RECENT STRATEGIC FARMING: FIELD NOTES PROGRAM COVERS HOW THIS GROWING SEASON’S PAST AND FORECASTED WEATHER CONDITIONS MAY AFFECT MINNESOTA SOYBEAN YIELDS
Each year, various crop tours crisscross the Midwest to estimate corn and soybean yields. In anticipation of these taking place soon, last week’s Field Notes session discusses what the Minnesota corn and soybean crops have been through and what they are forecasted to encounter and how this might affect yield. On the August 17, 2022 Strategic Farming: Field Notes episode, Seth Naeve, UMN Extension Soybean Agronomist, joined moderators and Extension educators Anthony Hanson and Dave Nicolai for a discussion of how this growing season’s past and forecasted weather may affect corn yield.
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Soybean and late planting. There is a reason why the old adage is “August and sept make beans”. Weather conditions have a large impact on yield potential, with timely rainfalls throughout pod-fill the most important factor. Unlike corn, which requires the accummulation of heat to grow and develop, Dr. Naeve says that, “a soybean crop is pushed to maturity.” In years like 2022 in which soybean is planted late, the period during which vegetative growth occurs gets shortened considerably. Consequently, fewer leaves are added to plants and rows may never close, reducing the overall photosynthetic capacity of the crop. With fewer leaves, less energy gets stored in them to be remobilized later on to form seeds during reproductive growth.
The “photosynthetic machinery” that makes up the soybean canopy can be more or less critical to yield. Dr. Naeve suggests that farmers know this fact all too well by watching their crop during the season and their yield monitors during harvest; they’ve harvested 2 foot tall crops that yield 60 bu/A and 5 foot tall crops that yield 35 bu/A. One thing is for certain: if we don’t have a nice and thick soybean canopy, we are just simply not going to top out yield potential. “If we don’t have the canopy there, we definitely limit the top yield potential. We can have 2 foot tall soybeans that yield 60, but they won’t yield 80.” according to Naeve.
Entering critical growth period. Our soybean crop is entering the most important period in its growth – halfway between the beginning and full seed growth stages (R5.5). At this growth stage a person looking at the crop can see the apical raceme (a group of tiny leaves and perhaps a cluster of flowers at the top of the plant). This apical raceme is kind of like a soybean crop’s “tassel”, telling us that the plant is done with vegetative growth and so will not form additional leaves. Says Naeve, “there are no new leaves being produced from now on. We’re stuck with the canopy.” The weather conditions between now and maturity or a hard frost will therefore have a very large impact on yield. We know what kind of canopy is going to be contributing to yield, but it is the weather between now and maturity that will determine pod count and seed size.
The various crop tours happening around this time of the year often count pods. Pod counts don’t really tell us what the yield will eventually end up being, but are very useful for identifying problem areas. The influence of seed size in final yield is even larger in soybean than corn and so combining pod count and using forecasted weather to guess about seed size is how the crop tours estimate
Effect on crop of cooler night temperatures. The cooler than normal temperatures in the near-term forecast will be particularly positive for some areas of Minnesota. Says Naeve, “Where we are dry, that’s going to keep us from ripping through so much water. But where we have had good rainfall, those cool temperatures are not really good at all for soybeans.” Soybeans don’t really like cold nights. Overnight through a process called respiration, soybeans move those starches from leaves into seeds. When night temperatures get down into the lower 50’s, respiration is slowed, and the crop isn’t as able to deal with this stored starch. When the following day rolls around, the starch from the previous day is blocking the plant’s ability to accumulate additional starch – causing the crop to essentially lose that day’s growth. These cool nighttime temperatures are one of the reasons why Minnesota soybeans don’t tend to yield as much as those grown further south. As the season progresses, cooler nighttime temperatures can limit the seed-fill period by cueing the soybean crop to begin to mature.
An audience member also asked about whether Dr. Naeve thought that narrower row soybeans would likely have a yield advantage in 2022. Dr. Naeve suggested that the yield advantage with narrower rows could be as 10 bu/a due to a combination of improved water use efficiency and light capture.
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Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!
