by Mike Staton

Many fields planted to double-crop soybeans in Michigan were recently damaged by frost.

The freeze event was not particularly early, but it occurred when the plants were immature. The main causes of the immature plants were delayed soybean planting because of a late wheat harvest and delayed maturity because of poor growing conditions this fall.

Frost-damaged soybeans are generally considered salvageable for grain harvest as long as the plants reached the R6 growth stage at the time the killing frost occurred. The R6 growth stage occurs when the beans completely fill one pod at one of the upper four nodes on the main stem on 50% of the plants in the field.

In dense, green soybeans, frost-freeze damage kills the upper leaves but rarely penetrates deeply into the canopy when temperatures remain above 30 degrees F. However, once the upper leaves have been damaged, subsequent freeze events will penetrate deeper into the canopy.

Once the plants reach the R7 growth stage, yield reductions because of frost-freeze injury will be minor. The R7 growth stage occurs when one pod on the main stem has attained its mature color on 50% of the plants in the field.

The following recommendations will help reduce the adverse effects if soybean fields were damaged by frost:

Adjust the combine. Frost-damaged beans will probably be wetter than normal and more difficult to thresh. The first step in adjusting for this condition is to reduce the concave clearance. If acceptable threshing still does not occur, increase the speed of the cylinder or rotor. Make incremental adjustments and check the progress after each adjustment.

Harvest at higher moisture contents. Soybeans that experienced severe frost-freeze damage extending well into the crop canopy will dry down slowly. In this case, avoid significant harvest delays by harvesting frost-damaged fields at moisture levels between 16% and 18%. Data from the University of Wisconsin showed that shatter losses of 0.2 bushel per acre per day occur after the beans reach 16% to 18% moisture. The beans will need to be dried to a safe moisture level for storage (12% for six months).

Electronic moisture meters will likely be inaccurate and tend to underestimate the moisture levels in green and immature soybeans, so remember to add 1 to 1.5 percentage points to the moisture meter readings when testing mixtures of green, immature and mature beans and adjust drying times accordingly.

Recheck the moisture content after drying and after a couple days to permit moisture equilibration. In fields where only the upper leaves were damaged by frost, wait and allow the beans to mature and dry to 14% to 15% in the field if possible.

Dry frost-damaged soybeans with ambient air. If only 2 to 3 percentage points of moisture need to be removed, if air temperature is above 60 degrees and below 75% relative humidity, no heat is required in drying bins equipped with full perforated floors and fans capable of producing 1 to 2 cubic feet per minute per bushel. However, drying will occur slowly. Drying times depend on initial moisture content, air flow, grain depth and weather conditions. Continuously run drying fans when the beans are above 15% moisture, and the average humidity of the air is below 70% to 75%.

Dry frost-damaged soybeans with a high-temperature dryer. Be careful if planning to dry soybeans in a high-temperature dryer because soybeans are more fragile than corn and are normally dried using temperatures below 130 degrees. Seed coat cracking and split beans increase with increasing temperature. For food-grade and seed beans, maintain the relative humidity of the drying air above 40% to protect the integrity of the seed coats and prevent splits, but drying will be extremely slow.

The air relative humidity is cut in half for each 20 degrees that the air is warmed, so keeping the relative humidity above 40% limits the drying temperature to about 20 degrees above outside temperature. Control the heat and humidity of the drying air by using short burner cycles or changing the burner jets.

Store frost-damaged beans. Elevators will discount loads containing green and immature soybeans and, in some cases, may reject entire loads if the damage levels are high. Discounts can be reduced by screening out the small beans, drying the rest to 12% moisture and storing them in aerated bins for a couple months.

Staton is a senior soybean educator at Michigan State University Extension. 

Source: MSU Extension