micronutrients, and he supplements fixed nitrogen with N from poultry litter, in an attempt to maximize pod numbers. We will discuss a "high-yield" soybean experiment we have underway, with funding from the Illinois Soybean Association, at Agronomy Day at Urbana on August 21.

Even if we are able to produce and retain a lot of pods on soybean plants, will the plants be able to fill these seeds to produce high yields? If the seed-filling process gets underway in early August, as it typically does, then the answer is usually yes, unless dry weather, loss of canopy to hail or insects, diseases that reduce water supply to the plant (SCN, SDS, brown stem rot, Sclerotinia white mold), or low temperatures reduce photosynthetic rates and result in small seeds or late pod abortion, or both. In a general sense, though, having more pods tends to increase the rate, and perhaps the duration, of photosynthesis in soybean, which reinforces the fact that high yield potential depends on high pod and seed numbers per plant.

Seed filling gets underway at the beginning of stage R5, which is identified as the point when there is a seed at least 1/8-inch (3 mm) long in a pod at one of the four uppermost nodes with fully expanded trifoliolates. The high-yield experiment at Urbana was planted on May 29 and is just now reaching stage R5. With normal (warm) temperatures, it takes about 35 days from full R5 to physiological maturity. When the seed-filling process starts in mid-August (and so extends past mid-September) and August temperatures are less than normal, we expect that this process will take at least 40 to 45 days. If we are successful in retaining higher-than-normal pod numbers, then filling the additional pods (the plants continue to flower and add nodes into mid-R5 stages) will take additional time.

In short, late-planted fields are not getting the temperatures needed to encourage development and retention of high pod numbers needed for high yields, and the fact that seed filling is being pushed into late August and September by continuing cool temperatures presents a serious challenge to the soybean crop this year. About the only way to see yields much above the projected 42 bushels per acre is to have above-normal temperatures in September, with adequate soil moisture. We still have good moisture in most places, but below-normal temperatures in mid-August are not a positive sign. We can still hope for a September with above-normal growing degree days, like the one we had in 2007. But if August temperatures predict September temperatures (both August and September GDD accumulations were more than 100 above normal at Urbana in 2007), the signs so far are not very positive.--Emerson Nafziger

Emerson Nafziger

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