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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 corn and soybean yield potential from later season drought stress?
R2 (full bloom) - Soybean plants respond to drought in this growth stage by aborting flower blossoms, 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 losses earlier in this growth stage.
R3 (beginning pod) - Temperature or drought stress at this time can cause a reduction in total pod number, number of beans per pod, or seed size. Drought-stressed soybean plants may abort flowers and pods.1 Normally, soybeans abort 60 to 75% of flowers.2 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 Leaf loss can also occur in severely stressed plants.
R4 (full pod) through R5 (beginning seed) - This period 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 the plants can not produce new blossoms and pods.1 The seeds per pod and seed size can also be reduced at this time. Severely stressed plants can lose leaves.
Drought-stressed soybean seed can be small (shriveled and wrinkled) and lower than average in protein.3 Soybeans produce seed protein in the later part of the growing season and consequently, conditions that shorten the growing season reduce protein levels. Oil content is not normally affected much by stress.3 However, oil yields per bushel may be lower because flat and small soybeans are harder to extract oil completely, leaving more residual oil in the soybean meal.
Drought stress can shorten the corn grain-filling period, increase lodging, and lower kernel weight.4 Water stress during several corn reproductive growth stages result in yield losses from 3 to 6.8% per day of stress (Table 1). Kernels near the tip of the ear are most susceptible to abortion during the first 2 weeks following pollination.
Severe drought stress that continues into the early stages of kernel development (blister and milk stages) can abort developing kernels. Once kernels have reached the dough stage of development, losses occur from reduced kernel dry weight accumulation. Severe stress during dough and dent stages of growth can decrease kernel weight and is often caused by premature black layer formation in the kernels.
Corn plants can tolerate stress during late grain filling because kernels can draw on stored carbohydrates from the stem to finish accumulating dry matter. However, remobilization of carbohydrates may weaken stems and increase susceptibility to stalk rots. Extreme drought can increase susceptibility of corn to aspergillus, which produces the mycotoxin known aflatoxin.
Sources: 1Staton, M. 2012. Moisture stress and high temperature effects on soybean yields. Michigan State University. 2Endres, G., and Kandel, H. 2015. Soybean growth and management quick guide. North Dakota State University. 3Hurburgh, C. 2003. Corn and soybean quality affected by late season drought. Iowa State University Integrated Crop Management. 4Lauer, J. 2006. Concerns about drought as corn pollination begins, Agronomy Advice, University of Wisconsin. Web sources verified 08/16/2018. 160629200038