Predicting when Michigan soybean fields will reach physiological maturity

Michael Staton<staton@msu.edu>, Michigan State University Extension - September 19, 2019

A new tool called SoyWater can help predict when soybean fields will reach physiological maturity, or the R7 growth stage.

Soybean plant at R7 growth stage. Photo by Mike Staton, MSU Extension.

Due to delayed planting problems and cooler than normal temperatures occurring this growing season, many producers are wondering if their late-planted soybean fields will reach physiological maturity before a killing frost. Physiological maturity (the R7 growth stage) is reached when one pod has attained its mature color on 50% of the plants in the field. In most cases, this occurs after nearly all the leaves turned yellow and are dropping off the plants (see photo). After the plants reach the R7 growth stage, the field will no longer be at risk of incurring yield losses due to frost or freeze damage.

Thanks to the combined efforts of Michigan State University Extension, the University of Nebraska at Lincoln and the Michigan Soybean Checkoff, Michigan soybean producers have access to a new tool called SoyWater that can be used to help predict when their fields will reach R7. SoyWater is a web-based irrigation scheduling program that has a soybean growth stage development model called SoySim embedded within it. SoyWater was developed at the University of Nebraska and has been adapted to incorporate weather data from the MSU Enviroweather stations. The program is free of charge.

SoyWater was used to determine the projected dates when soybeans planted on June 30 will reach physiological maturity in Lenawee, Gratiot and Sanilac counties (see table). These counties were selected as they are major soybean producing counties that experienced some of the worst planting delays in 2019. To demonstrate the effect that maturity group has on soybean maturity, the latest and the earliest adapted maturity groups for each location were entered. Based on the SoyWater projection, a 3.2 maturity group soybean variety planted in Lenawee County on June 30 is predicted to reach R7 on Oct. 19. However, historical weather records show there is a 50% chance that a killing frost of 28 degrees Fahrenheit will occur on Oct. 18. Remember that the R7 dates provided by SoyWater are estimates and may not precisely match the actual R7 dates in your fields.

Projected dates when soybeans planted on June 30, 2019, will reach physiological maturity in Lenawee, Gratiot and Sanilac counties.
County	Maturity group	*Projected date to reach physiological maturity (R7)	**Earliest projected harvest date
Lenawee	3.2	10-19	11-03
Lenawee	2.2	10-06	10-21
Gratiot	2.6	10-15	10-30
Gratiot	1.6	09-29	10-14
Sanilac	2.4	10-15	10-30
Sanilac	1.4	09-28	10-13

*Physiological maturity was predicted using the SoyWater irrigation scheduling program from the University of Nebraska at Lincoln.
** Earliest projected harvest dates were estimated by adding 15 days on the R7 dates predicted by SoyWater.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).