Economic assessment of weed management strategies in rice is dependent upon a quantitative estimate of the yield impact of a given weed population. To assist rice producers in making such assessments, a mathematical model was developed to predict rice yield reduction as a function of weed density and duration of interference. The nonlinear empirical model was a unique 3-dimensional adaptation of the Richards equation with 4 parameters. Using published data, individual parameter values were fitted for each of 6 weed species interfering with either conventional or semi-dwarf statured rice cultivars. The functional form of the equation produced surfaces that were qualitatively consistent with available data and experience regarding rice-weed biology. Hence, predictions from the model should be useful and reliable in assessing the economic impact of weeds and in determining the feasibility of alternative weed control treatments for various field scenarios.

Winter wheat cultivars that are competitive with weeds help control weeds in crop rotations. Ten winter wheat cultivars were evaluated for interference with summer annual grasses in the wheat and the subsequent grain sorghum crop in a winter wheat-ecofallow sorghum-fallow rotation in which there are two 10–mo fallow periods and two crops in 3 yr during 1983 to 1987. The medium–tall (100 to 109 cm tall) and medium–statured (90 to 99 cm tall) winter wheat cultivars (‘Buckskin’, ‘Siouxland’, ‘Lancota’, ‘Centurk 78’, and ‘Brule’) were more competitive than medium-short (80 to 89 cm tall) and short (68 to 79 cm tall) cultivars (‘Eagle’, ‘Homestead’, ‘Colt’, ‘Vona’, and ‘TAM 101’). Atrazine plus paraquat was applied to all cultivars 30 d after wheat harvest. When grain sorghum was planted in areas previously seeded with medium–tall and medium-statured winter wheat, summer annual grass weed biomass in sorghum was 61% less than in grain sorghum seeded into areas previously planted with medium-short and short wheat cultivars. Use of pendimethalin plus 2,4–D in winter wheat and glyphosate plus alachlor in grain sorghum eliminated differences in summer annual grass weed density and weed biomass among wheat cultivars. Sorghum grain yields were improved 7% when herbicides were used in the winter wheat and sorghum but value of the increase was less than cost of herbicides. Substituting less costly herbicides for herbicides used in this study still would not have been enough to pay for cost of herbicides for five cultivars. Grain sorghum grown on weed–free stubble of medium–tall and medium–statured winter wheat produced more grain than grain sorghum grown after medium–short and short-statured winter wheat by 5%. Volunteer wheat density during the fallow period following grain sorghum was lower in areas originally seeded to Centurk 78 and Siouxland wheat while volunteer wheat density was higher in areas planted to Homestead and Vona.