Evaluating Soybean Genetic Progress in Weed-Free and Weedy Conditions.

Previously we showed that soybean [Glycine max (L.) Merr.] genetic improvement rates increased as planting rates increased indicating that new cultivars were more tolerant to stresses associated with high plant density.  The current work was undertaken to evaluate genetic progress under stress associated with weedy conditions.  Twenty MG 0 to 00 cultivars released from 1934 to 2007 were grown at Ottawa, ON from 2006 to 2008 under weed free and naturally weedy conditions in a strip split trial.  Over all cultivars, yield loss due to weeds ranged from about 10 to 80% in different years.  Genetic improvement under weed free conditions ranged from 12.1 to 16.6 kg ha^-1 yr^-1, similar to previous estimates of genetic gain.  Under weedy conditions, estimates of genetic improvement decreased as weed pressure increased, ranging from 14.2 kg ha^-1 yr^-1 with 10 % yield loss to 1.2 kg ha^-1 yr^-1 with 80% yield loss.  New cultivars are not more tolerant to high weed stress.  Old cultivars are not more competitive with weeds compared to new cultivars as some organic producers hypothesize.