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.

Nomenclature: Redroot pigweed, Amaranthus retroflexus L. AMARE; snap bean, Phaseolus vulgaris L. ‘Matador’.