When are soybeans mature?
To achieve maximum yield, soybeans should have adequate water available through physiological maturity (PM). My definition of PM of a crop plant is that point where seed are at their maximum dry weight; i.e., translocation of photosynthate into the seed has completed.
For corn, PM is reached when the black layer has formed. This layer is easily recognizable. Also, the time to black layer formation in corn is predictable. PM in soybeans is not so easily defined or predicted.
The present determination of PM in soybeans is based on an ill-defined and ambiguous criterion of how seed appear in a pod. There is no clearcut indicator of this point that corresponds to the black layer in corn. This subjective determination of PM in soybeans creates a quandary, especially when a defined PM is needed to estimate when to terminate irrigation.
In my experience, R5 or beginning seed in soybeans is rather easily determined. Conversely, in my experience R6 or full seed and R7 or beginning maturity are not. This leads to the following thoughts.
- I know of no research that has established a seed component that indicates soybean seed maturity. Thus, a research project to determine whether or not there is such a chemical indicator of PM in soybeans would be justified.
- Researchers at the LSU AgCenter determined that a harvest aid applied at 40% average seed moisture content did not reduce seed yield and seed weight. This moisture content roughly coincides with stage R6.5, which they describe as “pod cavities have completely filled and all seed are separating from the white membrane inside the pod”. Thus, these research results indicate that soybean seed maturity apparently is reached at R6.5. But again, this stage is also subject to rater interpretation and thus may not be consistently determined.
- Using the above results, a better way to use PM as the guidepoint for soybean irrigation termination is to determine when this 40% average seed moisture content is reached in varieties from various soybean maturity groups-e.g. MG’s 3.5, 4.0, 4.5, 5.0, 5.5. Once this has been documented, the number of days from R5 to this point can be calculated.
- For example, if it is determined that the number of days from R5 to PM is 35 days, and it is known that a last irrigation applied to soybeans will provide adequate water for 10 days at the end of the season, then the last irrigation in this case can be applied 25 days after R5 to supply enough water to carry seed to PM. Using this method, there would be no need to be concerned about seed stages past R5.
With the increasing irrigation of soybeans and the concern about conserving the irrigation water supply in Mississippi, the above points are worthy of consideration to ensure that irrigation water is not wasted at the end of the soybean growing season.
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