Tuesday, September 13, 2011

Late-season drought brings soybean stress

The dry soil conditions experienced recently over much of Minnesota may limit soybean yields, according to Dave Nicolai, crops educator with University of Minnesota Extension.

"Some plants are ripening prematurely. Another symptom of moisture loss is when the soybean leaves flip over in an attempt to slow water loss thru the plant," said Nicolai. Reduced rainfall in the past four weeks in central Minnesota has resulted in less available soil water for plant growth.

Although nothing can be done about the amount of rainfall received, and little can be done to manage drought stress at this point, producers can analyze their practices to see if they may be able to minimize drought stress in future seasons. 

The soybean plant might use two strategies to cope with drought during seed filling: reduce (abort) the number of seeds, or reduce the size of the seed. Both seed abortion and reduction in seed size represent direct and irreversible reductions in yield potential. For example, soybean pods which may have included 3 seeds in a pod may now contain just 2 seeds. A reduction in seed size may also lead to harvest losses for growers if combines are not properly calibrated to harvest smaller seeds, according to Nicolai.

Extension educators throughout the state are noticing that drought-related symptoms on soybean fields are spotty, which is not unusual. Several factors contribute to the irregular nature of the drought-stress symptoms. The plants are not responding directly to the lack of rain, but to limited soil moisture. Soil works like a sponge, quickly soaking in water then slowly losing water through evaporation or plant transpiration. Differences in topography, soil type, soil depth, surface cover, the presence of drainage tile and soil compaction zones are some of the variables that determine how much water will be available to plant roots in a particular area of a field. 

Drought-stress symptoms can be related to management issues. "Small nutrient deficiencies might go unnoticed during a normal year, but become evident when plants are drought-stressed," said Nicolai.

Soybean producers who are seeing considerable symptoms in their fields may want to review their practices related to fertility, insect pressure, disease, weed competition, soil compaction, planting date and plant density.

  For more educational information and tools, visit www.soybeans.umn.edu, a cooperative effort among the University of Minnesota, University of Minnesota Extension, and the Minnesota Soybean Research and Promotion Council. - Dave Nicolai and Liz Stahl, crops educators with University of Minnesota Extension.

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