The cold snap over late last week and the weekend has raised questions on damage to soybean crops across the region. The effect of frost on the soybean crop depends upon the growth stage of the soybean plants. Frost-damaged soybeans are generally considered salvageable as long as the plants reached the R6 growth stage. Losses are uncommon at full maturity. Temperatures of 30°F to 32°F can easily damage the top leaves on a soybean plant, with 28°F temps causing significant damage occurs.
Assessing Plant Damage: Frost damage within a soybean field may vary considerably, depending on microclimate effects, landscape position in the field, canopy density, and other factors. Generally, thick plant canopies formed by narrow rows and high plant populations tend to hold the soil heat better and protect the lower portion of the plants and pods to some extent. In order to determine the yield potential that has been lost, you need to scout fields and determine the growth stage of your soybean crop. See photos below for staging. Much of the damage reported is limited to the tops of plants.
- The key growth stage needed for limiting damage is R6. Soybeans at Beginning Seed (R5), when seed is 1/8 of an inch inside the pod located on the main stem at one of the four upper most nodes, can be severely damaged, with losses of around 60-65% possible.
- Losses reduce significantly with increasing maturity. Through R6, losses reduce rapidly. By R7, when one pod on the stem is brown/tan, losses of only a few percentage points are expected.
- If only a light frost occurs, damage may be confined to the upper leaves in the canopy. Some maturity delay (several days) may be expected on damaged plants, and small pods may abort or fail to fill.
- If a more severe freeze occurs, leaves in the lower canopy may also be damaged, as well as stems and pods. Frost-damaged stems turn dark green to brown. Beans that were still green and soft at the time of the freeze will shrivel, reducing soybean yield (seed size and test weight), quality and drying rate.
If beans had reached physiological maturity (R7) prior to the freeze, these yellow beans should dry normally, and quality should not be affected.
Harvesting / Managing Freeze-Damaged Soybeans
The effect of frost on soybean grain quality is related to crop maturity and amount of killed leaf tissue. If soybean plants are killed before reaching maturity, some or all of the grain may be green, lower quality seeds, lower yield potential, and variable moisture content can also be expected. Care must be taken in harvest, handling, drying and storing of frosted soybeans. An early frost can cause slower field drydown.
- If soybeans have been frosted prior to maturity or have higher than normal moisture at harvest, combine settings may have to be adjusted to minimize harvest losses.
- Soybeans should be at 16% seed moisture or below for ideal threshing, and with early frost, some fields may be wetter than this, at least in spots, late into the season.
- Growers will also need to manage a balance between field dry down and shatter losses. Shatter losses of 0.2 bushels per acre per day occur after the beans reach 16 to 18 percent moisture.
- Wetter soybeans will require careful handling and drying. Slow drying can help reduce moisture levels, and may begin to turn some of the green seed coats to a normal mature color, but full color change on all green soybeans is not feasible.
Green and immature soybeans are considered damaged seed at the elevator.
Final Thought: As always, be sure to include crop insurance in communications as soybean fields are assessed and any damages determined.