Soybean Crop Update and Management Considerations

Trent Irby, Extension Soybean Specialist
By Trent Irby, Extension Soybean Specialist June 19, 2012 18:13

We are all thankful for the recent rains that were received throughout the majority of the state. These rains were certainly welcomed, especially in those areas that have remained dry for the last several weeks. Many places received adequate rainfall, with reports of up to 5 inches in portions of the state. However, some areas received less than ideal amounts of rainfall from these showers. Additional moisture will soon be needed.

According to the National Agricultural Statistics Service in Mississippi, the state’s soybean crop is 100% planted, 99% emerged, and 41% blooming as of the week ending June 17, 2012. The recent rains supplied enough moisture to allow some additional acreage of double crop soybean to be planted where wheat was harvested. Currently, disease and insect pressure is low. Overall, Mississippi’s soybean crop continues to look good.

At this point of the growing season, soybean management strategies that should be considered include preventative fungicide applications and irrigation. Determining soybean growth stage is critical when planning for fungicide application timing and irrigation scheduling. The commonly recognized soybean reproductive growth stages are as follows:

R1: First flower anywhere on plant R5.5: Soybeans filling half the space in the pod in upper four nodes
R2: Flower in the upper two nodes R6: Soybeans touching inside the pod in upper four nodes
R3: 3/16” long pod in upper four nodes R6.5: Pod and pod wall beginning to turn mature color
R3.5: 1/2” long pod in upper four nodes R7: Pod mature in color anywhere on plant
R4: 3/4” long pod in upper four nodes R8: 50% of the pods contain mature seed and are mature in color
R5: Visible seed in pod in upper four nodes    

The above table lists the identifying characteristics which can help you determine the reproductive growth stage of your crop. Keep in mind that indeterminate soybean varieties (most of our MG IV varieties) begin reproductive growth at the bottom of the plant and progress upward as the plant produces more nodes, while determinate varieties (most of our MG V varieties) begin reproductive growth uniformly up and down the plant. In other words, indeterminate soybean varieties may have immature pods in the lower portion of the plant with new flowers in the upper nodes. Determinate varieties will have the same size flowers/pods uniformly throughout the plant.

With respect to preventative fungicide applications, I have received numerous calls regarding product selection (strobilurin vs. triazole vs. pre- or tank-mix) and application timing. A fungicide application at the R3/R4 stage with a strobilurin has become almost standard as a management practice. Keep in mind that the term “preventative” is used in this situation because the target application timing revolves around the fact that no disease is present. These applications are made in order to minimize the effects that disease may have on the crop as it reproduces. Situations where a preventative fungicide application can be beneficial include continuous soybean, irrigated, and high yielding environments. Dr. Tom Allen recently submitted an article containing excellent information for consideration when planning an R3/R4 fungicide application. This article can be found at http://www.mississippi-crops.com/2012/06/15/preventive-soybean-fungicide-applications-products-rate-selection-timing/.

In terms of irrigation scheduling, there always seems to be some debate on initiation and termination. An important point to remember is that soybean can utilize a minimum of 0.25 inch of water per day during reproductive development. Consider the soil texture within your fields. A course textured soil will not retain as much plant available water as will a medium or fine textured soil. Generally speaking, it is a good idea to ensure that the soybean crop does not suffer from lack of water from R1 to R6. While our recent rains have provided moisture needs to the soybean crop for a few more days, it is likely that without additional rainfall in the near future, soybean irrigation will need to resume.

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Trent Irby, Extension Soybean Specialist
By Trent Irby, Extension Soybean Specialist June 19, 2012 18:13
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