Index selection for weed suppressive ability in soybean

The economic and environmental costs of weed management in soybean (Glycine max [L.] Merr.) have led to interest in developing weed suppressive soybean varieties to enhance traditional herbicide and tillage-based approaches. We evaluated 104 inbred progeny from three crosses among elite soybean lines to determine optimal selection criteria for weed suppressive ability (WSA). We grew the lines in 1996 and 1997 at Becker, MN, an irrigated sandy site, and Rosemount, MN, a rainfed silt loam site, in a split-plot, with and without white mustard (Brassica hirta Moeach). We measured soybean height 7 mk after emergence (WAE), light interception 5 and 7 WAE, specific leaf area 7 WAE, and date of full bloom. We harvested aboveground mustard biomass 8 WAE and calculated each soybean line's WSA as the difference between mustard biomass when grown in competition with that line and the overall mean mustard biomass. We estimated genetic correlations between soybean morphological traits, WSA, and the agronomic traits lodging, maturity date, and yield. Soybean early height's heritability (h(2) = 0.64) and genetic correlation with WSA (r = 0.81) made it an ideal selection criterion, Indirect selection on height increased predicted selection efficiency by 70% relative to direct selection on mustard dry weight. Restricted index selection combining information on early height and lodging or yield eliminated undesirable correlated responses of lodging and geld while maintaining genetic gain for early height and WSA. Nevertheless, continuing rapid gains in agronomic performance while incorporating WSA may be difficult.