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U of M expert says frost will affect soybean yields

Media contact: Catherine Dehdashti, U of M Extension, (612) 625-0237, ced@umn.edu

ST. PAUL, Minn. (9/15/2011) —The severity of early frost on Minnesota's crops may not be known for two or three days; however, yield and quality in immature plants will be affected, according to Seth Naeve, a soybean agronomist with University of Minnesota Extension.

Widespread frost was reported across the entire state of Minnesota on the morning of Sept. 15, approximately two weeks earlier than average for many southern Minnesota locations, according to Extension climatologist Mark Seeley. "Low temperatures in the upper 20s were reported as far south as Byron, Zumbrota, Worthington and Sherburn," said Seeley.

"Many cultural and environmental factors will affect the level of damage," said Naeve. "Late planting, long-season varieties, poor fertility or drainage, and continuing cool temperatures may exacerbate the effects."

In most crop species, a hard killing frost after physiological maturity has little effect on yields. Physiological maturity is defined as the point at which maximum dry matter accumulation has occurred in the seed. But crops are not ready for harvest at physiological maturity, since dry- down usually takes a longer period of time. Soybeans are usually harvested at moisture contents of 14 percent or less.

Maximum dry matter accumulation of soybeans has been reached when:

  • all leaves are yellow and about 60 percent of the leaves have dropped from the plant
  • pods are all yellow and more than 50 percent of the lower pods have turned brown
  • beans within the pods have about 60 percent moisture, show little evidence of green color, and may be shrinking
Soybeans are easily damaged by frost in the 28- to 32-degree range. "Temperatures of 28 degrees for any extended period of time can completely kill soybean plants," said Naeve.

The yield loss will be proportional to the growth stage of the plant. Soybean plants that are very near the point of maturity can be expected to weather the freeze with little impact on yield. However, soybean fields that are only at the R6 (full seed) stage with all green leaves will experience significant yield losses. Very late-planted or very long-season soybeans could experience yield reductions of up to 50 percent due to a long duration freeze tonight.

How can you recognize frost-damaged soybeans? Watch for these characteristics:

  • Green or elongated yellow soybeans that shrink to smaller than normal size after drying
  • Reduced oil content and quality
  • Higher moisture level (by 1-2 percent) than indicated by a moisture meter
  • Slower field dry-down
Soybeans left standing in the field may lose green color within two weeks of maturity, so allow for field dry-down if possible, even if the plants were only partially frosted.

A study conducted at the University of Minnesota indicated that if green beans were properly dried to low, safe storage moistures, they should keep in storage.

Naeve suggests that it may be a good idea to screen out small green soybeans as a means to reduce potentially large discounts due to damage. If you are storing soybeans which require drying, be sure to dry them (at temperatures lower than 130 degrees) to a low-moisture level in order to ensure safe storage. In the Midwest, the Midwest Plan Service generally recommends storage moistures of 12 percent or lower for clean, high-quality soybeans in aerated storage for up to one year. For damaged soybeans, the storage moisture content should be 11 percent or lower.

For more educational information and tools, visit www.soybeans.umn.edu, a cooperative effort among the University of Minnesota, University of Minnesota Extension, and the Minnesota Soybean Research and Promotion Council.


Sources: Seth Naeve, agronomist; Mark Seeley, climatologist, University of Minnesota Extension

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