Estimating Soybean Genetic Gain for Yield in the Northern United States-Influence of Cropping History

Mean on-farm U. S. soybean [Glycine max (L.) Merr.] yield increased at a rate of 23.4 kg ha(-1) yr(-1) between 1924 and 2010 due to a combination of genetic improvements, agronomic technologies, and climatic changes. To estimate annual rates of genetic yield gain in three northern U. S. soybean maturity groups (MGs) and determine if these estimates are influenced by cropping history, 45 MG II, 40 MG III, and 45 MG IV cultivars released between 1923 and 2008 were evaluated in split-plot trials conducted in Illinois in 2010 where the main plot prior cropping treatments were either 11 yr of continuous corn (Zea mays L.) or 11 yr of a soybean-corn rotation. The experiment-wide genetic yield gain estimate was 22.8 kg ha(-1) yr(-1) and, after covariate adjustment of yields for maturity, the estimate was 19.8 kg ha(-1) yr(-1). These estimates show that soybean genetic yield potential has been a large contributor to the rate of on-farm yield improvement. The rate of yield gain estimates were not significantly different (P = 0.38) between the two cropping history treatments on an experiment-wide basis or at the MG III and MG IV individual locations but were significantly different at the MG II locations, where yield gain for soybean following continuous corn was significantly greater compared with the soybean-corn rotation treatment. Modern cultivars were not observed to be able to close the yield gap between the two cropping history treatments used in this experiment.