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Recognizing the growth stage and management issues for each stage can help make decisions. High temperatures, moisture or nutrient deficiency, hail, insects, weeds, or diseases can interact to stress soybeans.
Bloom stages (R1 to R2). Flowering is initiated at the third to sixth node and progresses up and down the plant. By the R2 growth stage, soybeans have accumulated 25% of total dry weight and nutrients. Root growth, nutrient uptake, and nitrogen fixation are rapid during these stages. Six percent of yield potential can be at risk if 50% defoliation occurs during R2 may reduce yield potential by 6%.2 Multi-state research has shown that well timed insecticide applications for soybean aphid (SBA) are most beneficial during the R1 to R5 growth stage. The economic threshold for SBA is an average of greater than 250 aphids/plant, with 80% of the plants infested, and an increasing SBA population.3
Pod filling and seed development stages (R3 to R6). Temperature or moisture stress during the beginning pod growth stage (R3) can affect yield potential by reducing pod numbers, seeds/pod and seed size.2 Stress can increase abortion of flowers (60-75% is normal). Rapid pod growth and the beginning of seed development occur in the full pod stage (R4). R4 is the most critical stage for seed yield determination. Stress during the R4 to R6 growth stages will reduce yield potential more than stress at any other phase of development. Late pod formation (R4.5 to R5.5) is especially critical because flowering is complete and young pods and seeds are prone to abort if stress occurs.
Insecticide applications may be necessary during the R3 to R6 stages of soybean growth. If SBA populations are increasing and exceed 250 aphids/plant during R1 to R5 and insecticide application may be warranted and should be applied within 7 days.3 Bean leaf beetle (BLB) infestations can occur in July and August. Insecticide applications can be considered when defoliation exceeds 25% during pod fill and there are more than 50 beetles per row-foot.4 If pod feeding exceeds 10% of pods and more than 0.5 BLB/plant are present, consider an insecticide application during pod-fill.
Typically, a foliar fungicide application around the R3, beginning pod growth stage, can have the greatest impact on yield potential. Protecting the canopy during the R2 to R5 stages of growth can be beneficial. Plant demands for water and nutrients are high during the beginning seed stage (R5). Redistribution of nitrogen, phosphorous, and potassium, supplies about 50% of the plant’s needs. At the R5 stage the plant cannot compensate for stress. Complete defoliation can result in up to 80% loss of yield potential.2
There are no clearly defined thresholds for two-spotted spider mites in soybean fields. Spider mite damage usually starts in the lower canopy, during dry weather, and progresses to the middle and upper canopy. Heavy damage in the lower canopy and stippling in the middle canopy may require treatment.
During the R5 stage the plant attains maximum leaf area, peak nitrogen fixation and nutrient accumulation. The full seed growth stage (R6) is initiated when green seeds fill the pod at the top four nodes on the plant. Leaves may begin to yellow and fall from the lowest nodes of the plant.
Table 1. Effect of 4 days of visible moisture stress on soybean yield.Adapted from Wright, J. 2006. Predicting the last irrigation for corn and soybeans in central Minnesota. Minnesota Crop eNews.13;10;13;10;13;10;
Sources: 1 Casteel, S.N. 2011. Soybean early reproductive growth stages. Purdue University. 2 McWilliams, D.A.. 1999. Soybean growth and management quick guide A-1174. North Dakota State University. 3 Hodgson, E.W. 2012. Management recommendations for soybean aphids in the United States. Journal of Integrated Pest Management vol. 3. 4 Moechnig, M. 2013 South Dakota Crop Protection Guide. South Dakota State University. Web sources verified 7/30/2015. 170619130724