By Michael Staton, Michigan State University Extension Soybean Educator
Producers want to know how their soybean fields will be affected by the recent heat wave and lack of rain and the warmer and drier than normal conditions that are forecast to prevail for the remainder of July. Soybean yield losses are most likely to occur when moisture stress occurs during germination and reproduction. Inadequate soil moisture during germination causes uneven and spotty emergence. This is the reason why soybean agronomists recommend placing soybean seed into at least 0.5 inches of moist soil at planting. Soybeans that were planted later in June may have germinated under marginal soil moisture conditions.
Soybeans can tolerate moisture stress relatively well during the vegetative stages. Stress at this time reduces shoot growth, but not root growth. These conditions diminish water use by the plants and increase their ability to extract water from deeper in the soil profile. Planting early into good soil conditions is a recommended tactic for mitigating the adverse effects of moisture stress. This is because the plants will have deeper roots for extracting soil moisture and a larger crop canopy, which shades the soil and reduces soil moisture losses due to evaporation. This tactic seems to be working in 2020 as the early–planted fields are tolerating the moisture and high-temperature stress better than the late-planted fields.
Very high soil temperatures (90 degrees Fahrenheit) can cause decreased nodulation and nitrogen fixation to occur in soybeans. High soil temperatures are most likely to have occurred in later planted beans due to the reduced canopy cover and on coarse textured soils. Producers can evaluate nodulation by digging up some plants and inspecting the nodules. Well-nodulated soybeans should have seven to 14 nodules on the tap root at flowering. Nodules that are actively fixing nitrogen will be pink to red when cut open. Moisture stress also reduces nitrogen fixation. However, research has shown that supplemental nitrogen applications to drought-stressed soybeans are not profitable.
At this time, soybean fields planted in May are in the R2 (full bloom) growth stage. Soybean plants respond to drought in this growth stage by aborting flower blossoms and in severe cases, lower leaves are dropped. While flower production can occur for 30 to 40 days under good conditions, moisture stress and high temperatures will shorten the flowering period. A reduction in moisture stress any time during this period will enable the plants to produce and retain new blossoms to compensate for those aborted during R2.
During the R3 growth stage (one pod 3/16 inches long on the upper four nodes on the main stem having an unrolled leaf), drought-stressed soybean plants will abort both flowers and pods. Temperatures above 95 F have been shown to significantly decrease pod set. Leaf loss can also occur in severely stressed plants. If the moisture stress ends, soybeans will produce new flowers and pods up to the R5 stage (beginning seed). This is a critical time to begin applying irrigation water to soybeans.
Soybean yield losses will be the greatest when moisture stress occurs between the middle of the R4 growth stage (beginning seed swell in any pod on the plant) and the middle of the R5 growth stage (full seed in any pod on the plant). Stress at this time reduces the number of pods per plant as the plants are no longer able to produce new blossoms and pods. This is the major source of the lost yield. However, the number of seeds per pod and the size of the seed can also be reduced at this time. Leaf loss will continue in severely stressed plants.
Moisture stress occurring at the R6 growth stage (one pod having green seed that completely fills the pod on one of the upper four nodes on the main stem having an unrolled leaf) is largely due to a reduction in seed size. A reduction in pod number per plant can occur but is much less likely after R6. Stress occurring after the R7 growth stage (one normal pod on the main stem has reached its mature color) does not affect yield.