Separate field trials were conducted in 2007 and 2008 to investigate the effects of increasing densities of common ragweed or common cocklebur on total yield and forage nutritive values in tall fescue pastures. Common ragweed densities ranged from 0 to 188 plants m⁻², and common cocklebur densities ranged from 0 to 134 plants m⁻². Total biomass yields (weeds plus tall fescue) were determined in response to each weed density and species; pure samples of tall fescue, common ragweed, or common cocklebur were also hand collected from each plot at the time of the total biomass harvest. Near-infrared spectroscopy was used to predict crude protein (CP) concentration and in vitro true digestibility (IVTD) of the total harvested biomass, pure tall fescue, and pure weed species in each plot. Results indicate that biomass yields may increase by as much as 5 kg ha⁻¹ with each additional common ragweed plant m⁻² within a tall fescue stand. Additionally, CP concentration of the total harvested biomass, pure weed species, and tall fescue decreased by 0.2 to 0.4 g kg⁻¹ with each additional increase in common ragweed or common cocklebur plant per m⁻². As weed densities increased, IVTD of pure tall fescue samples increased only minimally (0.04%), regardless of the weed species. An increase in common ragweed density also resulted in the CP concentration of pure samples of common ragweed to decrease by 0.2 g kg⁻¹ for each additional plant per m² and by 0.4 g kg⁻¹ for each additional common cocklebur per m². Overall, results from these experiments indicate that plant biomass yield and nutritive values of the total harvested biomass are only marginally influenced by increasing common ragweed or common cocklebur densities.

Nomenclature: Common ragweed, Ambrosia artemisiifolia (L.); common cocklebur, Xanthium strumarium (L.); tall fescue, Festuca arundinacea Schreb