Even Irrigated Fields Can Struggle
MANHATTAN, Kan. – Soybeans typically can withstand drought stress reasonably well in the vegetative stage. However, the combination of drought and heat stress has been so extreme in much of Kansas this year that soybean leaves have begun to curl or drop, said Kraig Roozeboom, K-State Research and Extension crop production specialist.
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.
“Prolonged heat and drought stress 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,” Roozeboom said.
The decision depends on the stage of growth and condition of the plants, he explained.
“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, or the R5 to R6 stages of growth. Beginning seed fill is the optimal time to cut beans for hay in order to retain digestible nutrients,” he said.
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, he added.
“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 beginning seed fill stage, and before the soybeans lose too many leaves,” the agronomist said.
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.
On the other hand, soybeans with 50 to 90 percent leaves and a good number of pods at the R6 stage have a good chance of producing a decent crop if allowed to mature -- especially if timely rains occur, Roozeboom said.
“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,” he said.
The “gray area” is where there are plants with 30 to 50 percent of leaves still remaining, he said. 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.
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,” Roozeboom said.
By the early reproductive stage, the effects of prolonged heat and drought are critical, he explained. Under drought conditions, soybeans in early reproductive stages will have increased flower and pod abortion.
“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,” Roozeboom said
If no pods are set after the normal blooming period of three to six weeks, it is possible that the crop will not set any pods or make any seed yield. Determinate varieties have shorter blooming periods than indeterminate varieties.
“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,” Roozeboom said.
Because of extremely high July temperatures, irrigated fields are not immune to the effects of drought stress, the agronomist added. “With numerous days over 100 degrees, even irrigated plants can fail to set or fill pods.”