Snap beans are a common processing vegetable whose yield and quality can be reduced when a few weeds emerge with or soon after the crop. The effect of redroot pigweed's emergence time and density on snap bean growth and yield was studied. Redroot pigweed, at four densities, was seeded with snap beans (early) or at the first trifoliate leaf stage (late). In 1998 the yield loss at 8 redroot pigweed plants m−1 row was 42 and 58%, whereas in 1999 it was 39 and 48% for late- and early-planted redroot pigweed, respectively. The effect of redroot pigweed density on snap bean yield loss was predicted with the hyperbolic yield equation. Coefficient A (percent yield loss as weed density approaches infinity), determined from the hyperbolic equation, varied from 47 to 63% and coefficient I (percent yield loss as weed density approaches zero) varied from 10 to 32% depending on the year and time of weed emergence, with the greater values for early-emerging redroot pigweed. Snap bean pod number and biomass were reduced as the density of early-emerging redroot pigweed increased. Regardless of the density, late-emerging redroot pigweed had less effect on snap bean growth and yield than did early-emerging redroot pigweed. The hyperbolic yield loss equation may be useful for growers to predict the effect of redroot pigweed in their fields on snap bean yields.

Field experiments were conducted to determine the effect of large crabgrass densities of 0.5 to 8 plants m−1 of row and emergence time on snap bean yield. Large crabgrass was planted either along with snap beans (early) or when the first trifoliate leaf of snap beans was opening (late). Observed yield loss ranged from 46 to 50%, and predicted yield loss ranged from 53 ± 29.3% to 63 ± 18.3%. Relative leaf area was correlated to snap bean yield (r 2 = 0.88 to 0.92). The relative damage coefficient (q), an indication of the competitiveness of large crabgrass with snap bean, was 1.65 ± 1.03 and 1.26 ± 0.72 for early- and late-emerging large crabgrass, respectively. Early-emerging large crabgrass reduced snap bean biomass 10 to 28% and snap bean pod numbers 44 to 60%, depending on the density. Because of intraspecies competition, leaf area index and number of seed for large crabgrass were reduced with increasing density. Emergence of > 2 plants m−1 of large crabgrass with snap beans should be controlled to avoid significant yield loss.