Managing drought-stressed soybeans
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.