Soybeans reaching critical point for moisture

By Matt Reese

With hope gone for high corn yields in many parts of the state, attention has shifted to the needs of the soybean crop as it enters the time of the growing season when moisture is most needed. Soybeans have pushed through the tough conditions in many fields but will still need some rain to perform in 2012.

“With most of the state experiencing at least moderate drought conditions and high temperatures, soybeans are exhibiting

Dry soybeans have struggled all season long, but are now getting to a critical need for moisture.

symptoms of water stress. A visual indication of soybean water stress includes flipped leaves,” said Laura Lindsey, the new Ohio State University Extension soybean specialist. “The flipped leaves expose a silver-green underside which reflects light. In more severe cases, the outer leaves of the trifoliate will close together to reduce the leaf area exposed to sunlight and reduce water loss. Water-stressed soybeans will grow slower and have smaller leaves compared to soybeans growing with adequate soil moisture.”

The yield potential for soybeans is influenced by the number of pods per plant, the number of soybeans per pod and the seed size.

“Stress conditions during soybean reproduction can reduce yield by affecting one or more of these yield components,” Lindsey wrote in a recent CORN Newsletter. “Vegetative growth, flowering, pod development, and seed filling stages overlap allowing the plant to compensate for short periods of stress.”

Many soybean fields around the state started flowering in early July. With dry weather following this stage, yields can suffer.

“In a normal year, 60% to 75% of soybean flowers will abort, but this number can increase in a stressful year,” she said. “However, flowering can occur through the beginning of the R5 growth stage (beginning seed). If water stress is alleviated prior to the R5 growth stage, some flowers and pods can still be produced compensating for the flowers that were aborted earlier. If water stress persists, soybean yield will be reduced, especially as plants enter the R4 growth stage (full pod). Yield reductions at this time result mainly from reductions in total pod number per plant. At the R6 growth stage, water stress will cause a reduction in seed size.”

To make strong yields for soybeans even more challenging, a number of pests and diseases are also at work in the fields. Ohio State University Extension entomologist Anne Dorrance encourages farmers to scout for signs of soybean cyst nematode (with symptoms similar to nutrient deficiencies), Phytophthora stem rot (with chocolate brown cankers on the stem), white mold and frogeye leaf spot (circular spots on leaves).

With the dry conditions, two-spotted spider mites are also showing up. If spraying for spider mites, entomologists urge caution to protect wild bees.

“With drought conditions, the likelihood of bees coming to soybeans to forage is higher than usual,” wrote Ron Hammond and Any Michel in a recent CORN Newsletter. “According to Ohio law, if a pesticide is toxic to bees, it is the applicator’s responsibility to contact the beekeepers with registered apiaries within a half mile of the target area if it is more than a half acre in size and the crop is in flower. Contact the Ohio Department of Agriculture for more information on apiaries in your area. If at all possible, applications should be made early or late in the day when bees are less likely to be out foraging.”

In addition, Japanese beetles are out early and in high numbers this year and growers need to watch closely for the 15% to 20% defoliation threshold in soybeans.

“Adult Japanese beetles and the first generation of adult bean leaf beetles are occurring at least two weeks earlier than normal. And because of the dry weather in much of the state, soybeans are not growing well and are much smaller in size with low levels of leaf area,” wrote the entomologists. “Although we still believe it best to keep the defoliation thresholds at the current levels, growers might want to be conservative with these thresholds and make a decision to treat sooner than they would in a ‘normal’ year with adequate moisture and crop growth.”

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