Managing drought-stressed soybeans
Soybeans typically can withstand drought stress reasonably well in the vegetative stage. But the combination of drought and heat stress has been so extreme in some areas that soybean leaves have begun to curl or drop. In those cases, it’s already time to consider whether to leave the soybeans in the field and hope for the best or cut them for hay. The decision depends on the stage of growth and condition of the plants.
Drought symptoms appear early as leaf wilting and reduced growth. Nodule formation, development, and nitrogen fixation are reduced when soil temperatures rise above 90 degrees F. In general, soybeans can tolerate short periods of high temperatures if supplied with adequate moisture, but the crop cannot tolerate high temperatures indefinitely. The ideal temperature for soybean growth and development is around 86 degrees F. Temperatures above 95 degrees F can reduce seed set.
Prolonged heat and drought stress causes considerable leaf loss and yield reduction in soybeans. If the crop is so drought-stressed that it’s losing leaves or not setting pods, it may be time to cut it for hay. This might have particular appeal for livestock producers who are facing dry pastures and supplemental feed costs.
Soybeans with 50 to 90 percent leaves and a good number of pods at R6 have a good chance of producing a decent crop if allowed to mature -- especially if timely rains occur. In that case, it would probably best to harvest the crop as normal, even though some of the leaves and flowers have dropped due to stress. This is still a gamble, and good yields are not guaranteed even if the plants are in good shape at R6. Stress during rapid pod growth reduces the number of beans per pod and reduces bean size. Pod filling is the most susceptible time for drought injury to the soybean crop.
If possible, it’s best to hold off on making any decisions about cutting soybeans for hay until the plants are moving into seed fill (R5 to R6). Beginning seed fill (R5) is the optimal time to cut beans for hay in order to retain digestible nutrients. However, holding off until this stage of growth may not be possible if plants in the vegetative stage are dropping half or more of their leaves already. If too many leaves are dropped, the plants have reduced value as a hay crop. Producers may need to make the decision to cut for hay while the plants are still in the vegetative stage, before the R5 stage, and before the soybeans lose too many leaves.
Soybean plants that still have 30 percent of their leaves can produce 0.75 to 1.25 tons (dry matter) of hay per acre, with about 13 percent protein and 48 percent in-vitro dry matter digestibility. The more leaves a plant has, the more hay tonnage it will produce.