New and old soybean cultivar responses to plant density and intercepted light
Genetic gain for new soybean [Glycine max (L.) Merr.] cultivars is substantial, and research suggests this is partially explained by greater tolerance to increased plant densities. We conducted an experiment to determine if greater tolerance to increased plant density and more efficient use of cumulative intercepted photosynthetically active radiation (CIPAR) partially explained the yield difference of two old soybean cultivars and two new cultivars. Soybean cultivars were grown in 38-cm spaced rows at final plant densities of 4.9, 14.8, 24.7, 34.6, and 44.5 plants m–2 for 2 yr in Ames, IA. New cultivars averaged 914 kg ha–1 and 4.9 g plant–1 greater yield compared with old cultivars averaged across years and plant densities. Yield plateaus occurred at plant densities of 23.0 plants m–2 for new cultivars and 19.9 plants m–2 for old cultivars. Each cultivar group showed a similar yield decline per plant to increased plant density. New and old cultivars showed a similar response to increasing values of CIPAR, attaining a 95% relative yield plateau at 466 MJ m–2. Soybean yield response to increased plant density and CIPAR has not changed over time, and genetic gain from new cultivars is associated with more efficient use of light.