Soybean

Soybean [Glycine max (L.) Merr.] yield improvement arises from continual producer adoption of genetic technology-the annual releases of new cultivars that express ever-greater genetic yield potential, and agronomic technology-the periodic transformations in crop and soil management practices that enhance environmental yield potential. The contribution of both factors, and their interaction, to soybean yield is assessed in this chapter. In the United States, the rate of on-farm yield improvement from 1924 to 2012 is 23.3 kg ha^-1 yr^-1, whereas the Brazilian rate from 1977 to 2013 is 43.5 kg ha^-1 yr^-1. When a segmental regression model was applied, the post-1983 U.S. rate increased 25% to 29.4 kg ha^-1 yr^-1. The genetic rates of yield improvement in the United States for Maturity Group (MG) II, III, and IV cultivars, estimated by growing cultivars from different eras in common environments, were 23.1, 22.8, and 19.5 kg ha^-1 yr^-1. In the states in which these MG cultivars are adapted, the on-farm rates are 27.3 (MG II & III) and 21.4 (MG IV) kg ha^-1 yr^-1. Statistical modeling indicates that the more recent genetic improvement rates for the three MGs are 31.1, 29.4, and 26.5 kg ha^-1 yr^-1. Comparison of on-farm yield improvement and genetic yield improvement in high-yield irrigated production environments suggests that about two-thirds of the on-farm yield improvement is likely arising from the contribution of continual release of ever-higher yielding soybean cultivars that are quickly adopted by soybean producers. Periodic advances in agronomic practices adopted by producers likely account for the other one-third, but recent publications have documented another contributor-synergistic genetic x agronomic interactions, wherein the difference between the yields of modern and obsolete cultivars widens when the agronomic productivity is enhanced by better crop management practices (e.g., earlier planting date). The impact of rising atmospheric CO₂ levels on soybean yield was judged to be small, but it will be cumulative over time.