Weeds can influence the economics of alfalfa (Medicago sativa L.) production by reducing forage yield and nutritive value or by contaminating hay. Field studies were conducted in Idaho in 2021 and 2022 to evaluate the effect of weed control treatments on alfalfa forage accumulation, weed biomass, and nutritive value. In addition, the relationship between the proportion of individual weed species biomass and alfalfa nutritive value was assessed. These studies included eight different herbicide and herbicide combination treatments, including the untreated check. Treatments were comprised of preemergence, early postemergence (after 80% alfalfa emergence), and postemergence (third trifoliate alfalfa) herbicide applications. Data collection included weed control efficacy, weed and alfalfa biomass, and alfalfa nutritive value. Additional samples were collected and combined in these alfalfa to weed biomass proportions (percentage by weight): 0/100, 20/80, 40/60, 60/40, 80/20, and 100/0, for wet chemistry analysis of forage nutritive value to evaluate the relationship between the proportion of individual weed species biomass and alfalfa nutritive value. The acetochlor-only treatment provided less than 50% weed control, while the EPTC (S-ethyl-N,N-dipropylthiocarbamate)-only treatment provided 54%-81% weed control. The control provided by acetochlor and EPTC was less than that provided by treatments containing imazamox and imazamox plus bromoxynil. Weed biomass in forage (23%-55% of total biomass) due to poor or no weed control reduced crude protein, increased fiber concentrations, and reduced the relative feed value. The relationship between the proportion of individual weed species biomass and alfalfa nutritive value was linear for all weed species evaluated. Poor weed control increased weed biomass and reduced forage nutritive value.Effective weed control reduced forage accumulation due to reduced weed control and herbicide injury to alfalfa.A linear relationship between weed biomass proportion and mixed stand nutritive value is described.The risk of nitrate poisoning may increase when alfalfa hay has 60% or more biomass from certain weeds.
