FIELD-EVALUATION OF SOYBEAN (GLYCINE-MAX) GENOTYPES FOR WEED COMPETITIVENESS

In the first of 2 field studies, weed biomass and soybean seed yield w ere used to evaluate 16 soybean genotypes for competitive ability agai nst 12 weed species at Rosemount, MN, in 1992 and 1993. The yield and ranking of soybean genotypes often varied with the weed species. Grass weed species reduced yields the most, and small-seeded broadleaf weed s reduced yields the least across years. 'Parker' was highly competiti ve, as it suppressed weed biomass and produced high soybean yield. 'Ka to,' 'Kasota,' 'Dawson,' and 'Glenwood' minimized weed biomass and mai ntained soybean yield while in competition with grass weeds but yielde d poorly relative to other soybean genotypes in weed-free conditions. 'Lambert' produced high soybean yield in weed-free conditions, but yie ld dropped markedly when in competition with grass weeds. 'Grande,' 'H eifeng 25,' and 'Norman' soybeans were poor competitive genotypes in w eedy situations and low yielding in weed-free conditions. A 2nd field study conducted at Rosemount and St. Paul, MN, during 1993 evaluated 1 6 soybean genotypes under 4 levels and durations of weed pressure for weed competitiveness. Parker, 'Sturdy,' and M89-794 were most competit ive in suppressing weed biomass and producing high yields. Lambert yie lded fairly well but allowed high weed biomass. M89-1743, M89-1006, 'A rcher,' and 'Ozzie' yielded poorly and did not suppress weed biomass p roduction. No relationship was found between weed competitiveness and soybean canopy area, height, and volume measured 30-45 d after plantin g (DAP).