Management options for drought-stressed soybeans
Soybeans typically can withstand drought stress reasonably well in the vegetative stage. Many fields have been blooming now for several days, but are not setting pods. 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.
click image to zoomPhoto by Doug Shoup, K-State Research and ExtensionEffect of drought and heat stress on soybeans in Sumner County, 2011. Prolonged heat and drought stress can cause 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 R4 (full pod) 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 for grain, 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 R4. 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.
Last summer, the stress period did not begin as early and fields in northeast Kansas still looked good into early August. A close examination of those fields revealed that very few pods had been set. Once a good rain or two occurred and temperatures dropped for a few days, pods set, began to fill and the plants immediately went into drought stress because of the lack of stored soil moisture and continued high heat. Timely rainfall in the extreme northern areas of the state kept these plants sustained and good to excellent yields still were harvested in most cases.