Growth Stages R4-R6: Protect Soybeans from Stress

Published on 7 Aug 2017, 08:00 AM • by: Kevin Nelson, North Central CCA Soy Envoy • 724 Views

As we head into pod fill, stress impacts pod survival, development and YIELD.

August is the month that puts the finishing touches on the soybean crop. In soybeans the R4 to R6 growth stages are when the plant starts really accumulating dry matter in pods and seeds, producing the oil and protein that are the soybean plant’s ultimate cash product. Stress during the R4-R6 period can cause more reduction in yield potential than at any other growth stage. Stress can happen in the form of competition from weeds or pressure from insects, diseases and nematodes, or from environmental factors such as heat stress or drought. While soybeans seem to tolerate short periods of stress better than some crops, any extended stress during these stages impacts the ability of soybean to reach full yield potential. 

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The full pod stage, R4, begins with one of the four uppermost nodes on the main stem having a pod that is ¾ inch long. Pod growth is rapid and the final survival pod number is being determined. Pod dry weight increases from R4 into R5 as seed development begins. If plants are under prolonged stress, they move nutrients from other areas to the pods to compensate. This can open leaves and roots to invasion by diseases. It takes an average of 9 days to move through R4 to R5.

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The beginning seed stage, R5, occurs when there is a seed 1/8-inch long in a pod at one of the four uppermost nodes on the main stem. Seed filling requires water and nutrients. The plant redistributes nutrients to provide about half of the nitrogen, potassium and phosphorus needs of the grain. The rest comes directly from the roots.

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The presence, or lack, of August rains has a huge impact on soybean yield during this stage. The plant reaches its maximum height, number of nodes and leaf area. Nitrogen fixation peaks and begins to decline. Seeds begin to accumulate dry weight. The R5 Growth Stage takes an average of 15 days to advance to R6.

The full seed stage, R6, begins when there is a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem. Dry weight accumulation in seeds is now very rapid. About halfway through R6, about 80% of the total seed dry weight has been accumulated. During the late R6 stage, three to six trifoliates may drop from the lower nodes of the plant. It normally takes about 20 days to move through R6 to R7.

Even before we approach these core stages of yield production in soybeans, we need to consider all the ways stress can impact the plant and do whatever we can to mitigate those stress factors before they can happen.

Some of these practices include irrigation, insecticide and/or fungicide treatments at the R3 stage to control stress-causing insects or diseases now so they don’t affect the crop through the final stages. Some growers have been trying R3 nitrogen, potassium or micronutrient applications to reduce stress and keep the plant from moving as much of these nutrients from leaves, stems and roots to the seed. Foliar micronutrients, biostimulants or other biologicals have been touted as stress relievers and yield enhancers.

Whatever practice you want to try, remember that the goal is to protect the soybean plant from experiencing stress, especially during the R4-R6 growth stages.

CCA Kevin Nelson is a 4R NMS at Northern Partners Cooperative. Kevin works with the Agronomy Sales Team and growers in North Central Illinois. He is a 2017 Illinois Soybean Association CCA Soy Envoy.

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