Soybean improvement via plant breeding has been critical for the success of the crop. The objective of this study was to quantify genetic change in yield and other traits that occurred during the past 80 yr of North American soybean breeding in Maturity Groups (MGs) II, III, and IV. Historic sets of 60 MG II, 59 MG III, and 49 MG IV soybean cultivars, released from 1923 to 2008, were evaluated in field trials conducted in 17 U.S. states and one Canadian province during 2010 to 2011. Averaged over 27 MG II and MG IV and 26 MG III site-years of data, the estimated rates of yield improvement during the 80 yr were 23 kg ha -1 yr -1 for MGs II and III, and 20 kg ha -1 yr -1 for MG IV cultivars. However, a two-segment linear regression model provided a better fit to the data and indicated that the average current rate of genetic yield gain across MGs is 29 kg ha -1 yr -1 . Modern cultivars yielded more than old cultivars in all environments, but particularly in high-yielding environments. New cultivars in the historic sets used in this study are shorter in height, mature later, lodge less, and have seeds with less protein and greater oil concentration. Given that on-farm soybean yields in the United States are also increasing at a rate of 29 kg ha -1 yr -1 , it can be inferred that continual release of greater-yielding cultivars has been a substantive driver of the U.S. onfarm realized yield increases.

SummaryLinear increases in light interception, energy conversion, and partitioning efficiencies have driven past yield gains in soybean. In modern cultivars, canopy light interception and harvest index are reaching theoretical maximum values.

Soybean is the world’s leading source of vegetable protein and demand for its seed continues to grow. Breeders have successfully increased soybean yield, but the genetic architecture of yield and key agronomic traits is poorly understood. We developed a 40-mating soybean nested association mapping (NAM) population of 5,600 inbred lines that were characterized by single nucleotide polymorphism (SNP) markers and six agronomic traits in field trials in 22 environments. Analysis of the yield, agronomic, and SNP data revealed 23 significant marker-trait associations for yield, 19 for maturity, 15 for plant height, 17 for plant lodging, and 29 for seed mass. A higher frequency of estimated positive yield alleles was evident from elite founder parents than from exotic founders, although unique desirable alleles from the exotic group were identified, demonstrating the value of expanding the genetic base of US soybean breeding.

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.

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