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Effect of Duration and Type of Natural Weed Infestations on Soybean Yield¹

The time of weed removal from a crop may be as important as the extent of removal. A 4-yr field study was conducted at two locations each year to define the extent of soybean [Glycine max (L.) Merr.] yield and population reduction occurring from the presence of natural weed populations. Soil types were Typic Ochraqualfs and Typic Albaqualfs. ‘Williams 79’ soybeans were planted at approximately 30 seeds m^{– 1} of row using 76-cm spacing between rows. Grass, broadleaf, or grass plus broadleaf weeds were allowed to interfere 0, 1, 2, 3, or 4 weeks, or all season (control), prior to their removal. Weed populations in the control plots ranged from 16 to 148 plants m^–2 in 1981 and increased to 104 to 1172 plants m^–2 in 1984. Allowing grass, broadleaf, or grass plus broadleaf weeds to interfere with soybeans for up to 4 weeks did not reduce soybean yields under ample soil moisture conditions. However, under drought conditions or extremely high weed infestations, yield losses with 4 weeks of interference were observed. No differences were found between the competitive effects of grasses or broadleaves on soybean yield with 4 weeks or less of interference. Significant yield reduction occurred every year at both locations when grass plus broadleaf weeds interfered all season. Yield reductions from all-season grass-only or broadleaf-only weed interference occurred most years, but were dependent on location, weed density, and environmental conditions. Soybean populations were reduced where weeds were most dense and were associated with yield decreases.

Key Words: Interference • Grass weeds • Broadleaf weeds • Glycine max (L.) Merr. • Soybean population

² Former research associate (currently project leader, Herbicide Research, Dow Chemical U.S.A., Walnut Creek, CA 94598-0902), professor, and former graduate research assistant, respectively, Plant and Soil Sci. Dep., Southern Illinois Univ., Carbondale, IL 62901.

Received for publication November 19, 1984.