Genetic Improvement Rates of Short‐Season Soybean Increase with Plant Population

Studies with a series of historic soybean [ Glycine max (L.) Merr.] cultivars have found genetic improvement rates for seed yield of about 0.5% per year since soybean was first cultivated in Canada. Work with maize ( Zea mays L.) has shown seed yield differences between old and new hybrids are more pronounced under higher seeding rates. The objective of our research was to determine the effect of plant populations on the rate of genetic improvement of short‐season soybeans. Soybean cultivars released from1934 to 1996 were grown in 40‐cm‐wide rows at several seeding rates (Exp. 1, 42 cultivars grown at 25, 50, 75 seeds m −2 in 1998 and 1999; and Exp. 2, seven cultivars grown at 25, 50, 100, 150, 200 seeds m −2 in 1999 and 2000) in two fields for 2 yr at Ottawa, ON. Genetic improvement rates were based on cultivar year of release and were compared across plant populations. In Exp. 1, seed yields did not reach a plateau. In Exp. 2, a seed yield plateau was reached and genetic improvement rates generally increased with plant population. Maximum rates of genetic improvement occurred at plant populations three to four times greater than current commercial practice (40 plants m −2 ). A plateau for seed yield was reached at lower plant populations compared to plateaus for genetic improvement rates which were reached at higher plant populations (>80 plants m −2 ). Higher plant populations also resulted in earlier maturity, increased plant height, and increased lodging. Seed protein increased and seed oil decreased with higher plant populations. New soybean cultivars appear more tolerant to plant population stress than older cultivars.