No. 16 Article 7/July 23, 2010

Soybeans Are Shaping Up Nicely, But Are You Scouting Pod-Set?

The soybean crop is growing and developing on a nice pace across the state. There were mild concerns last week about high temperatures and depleting soil moisture in shallow root zones, but in most areas there was enough soil moisture that plants showed few signs of wilting and other stress symptoms. The return to scattered thunderstorms this week has relieved excessive heat indexes and has delivered needed rain to some areas. The July 18 USDA Crops report had soybeans at 64% blooming and 15% setting pods, percentages similar to the 5-year averages. There have also been no widespread reports regarding diseases or insects, but this is an important time to be in your soybean fields scouting for weed control failures, insects, and diseases and generally getting to know the progress of pod setting.

Yield is determined throughout the entire upcoming R3 to R6 window. However, as soybeans transition from flowering to pod-set over the next few weeks, these are critical developmental stages to determine the number of pods that will be retained (not aborted) on the plants. The number of pods that are set in the R3 and R4 growth stages plays a major role in determining the "sink" of the plant prior to R5 and R6 (the seed-filling stages). There is some speculation that as we want to increase soybean yields, we need to focus on building a bigger sink by limiting stresses and increasing the number of pods set in R3 and R4. This isn't the only answer, because we know that seed size is determined in R5 and R6 and is still an important component of yield, but as we continue research into high yields, conditions at R3 and R4 are certainly of interest.

While the source-to-sink relationship is difficult to map clearly in soybean, what we have learned is that "improving" late-season canopy appearance does not always relate to more yield. I have seen this, and I have heard from growers this frequently occurs. A limited sink is an obvious answer to this problem, although it is not clear. The photos here show a 2008 irrigation experiment at Dixon Springs Agriculture Center near Simpson. The late-season canopy was improved for greenness by both irrigation and fungicide, but there were no yield increases.

Greening-of-canopy effects of irrigation on soybean in September 2008 in a trial at Dixon Springs Agriculture Center. (Photo courtesy of Steve Ebelhar.)

Greening-of-canopy effects of foliar fungicide on soybean in September 2008 in a trial at Dixon Springs Agriculture Center. (Photo courtesy of Steve Ebelhar.)

While I don't have the answer for how to improve pod numbers on every acre, learning more about your field conditions during these reproductive stages is a good place to start.--Vince M. Davis

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