I have received this question a couple of times this week from growers and retailers with indeterminate soybeans. According to the soybean weed control section of the 2022 UGA Pest Management Handbook, paraquat can be applied to soybeans based on the following instructions. The paraquat rate according to this reference is 0.25 lb ai per acre which is 16 oz of 2 lb formulation.
Indeterminate Varieties: Apply when at least 65% of the seed pods have reached a mature brown color or when seed moisture is 30% or less.
Determinant Varieties: Apply when soybeans are fully mature (full pod development, 50% leaf drop, and the remaining leaves are yellow).
Mature cocklebur and drought stressed weeds are not adequately controlled by this treatment. Do not apply to immature soybeans. Add a nonionic surfactant at 0.25% v/v. May be ground or aerially applied. Tank-mix with Aim if annual morningglories are also a problem (except smallflower).
Other references are avaliable on the is subject
Critical timing for soybean desiccation by Dr. Reagan Nolan.
Dr. Prostko informs us that ”Early applications (>40% seed moisture) will likely result in significant soybean yield loss as a result of reduced seed weights (Figure 1). The 40% seed moisture content roughly coincides with the R6.5 to R7 stages of growth. R6.5 = Full Seed = all normal pods on 4 uppermost nodes have pod cavities filled. R7 = Beginning Maturity = one normal pod on main stem of all plants has reached mature color (Figure 2). “
If you are not familiar with soybean growth stages.
If you have any questions please contact your local county Extension agent.