Field studies were conducted in 1997 and 1998 at Manhattan and Topeka, KS, to examine the competitive effects of redroot pigweed, Palmer amaranth, and common waterhemp on soybean yield. The experiments were established as a randomized complete block design in a factorial arrangement of three pigweed species, two pigweed planting dates (soybean planting and cotyledon stage), and seven weed densities (0.25, 0.5, 1, 2, 4, and 8 plants m⁻¹ of row, plus a weed-free control). The effect of weed density on soybean yield loss, pigweed biomass, and pigweed seed production were described using a rectangular hyperbola model. Soybean yield loss varied between locations depending on the weed species, density, and time of emergence. Yield loss increased with weed density for each species and location with the first pigweed emergence time. The maximum soybean yield loss occurred at the first planting and 8 plants m⁻¹ of row density, and was 78.7, 56.2, and 38.0% as determined by the model for Palmer amaranth, common waterhemp, and redroot pigweed, respectively. The second planting of pigweed did not significantly reduce soybean yield. The relative ranking of the pigweed species biomass was Palmer amaranth > common waterhemp > redroot pigweed. Maximum seed production for Palmer amaranth, common waterhemp, and redroot pigweed was 32,300, 51,800, and 9,500 seeds m⁻². Palmer amaranth produced a larger quantity of seed than did common waterhemp or redroot pigweed at low weed densities (0.25 to 4 plants m⁻¹ of row). But common waterhemp seed production equaled or surpassed Palmer amaranth at high weed densities.

Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; common waterhemp, Amaranthus rudis Sauer AMATA; redroot pigweed, Amaranthus retroflexus L. AMARE; soybean, Glycine max (L.) Merr.