Managing drought-stressed soybeans

Source: Kraig Roozeboom, Row Crop and Cropping Systems Specialist and Doug Shoup, Southeast Area Crops and Soils Specialist, Kansas State University | July 27, 2011

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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.

Like most legumes, soybean hay quality is fairly good, ranging from 15 to 20% protein.

Soybean hay quality is roughly comparable to mature alfalfa. Once soybeans reach full pod and seeds begin to swell, quality begins to decrease because soon after this stage, leaf yellowing and leaf drop are initiated. In addition, the highest quality part of the plant is the leaf, and the least palatable portion is the stem, which will often be left as waste if fed directly to cattle in a bale. Grinding the hay or ensiling the soybean will increase the total dry matter consumed by the animal.

The “gray area” is where there are plants with 30 to 50 percent of leaves still remaining. Those have the capability of filling pods if it rains and of making a soybean harvest that is worth more than the price of the hay. But that's a pretty big “if” in parts of Kansas this year, given how spotty the rains have been and how long the crop has been subjected to triple-digit temperatures.

The producer’s decision this year will depend partly on when the soybeans were planted. Soybeans that were planted June or early July are probably still young enough to withstand drought stress for several more weeks without dropping leaves. Soybeans planted in May or early June will be more vulnerable to rapid leaf loss at this time of year.

By the early reproductive stage, the effects of prolonged heat and drought are critical. Under drought conditions, soybeans in early reproductive stages will have increased flower and pod abortion. In later reproductive stages, prolonged drought will cause pods to be small, with fewer and smaller (or shriveled) seeds than normal.

Soybeans can tolerate short periods of heat and drought at this time by aborting flowers and forming more later. But the crop will not bloom indefinitely and under prolonged heat and drought may be unable to recover. Stress during flowering reduces the length of the flowering period. Determinate soybean varieties will normally bloom for 3 to 4 weeks or so under excellent conditions, and indeterminate varieties will bloom for 4 to 6 weeks, but these periods of bloom will be less than that under stress conditions.

If no pods are set after the normal blooming period, it is possible that the crop will not set any pods or make any seed yield. If fields have no pods set at all by the time they have reached the end of their blooming period, the crop should be hayed.

Because of extremely high July temperatures, irrigated fields are not immune to the effects of drought stress. With numerous days over 100 degrees F, even irrigated plants can fail to set or fill pods.

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