Stress During Soybean Reproductive Development

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Adequate moisture during spring and early summer followed by dry conditions in August seems to be a recent trend in the region. How vulnerable is soybean yield potential? 13;10;R2 (full bloom) - Drought can cause flower abortion, and in severe cases, lower leaves are dropped.1 Flower production can occur for 30 to 40 days under good conditions. Moisture stress and high temperatures can shorten the flowering period. A reduction in moisture stress during this period can help plants produce and retain new blossoms to compensate for earlier losses.

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R3 (beginning pod) - Temperature or drought stress at this time can reduce total pod number, number of beans per pod, or seed size. Normally, soybeans abort 60 to 75% of flowers.2 Drought-stressed soybeans may abort flowers and pods.1 Any stress can increase floral abortion and significantly affect yield potential. If stress is alleviated, there may be partial compensation for earlier losses but as the plant matures from R1 to R5.5, the ability to compensate decreases.2 Temperatures above 95° F have been shown to significantly decrease pod set.1 Severely stressed plants may lose leaves.

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R4 (full pod) through R5 (beginning seed) - This is the most crucial period for seed yield. Stress during late pod formation at R4.5 to early seed fill at R5.5 is the most critical time.2 Stress reduces the number of pods per plant and plants can not produce new blossoms and pods.1 Seeds per pod and seed size can be reduced at this time. Severely stressed plants can lose leaves.

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Drought-stressed soybean seed can be small, shriveled, and wrinkled with lower than average protein.3 Soybeans produce seed protein in the later part of the growing season; consequently, conditions that shorten the growing season can reduce protein levels. Oil content is not normally affected much by stress.3 However, oil yields per bushel may be lower because it’s harder to extract oil from flat and small soybeans, and more residual oil could be left in the soybean meal.

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Soybean Stage of GrowthYield Reduction
1st week/flowering 8%
1st week/pod development 19%
3rd week/pod development 36%
2nd to 4th week/seed-filling 39 to 45%
5th week/seed-filling 12%
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Table 1. Effect of 4 days of visible severe moisture stress on soybean yield.Source: Wright, J. 2006. Predicting the last irrigation for corn and soybeans in central Minnesota. Minnesota Crop eNews.

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