Cover crops are becoming increasingly common in cotton as a result of glyphosate-resistant Palmer amaranth; hence, a field experiment was conducted in 2009 and 2010 in Marianna, AR, with a rye cover crop used to determine its effects on the critical period for weed control in cotton. Throughout most of the growing season, weed biomass in the presence of a rye cover crop was lesser than that in the absence of a rye cover crop. In 2009, in weeks 2 through 7 after planting, weed biomass was reduced at least twofold in the presence of a rye cover compared with the absence of rye. In 2009, in both presence and absence of a rye cover crop, weed removal needed to begin before weed biomass was 150 g m⁻², or approximately 4 wk after planting, to prevent yield loss > 5%. Weed density was less in 2010 than in 2009, so weed removal was not required until 7 wk after planting, at which point weed biomass values were 175 and 385 g m⁻² in the presence and absence of a cover crop, respectively.

Nomenclature: Glyphosate, N-(phosphonomethyl)glycine; Palmer amaranth, Amaranthus palmeri (S.) Wats. AMAPA; cereal rye, Secale cereale L. ‘Wrens Abruzzi’; cotton, Gossypium hirsutum L. ‘Stoneville 4554 B2RF’.