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

Abstract

Surface runoff or irrigation water contaminated with herbicides may cause crop damage and result in inadvertent residues in the crop. Rotational crops may also be damaged due to the persistence of leached herbicides within the root zone. An 11-yr field experiment was carried out to address these issues through repeated flood-irrigations of alfalfa (Medicago sativa L. 'Roamer') with water containing various concentrations (0, 10, 100 and 1000 µg L−1) of monuron or simazine. The experimental site, on an alluvial clay soil, 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 applications of water containing up to 100 µg L−1 of either monuron or simazine did not show any harmful effects on the crop. However, the 1000 µg L−1 treatments caused cumulative yield reductions with greatest deleterious effect being caused by simazine (up to 55% yield reduction). After 7 yr of irrigation, inadvertent residues of both herbicides were consistently detected in the crop, but only for the 1000 µg L−1 treatments. Average concentrations of monuron in the alfalfa foliage were 0.94 and 1.76 mg kg−1 for the first 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 with half of the total being present in the top 0.15 m, and they were detectable only for the 100 and 1000 µg L−1 treatments. Only soil residues of simazine from the 1000 µg L−1 rate of application reduced oat yields extensively. These yield reductions occurred only during the first 2 yr under dryland production. Key words: Herbicide-contaminated irrigation water, crop damage, herbicide residues, alfalfa, simazine, monuron