Crop & Pest Report


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Soybean Damaged by Frost (09/24/20)

Many areas in North Dakota experienced frost around September 8 and 9th.

Many areas in North Dakota experienced frost around September 8 and 9th. Soybean plants are easily damaged by temperatures in the 28 to 32 F range. With a dense canopy, a light frost may kill the upper leaves but the frost may not penetrate into the lower canopy (Photo 1).

plsc.5.soybean damaged by frost 1

Research from Wisconsin found that soybean had reduced yields when frost occurred at or before the R6 growth stage. The R6 growth stage is defined as ‘full seed,’ the stage at which time pods contain green seeds that fill the pod cavity at one of the four uppermost nodes on the main stem with a fully developed leaf and occur in more than half the plants in the field. The greatest yield losses occurred when frost damaged plants at the R5 growth stage. The lower number of beans per plant and reduced bean size contribute to overall yield loss (Table 1).

plsc.6.soybean damaged by frost.table 1

If the plants reached the R7 growth stage (Photo 2), yield reductions due to below freezing temperatures are limited. The R7 growth (beginning maturity) occurs when one normal pod on the main stem has reached its mature pod color. A frost between R6 and R7 may or may not affect yield, depending on temperature and duration of freeze.

Physiological maturity (growth stage R8) is reached when 95% of the pods have obtained the mature pod color, which can be brown, golden, yellow, or gray depending on the variety (photos 3 and 4). The seed moisture will be around 35%. At physiological maturity the seeds will not add anymore dry weight and just have to dry down for harvest.

Maturity can be determined on a plant or field basis but often there is variability in the maturity in the field due to differences in the growing conditions. At 95% mature pod color on a plant it may take 4 to 6 days to have all pods reach the mature color. It will take about another 2 weeks (depending on temperature and humidity) to get down to harvestable moisture levels. Harvest delayed to less than 13% moisture may cause an increase in pre-harvest shatter losses, sickle-bar shatter loss during harvest, an increased number of split beans, as well as a loss of soybean weight to sell.

plsc.7.soybean damaged by frost 2

plsc.8.soybean damaged by frost.last two

Frost-damaged soybeans generally can be harvested (after dry down) as long as the plants reached the R6 growth stage at the time the killing frost occurred. Frost-damaged soybeans may have higher moisture content and possibly are more difficult to thresh. Adjust the combine by reducing the concave clearance and adjust the cylinder speed if needed. Remove as much chaff and green plant material as possible before storing beans.

Severely frost damaged plants may dry down more slowly. The beans will need to be dried to a safe moisture level for storage (12% for 6 months). Electronic moisture meters may underestimate the moisture content in green and immature soybeans and actual moisture content may be 1.0 to1.5 percentage points higher. Some green beans will turn yellow after 30 to 40 days of storage.

Green and immature soybeans are considered damaged seed at the elevator. Elevators may discount loads with green and or immature soybeans and in some cases may reject entire loads if the damage levels are high.



Hans Kandel

Extension Agronomist Broadleaf Crops



Greg Endres

Extension Cropping Systems Specialist

NDSU Carrington Research Extension Center

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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