Soybean yield is derived from plants per acre, pods per plant, seeds per pod, and weight of seeds. Of these components, plants per acre is the one most controlled by the producer through seeding rate.

Assuming a reasonable or acceptable plant population density that will result from planting high quality seed at a pre-selected rate into usually ideal conditions for emergence, this leaves pods per plant, seeds per pod, and weight of individual seeds as the yield components controlled by the plant and its environment.

These latter three soybean yield components are sometimes lumped into two basic ones-seeds per acre (combines pods per plant and seeds per pod) and seed size, which is usually expressed as seeds per pound.

The goal of a soybean producer is to maximize the number of seeds per acre and weight of individual seeds to realize maximum yield potential of a particular variety. The controlling factor in this effort is usually water, assuming proper soil fertility and control of pests and weeds. Lack of optimum water at pod set and during seed fill will result in decreased yields because of fewer and smaller seed.

A producer’s capability to irrigate should ensure that the maximum number of pods are set, that the maximum number of seed per pod develop, and that the seed that are developed will attain their maximum size for a given variety. Thus, the critical decisions involved in irrigation management for soybeans are when to start and when to stop in order to achieve the maximum number of seed and the maximum weight of individual seeds.

The decision of when to start irrigating soybeans can be easily determined, especially since early-season rainfall in the Midsouth is usually adequate to maintain necessary soil moisture.

So, irrigating early- and normal-planted soybeans is rarely if ever necessary before they start blooming, which is R1. In fact, for early-planted soybeans, there is ample evidence that starting irrigation before R3 or beginning podset is not required when rainfall during vegetative development is normal. Also, both R1 and R3 are easily determined.

Remember, the critical yield components of soybeans are number of seeds per acre and weight of individual seeds. Thus, it is my opinion that the hardest decision is when to stop or terminate soybean irrigation so that the high seed load established by early irrigations will attain maximum weight per seed. The importance of attaining maximum seed size to achieve maximum soybean yield is presented here.

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.