Soybeans Compensate at Low Seeding Rates but not at High Thinning Rates

Soybean [Glycine max (L.) Merr.] seed costs have more than tripled, so high seeding rates are expensive insurance against poor emergence associated with planting conditions or pest damage to established plant stands. Drilled soybeans (0.19 m spacing) were evaluated in New York (NY) in 2006 and 2007 at three seeding (358,000; 469,000; and 580,000 seeds ha-1) and four thinning rates (0, 10, 25, and 50% at the sixth node stage) to determine if yields can be maintained at lower seeding rates. Soybean plants responded linearly to seeding rates with 20% more grams of biomass, 0.9 more side branches, 21% more pods, and 21% more seeds per plant at 358,000 seeds ha-1 vs. the recommended 469,000 seeds ha-1 (21.1 g, 1.4 side branches, 30.6 pods, and 70.1 seeds per plant), resulting in similar biomass accumulation (555 g m-2) during seed development (R5 stage), pod density (817 vs. 805 m-2), and seed density (1883 vs. 1816 m-2, respectively). Seeds per pod (2.30) and seed mass (170 mg) did not vary and all seeding rates yielded similarly (3.1 Mg ha-1). Thinning rates resulted in linear reductions in biomass accumulation and pod density with no effect on seeds per pod or seed mass. Thinning reduced yields linearly (3.3 Mg ha-1 for no-thinning and 2.8 Mg ha-1 for 50% thinning). Seeding by thinning rate interactions were not observed so plant density reductions after stand establishment posed no greater risk at lower seeding rates. Growers who drill soybeans in the northeastern United States should consider lower seeding rates.