Soybean Seeding Rate vs. Preplant Herbicide Application

High soybean seeding rate has less impact on yield than preplant/preemergence herbicide application.

Preplant/preemergence herbicide application reduces weed development during the critical early growth period in soybeans.

Soybean canopy development helps suppress emergence and growth of late-germinating weeds. But do higher soybean seeding rates produce a larger, denser crop canopy that results in fewer weeds?

Not necessarily, according to a University of Wisconsin (PDF) study. Researchers compared soybean seeding rate (from 189,919 to 59,974 seeds per acre) and use of a preplant/preemergence residual herbicide application and the impact on weed growth, soybean canopy development and yield. Results from two seasons showed no significant difference between seeding rates in weed development (weed density, height and biomass) at the time of postemergence herbicide application.

In contrast, use of a preplant/preemergence herbicide significantly reduced weed development over the same critical early growth period. Densities of lambsquarters, ragweeds and annual grasses were decreased by 94, 89 and 91 percent, respectively, compared to a postemergence-only treatment.

Weed competition can begin to impact yields three weeks after soybean plants emerge, according to
South Dakota State University
research, so early weed control is important to maximize yields.

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