Relevance. The appearance of weeds in the fields of farms does not always imply their complete destruction, since from an economic point of view this may be ineffective. Therefore, in order to predict the effectiveness of crop contamination control in specific soil and climatic conditions, an urgent task is to develop and substantiate models of weed harmfulness and calculate economic thresholds for their harmfulness.Methods. The harmfulness of gray bristle (Setaria glauca (L.) Beauv) and green bristle (Setaria viridis (L.) Beauv) in the conditions of the Agro-Invest Plus LLC enterprise in the Karsunsky district of the Ulyanovsk region in 2021–2022 on the yield of Despina barley was studied. The economic harmfulness thresholds were calculated against the background of the use of herbicides Ovsyugen Super, CE — 0.5 l/ha; Axial 50, CE — 0.9 l/ha; Puma Super, EMV — 0.9 l/ha; Avantik Turbo, MD — 0.5 l/ha. The scheme of the experience included: Option 1 — without weeds (control), 2–6 options — with the number of weeds, respectively, 10 pcs/m2, 20 pcs/m2, 30 pcs/m2, 40 pcs/m2, 50 pcs/m2, option 7 — with natural blockage. The accounting area of the plot is 1 m2, the location is systematic with an offset, the repetition is 6 times.Results. Based on field experiments, predictive models have been developed to determine the harmfulness of weeds, which make it possible to establish economic thresholds for their harmfulness in the conditions of a particular farm. The harmfulness of the bristle according to the linear function was 0.106–0.147 c/ha, depending on the amount (pcs/m2) or 0.322–0.291 c/ha by weight (g/m2). According to prognostic models, the proportion (%) of changes in barley grain yield from contamination ranged from 79 to 85%. Studies have shown that chemical weeding is advisable in the presence of bristles in barley phytocenoses from 8 to 15 pcs/m2.
