Drought conditions are intense across North Dakota and South Dakota, which combined accounted for about 6% of 2020 soybean production last year. There are also some worrying drought conditions across parts Iowa and a large portion of Minnesota, which sit at number 2 and 3 respectively in the top soybean production state rankings. While soybeans are pretty a resilient crop, prolonged dry conditions during both germination and the reproductive growth stages can be particularly detrimental, reducing stands and ultimately limiting yields.
Soybean yield is a function of the number of plants per acre, pods per plant, seeds per pod, and the size of the seed. Each of these yield components is fixed in the above order during specific timeframes of plant development. Moisture stress confined to a specific growth stage reduces the yield component that is established during that phase. Because soybeans flower over several weeks, they can compensate for periods of drought better than crops with a short pollination period. The crop is the most susceptible during the reproductive stages of growth as is corn. Below are more details about soybean growth stages and the impact dry conditions might have on the crop as we move through the growing season.
Soybean Growth Stages:
- Germination – Germination begins with the soybean seed absorbing 50% of its weight in water. The radical (or primary root) grows from the swollen seed and elongates downward. The hypocotyl begins elongation upward toward the soil surface, pulling the cotyledons along.
- Emergence (VE) through Sixth Trifoliate (V6) – Typically, VE stage occurs 5-14 days after planting depending on soil moisture levels, soil temperatures, and planting depth. Soon after the cotyledons are fully exposed, unifoliate leaves emerge at the second node and begin creating energy through photosynthesis (the VC stage). Development and full extension of the first trifoliate leaflets (node 3) establishes the V1 stage of growth, and with each fully developed trifoliate on the main stem, another V stage is established. Plants develop new growth stages about every 3 days, depending on environmental conditions, until reaching the final V6 stage.
- Reproductive Bloom Stages R1 and R2 – Flowering begins on the third to sixth node, continues up and down the main stem, and eventually moves to the branches. The plant has accumulated about 25% of its total dry weight by R2 and nutrients and about 50% of its mature height. Nitrogen fixation by root nodules is increasing rapidly. Moisture stress during the reproductive stages will lead to fewer, smaller, and poor quality seeds. Once the plant starts to flower it needs significant water right through to seed fill.
- Pod Development R3 and R4 – The R3 stage is marked when the plant has a pod on at least one of the upper four nodes of 3/16-inch long or longer. Heat or moisture stress at this stage can reduce pod numbers, seed number per pod, or seed size, which may reduce yield potential. The ability for soybean plants to recover from temporary stress decreases from R1 to R5. Favorable growing conditions during this period may result in greater pod number and increased yield potential. Conversely, stress during this period (and through R6) can cause more reduction in yield potential than at any other growth stage. Timely rainfall or irrigation may help reduce the potential for yield loss.
- Seed Development R5 and R6 – Beginning seed (R5) is marked when at least one 1/8-inch long seed is present in a pod at one of the four upper-most nodes. About half of the nutrients required for seed filling come from the plant’s vegetative parts and about half from N fixation and nutrient uptake by the roots. Nitrogen fixation peaks. Plants attain maximum height, node number, and leaf area at this stage. The R6 stage market the beginning of the full seed stage. At least one of the four upper nodes should have a pod with a green seed filling the pod cavity. Total pod weight peaks and leaves begin to yellow.
- Maturity R7 to R8 – Maturity begins with at least one normal pod on the main stem reaching its brown or tan mature color. Seed dry matter begins to peak. Seeds and pods begin to lose green color. When at least 95% of the pods on a plant have reached their mature color, the plant is fully mature.
Stress Symptoms: Soybean respond to drought stress by flipping their leaves over so the underside of the soybean leaf is turned up. A less obvious sign of drought stress in soybean is diminished vegetative growth which normally occurs prior to leaf flipping. In severe drought conditions, the leaf trifoliates will close or clamp together with the center leaflet being sandwiched between the outside leaflets. Another serious consequence of moisture stress is the reduction of biological nitrogen fixation. Plants that lack water will shut down nitrogen fixation in the root nodules. Plants weakened by stress are also more susceptible to disease and insects. Insect feeders like soybean aphids do considerably more harm in a dry year than in a wet year.
Effects on Vegetative Soybeans: Vegetative growth of soybean during drought is diminished. Drought stressed soybean are often shorter with smaller leaves due to a lack of water, nutrient availability, and nutrient uptake. Soybean root growth increases during drought conditions because plant carbohydrates are shifted to root growth. When adequate rainfall or soil moisture returns, vegetative growth will resume until the mid-seeding filling stage (R5.5). Under severe drought stress, soybean flowering may occur earlier than normal in an effort to produce seed before premature death.
Effects During Grain Fill: Drought effects on soybean are generally not as severe as corn. This is a result of overlapping of development stages. When short-term drought stress results in flower or pod abortion, new flowers and pods will set when conditions improve. During prolonged drought stress, or when the stress occurs during pod set and seed filling stages, the compensatory ability is not as likely to occur. Drought can reduce pod number by up to 20 percent as a result of flower and pod abortion. Seeds per pod and seed size can also be affected by drought stress but to a lesser extent than the number of pods. Drought stress often results in earlier maturity or shortening of the grain filling period resulting in lower seed weights and yields. (Sources: Iowa State University, South Dakota State Extention, Michigan State University)