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Field Notes program covered how past and forecast weather conditions may affect corn and soybean yields

Angie Peltier, Extension educator - crops

Each year various crop tours crisscross the Midwest to estimate corn and soybean yields. In anticipation of these taking place soon, the August 17th Field Notes session discussed what the Minnesota corn and soybean crops have been through, what they're forecasted to encounter, and how this might affect yield. The program featured UMN Extension soybean agronomist, Seth Naeve, and UMN Extension corn agronomist, Jeff Coulter and was hosted by UMN Extension educators Dave Nicolai and Anthony Hanson.

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What the crop has been through 

The hot and dry conditions during and after pollination affected pollination and kernel loss. Peeling back husks may reveal some missing kernels along the length of the ear because the ovules were not fertilized by pollen. It might also reveal lost kernels at the ear tip (i.e., tipback), where kernels were set but dried out and were lost due to moisture stress after pollination. “In some of those areas that were dry, we definitely had some pull back on the tips of the ears where we lost some of those kernels that were set,” shares Dr. Coulter. While recently forecasted rainfall will certainly help us during the grain-filling period, much of it was a little on the late side for saving kernels at the ear tips. Kernels can continue to be lost throughout the milk stage.

Daily heat units that accumulate after planting (growing degree days, GDD), based on daily high and low air temperatures, can be used to estimate how long it will take for the crop to reach key points in growth and development, including physiological maturity or black layer. GDD accumulation in areas around the state suggests that the crop is between 0 and 10 days behind normal. An online tool is available to estimate when your corn crop is likely to mature. Alternatively, a general guideline is that corn reaches maturity at about 60 days after silking. This year, much of the corn began silking on July 20-25.

While warm and sunny days, coupled with cool nights are optimal for corn during the grain-filling period, the recent cooler-than-normal temperatures reduce water usage by the crop, which should be a benefit for corn in areas that are droughty or abnormally dry. If it remains cooler than normal, fields that were delayed in planting are going to need the first hard freeze to hold off until early October.

Yield estimates 

According to Coulter, “At this point, if we go out in the field and start pulling ears and looking at them, we can get a real accurate estimate of the number of kernels per acre, as those numbers aren’t going to change once we’ve moved past the milk stage. But the thing that we aren’t able to get a real solid estimate on is kernel size.” 

 The primary grain-filling period is from the dough stage through maturity, or from about mid-August through September. When grain-filling conditions, largely soil moisture, are average a corn crop can have about 75,000 to 80,000 kernels per bushel. But when there is intense drought during grain filling, corn may have 90,000 kernels per bushel because individual kernels are smaller. 

Under highly favorable conditions during grain filling, the kernels will be larger and there may be 70,000 or fewer kernels per bushel. A change from 70,000 to 90,000 kernels per bushel can equate to a yield difference of around 60 bushel per acre – this helps to explain some of the large differences in yield that occur from year to year. If it remains moist with average or cooler-than-normal air temperatures, grain-filling conditions are expected to be average to above average, so kernel size may be average to larger than average. However, in areas that remain dry, kernel size is expected to be smaller than normal.


There is a reason why the old adage is “August and September make beans." Weather conditions have a large impact on yield potential, with timely rainfalls throughout pod-fill the most important factor.

Soybean and late planting

Unlike corn, which requires the accumulation of heat to grow and develop, Dr. Naeve says that, “a soybean crop is pushed to maturity.” In years like 2022 in which soybean was 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 yield.

Effect of cooler night temperatures on crop 

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.

Effect of narrow rows  

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!

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