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WEED MANAGEMENT

Parameters for Predicting Giant Foxtail Cohort Effect on Soybean Yield Loss

^a Dep. of Agronomy, Univ. of Missouri, 210 Waters Hall, Columbia, MO 65211
^b Dep. of Horticulture, Univ. of Wisconsin, Madison, WI 53706
^c Dep. of Agronomy, Univ. of Wisconsin, 1575 Linden Drive, Madison, WI 53706

^* Corresponding author (conleysp{at}missouri.edu).

Received for publication May 25, 2002. Quantifying the effect of cohort emergence time on weed–crop interference is critical for development of accurate crop yield loss models. Therefore, research was conducted to assess late-season weed shoot biomass, weed density and cohort emergence time, weed relative leaf area, and weed relative volume as predictors of soybean [Glycine max (L.) Merr.] yield loss. Field experiments were conducted in 1999 and 2000 at the University of Wisconsin Arlington Agricultural Research Station near Arlington, WI. The main-plot effect was giant foxtail (Setaria spp.) cohort emergence time at VE, VC, V1, and V3 soybean. The subplot effect was giant foxtail density at 0, 4, 16, 36, and 64 plants m^-2. Linear and nonlinear regression equations were used to describe soybean yield loss based on measurements of giant foxtail shoot biomass, volume, leaf area, and density and cohort emergence time. Based on weed density and cohort emergence time, coefficient estimates used to describe soybean yield loss were similar between years. Based on weed relative leaf area, coefficient estimates for the giant foxtail damage coefficient (q_L) and maximum soybean yield loss (m_L) differed between years. Based on weed relative volume, coefficient estimates for the giant foxtail damage coefficient (q_V) and maximum soybean yield loss (m_V) were similar between years. Soybean yield loss increased linearly as giant foxtail shoot biomass increased. The stability of the density and cohort emergence time coefficients and the weed relative volume coefficients indicated these parameters may prove more useful that weed relative leaf area for predicting soybean yield loss.