Effect of Annual Grass Cohort Age and Clipping on Herbicide Efficacy

Bermudagrass is grown extensively in the southern United States as a livestock forage. Annual grass weeds can be problematic in bermudagrass hay production, few effective control measures are available. Bermudagrass hay is harvested approximately every 28 d. Because most annual grass weeds germinate whenever environmental conditions become appropriate, control options must address a target weed population that varies in age, and may or may not have been mowed during previous harvests. Cohort age has rarely been included in control studies that combine herbicides and mowing. We tested this idea in a greenhouse study. Barnyardgrass, large crabgrass, and green foxtail were seeded on a weekly schedule. The following four treatments were initiated when five cohort ages ranging 8 to 36 d were obtained: clipping only, herbicide only, clipping followed by (fb) herbicide, and a nontreated control. Time interval between clipping and herbicide application was 5 d. Plants were clipped at 5 cm above the soil surface. Herbicide application was a tank mixture of nicosulfuron + metsulfuron at 39.3 + 10.5 g ai ha⁻¹, respectively. Shoots were harvested 21 d after the herbicide treatment, and dry weights of the first three treatments were expressed as percent biomass reduction relative to the age-equivalent nontreated control. Regression analyses indicated a similar response pattern for all three grass species within treatments. For clipping alone, biomass reduction increased as cohort age increased. Conversely for herbicide alone, biomass reduction decreased with age. However, biomass reduction with the clipping fb herbicide treatment was only nominally influenced by grass cohort age, providing ≥ 84% biomass reduction across all cohort ages and species.Nomenclature: Metsulfuron; nicosulfuron; barnyardgrass; Echinochloa crus-galli (L.) Beauv.; green foxtail; Setaria viridis (L.) Beauv.; large crabgrass; Digitaria sanguinalis (L.) Scop.; bermudagrass [Cynodon dactylon (L.) Pers].