Herbicide residues and yield effects from repeated flood-irrigations of alfalfa with water containing monuron or simazine

Surface runoff or irrigation water contaminated with herbicides may cause c rop damage and result in inadvertent residues in the crop. Rotational crops may also be damaged due to the persistence of leached herbicides within th e root zone. An 11-yr field experiment was carried out to address these iss ues through repeated flood-irrigations of alfalfa (Medicago sativa L. 'Roam er') with water containing various concentrations (0, 10, 100 and 1000 mu g L-1) of monuron or simazine. The experimental site, on an alluvial clay so il, was flood-irrigated for 8 yr with a total of 32 irrigations, then oat ( Avena sativa L. 'Harmon') was grown for 3 yr under dryland conditions as a sensitive bioassay crop. Based on alfalfa forage yield, repeated applicatio ns of water containing up to 100 mu g L-1 of either monuron or simazine did not show any harmful effects on the crop. However, the 1000 mu g L-1 treat ments caused cumulative yield reductions with greatest deleterious effect b eing caused by simazine (up to 55% yield reduction). After 7 yr of irrigati on, inadvertent residues of both herbicides were consistently detected in t he crop, but only for the 1000 mu g L-1 treatments. Average concentrations of monuron in the alfalfa foliage were 0.94 and 1.76 mg kg(-1) for the firs t and second cuts, respectively, whereas corresponding values for simazine were 0.31 and 0.64 mg kg(-1). Approximately 4 and 12% of the total amounts applied remained in the soil profile for monuron and simazine, respectively . Herbicide residues to 1.5 m soil depth decreased with increasing depth wi th half of the total being present in the top 0.15 m, and they were detecta ble only for the 100 and 1000 ug L-1 treatments. Only soil residues of sima zine from the 1000 mu g L-1 rate of application reduced oat yields extensiv ely. These yield reductions occurred only during the first 2 yr under dryla nd production.