Fungicide Management Does Not Affect the Rate of Genetic Gain in Soybean

On-farm U.S. soybean [Glycine max (L.) Merr.] yields have increased at an annual rate of 23.3 kg ha(-1) yr(-1) since the 1920s. These gains have come from a variety of sources including genetic, agronomic, and environmental changes. Genetic gains arising from breeding efforts have likely contributed the most to the U.S. soybean yield increase; however, the relative contribution of each source of gain is difficult to estimate. The objectives of this study were to compare yield of soybean varieties with different year of release, understand the effects of fungicide applications on soybean seed yield, and evaluate the composition of soybean cultivars chosen to represent historically significant releases in maturity groups (MGs) II and III released during the last 85 yr. A set of 116 cultivars in these two MGs, released from 1923 to 2008, received a fungicide seed treatment followed by foliar applications at R1, R3, and R5 and were compared to non-treated controls. Seed composition changed over time with protein concentration decreasing 2.1 g kg(-1) for every g kg(-1) increase in oil concentration. The significant interaction between fungicide treatment and MG III cultivar release year for plant stand revealed that such treatments were more beneficial with respect to obsolete cultivars of MG III, though this plant stand interaction did not translate into a significant yield interaction. The rate of genetic yield improvement made by breeders was not influenced by fungicide management and matched the observed rate of on-farm yield improvement that occurred during the same period.