The weather over the past few days brought a wave of frosts and freezes to parts of the state. For those growing full-season soybeans, this may be the first time in many years where an early frost affected still-maturing soybeans. For those growing double crop beans after small grains, a frost at this time is likely ahead of schedule and the plans of a good crop may be compromised. Fortunately, some assessments can be made now, so you know what to expect later.
An early frost on soybeans can greatly diminish soybean yield, as pods may have not had enough time to fill completely. The upper leaves of soybean plants are easily damaged by frost in the 30° to 32°F range, and temperatures under 30°F for any extended period can completely kill stems and lower leaves, resulting in death of the whole plant. Soybeans planted in narrow rows (15 inches and under), may have slightly more tolerance to light frosts than those planted in wider rows. Thin stands are more likely to be affected and injured by frost as well. Frost damage in a soybean field can vary considerably due to microclimate effects and the landscape position in the field.
Assessing a Field
The true effects of frost damage cannot be observed until after a couple of days, so one needs to wait before assessing a field. If the crop suffered a light frost, damage will be seen in the upper canopy, with wilted, dried leaves attached to the plant. Leaves in the lower portion of the canopy should appear normal. In those cases, a maturity delay of several days is possible and small pods near the top of the plant may abort or not fill normally. If a more severe freeze occurs, damage may be present in leaves, stems, and pods in the lower canopy. Frost-damaged stems will turn a dark green to brown color and immature beans will shrivel, reducing soybean yield, test weight, and drying rate.
The make or break point for a complete freeze of soybean plants is the growth stage R6, or when beans completely fill a pod at one of the upper four nodes on the main stem on at least 50% of the plants in a field. A frost at this stage may or may not affect yield, depending on the temperature and duration of the freeze and whether the plants are in the early or later parts of this stage. However, if a frost occurs before this stage (the seed has not completely filled the pod), reduced yields are almost certainly due to reductions in bean size and pods per plant. Beans experiencing frost at the latter end of R6 when leaves are coloring may suffer yield losses as little as 5%, while those in the early part of this stage may suffer losses of over 50%.
If a frost occurred at the growth stage R7 or when one pod on the main stem reaches its mature pod color, beans should dry normally, and yield and quality should not be affected much. Additionally, it is unlikely that bean oil and protein content will be reduced. While mature pod color can vary between varieties, a good indicator are yellow-colored pods sprinkled with brown. A better check is to open pods and check for the separation of beans from the white membrane inside the pod
Harvesting and Drying
Frosted beans tend to have near normal development near the bottom of the plant, but at the tops green or yellow, elongated lima bean-like seeds can be seen. These can be slow to dry, resulting in a grain mass ranging from very dry seeds to very wet seeds. In fields where only the upper leaves were damaged by frost, one can wait and allow the beans to mature and dry to 14% to 15% in the field, if possible. However, if fields experienced a freeze throughout the entire canopy, is it best to harvest frost-damaged fields at moisture levels between 16% and 18% to avoid shatter loss. Extra attention should be paid to combine settings, too. If beans are at “normal” harvest moisture content, keep the cylinder speed to a minimum to avoid bean crackage. If beans are wetter than normal, reduce concave clearance first, then increase RPM until acceptable threshing occurs, making incremental adjustments and checking your progress after each adjustment.
Electronic moisture meters are likely to underestimate the moisture levels in green and immature soybeans, so add 1.5 percentage points to moisture meter readings when testing mixtures of immature and mature beans and adjust drying times accordingly. Recheck the moisture content after drying and after a couple days to permit moisture equilibration.
When drying soybeans, excessive heat causes seed coats to crack and beans to split. To avoid splits while drying, it is important to maintain the relative humidity of the drying air above 40%, which unfortunately limits heat input and drying capacity. For example, outside air at 50°F and 80% relative humidity can only be heated to 70°F to maintain humidity above 40%. Therefore, drying with air heated above 160° to 180°F is not an option when drying soybeans. If splits are not as much of a concern, drying air temperatures between 120° to 140°F can be used. Heating temperatures in bin dryers can be lowered by using short heat on-off cycles or changing the burner jets to low-fire types. Natural air above 60°F and below 75% humidity will require no supplemental heat to remove 2 to 3 points of moisture. The surface color of green-colored beans may change little in drying and storage, however green beans dried in the field or in dryers do not appear to present a greater storage risk.
Grain Quality and Marketing
Soybeans are graded by USDA standards to determine the quantity of damaged seeds splits, foreign material, and off-color (green) beans, and loads with a musty or sour odor. In the case of frosted beans, loads could be discounted for most or all the above criteria. Discounts are possible for green beans at processors and elevators and loads could be rejected if damage levels are high. Therefore, one should not harvest green or immature soybeans wet and market them immediately at harvest. To reduce the potential for discounts dry beans to 12% moisture, storing them in aerated bins for a couple months. Additionally, it may be worthwhile to screen out small beans before binning or delivery.
So when asked if a field is worth harvesting, the answer will depend on a variety of factors. The best course of action is to first wait and see how bad the frost damage was and then determine how to best get the beans harvest stored and sold without taking a hit on yield and price. If this looks promising, it may be just like many other years. However, if Mother Nature decided that indeed this was the end of your growing season, there may not be a lot of good news for you this fall.