Impact of Dry Conditions on Soybeans
Prolonged dry conditions can reduce soybean yield at any stage of development, but plants are more susceptible during germination and the reproductive growth stages. Dry soil conditions during germination will reduce plant stands. Severe moisture stress is evident by leaf wilting and under extreme conditions may lead to plant death. A lack of moisture during the vegetative growth stages (May and June) is usually not yield reducing if enough moisture was present for emergence. The crop is the most susceptible during the reproductive stages of growth as is corn. 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. In Ontario the growing conditions during the second half of July and August are critical to soybean yield.
Soybean yield is a function of the number of plants/acre, pods/plant, seeds/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. On the other hand, water abundance during a specific phase enhances a specific component. Stress early during the reproductive stages reduces seed number. However, if moisture returns that reduction is offset by enhanced individual seed size or mass. Stress later during the reproductive phases greatly depresses seed mass, the last yield component to be fixed during reproduction (Table 1).
Means followed by different letters are significantly different, LSD (P = 0.05). Reaper and Purcell, 1999.
Soybean varieties grown in Ontario have an indeterminate growth habit. The indeterminate cultivars flower from the base of the plant upward as the plant grows vegetatively as compared to determinate cultivars which flower after vegetative growth stops. Therefore, soybean varieties adapted for Ontario have an overlapping vegetative and reproductive growth periods. The result is that indeterminate varieties have an extended flowering period (end of June into early August).
This extended flowering period gives us added built-in protection or insurance against stressed conditions. Yield that may be lost due to flower abortion early in the reproductive stages can be compensated for by flowers and pods that will be produced later in the season. If plants go through the entire flowering period with insufficient moisture significant yield losses will occur. Even if water is supplied past flowering yield losses cannot be regained.
Stress Induced Symptoms
Inadequate water reduces nutrient availability, nutrient uptake and movement within the plant. One of the most evident water stressed nutrient deficiencies is potassium as is flipped leaves exposing the underside of the leaf. 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.
There are no management strategies to alleviate moisture stress once it occurs. The following practices have been shown to lighten the impact of moisture stress:
What About Fungicides?
Although there is scientific data showing fungicide applications can reduce stress caused by diseases, there are no conclusive scientific publications or consistent results showing a fungicide application will reduce environmental stresses caused by dry conditions, hail, etc. in the absence of disease. What will determine yield in this case is the variety/hybrid (genetics) grown, weather conditions, crop nutrition and soil structure.
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