MOVEMENT OF SIMAZINE IN RUNOFF WATER FROM CITRUS ORCHARD ROW MIDDLES AS AFFECTED BY MECHANICAL INCORPORATION

In California, preemergence herbicide residues have been measured in r unoff water from citrus orchards that resulted from winter rainfall, T his study measured the effect of rainfall on the redistribution of her bicides within a citrus orchard and the effect that shallow mechanical incorporation had on residue movement. Simulated rainfall treatments were applied to plots within a citrus orchard where simazine was appli ed only to row middles, Simazine (2-chloro-4, 6-bis [ethylamino]-s-tri azine) movement in runoff water was compared between middles that were either undisturbed, the normal orchard practice, or subject to shallo w mechanical incorporation. In undisturbed middles, simazine concentra tion in runoff water collected from the first of two simulated rain ev ents averaged 0.87 mg L-1; simazine concentration in runoff water from a second event applied 7 d later averaged 0.40 mg L-1. Shallow mechan ical incorporation of row middies decreased runoff mater volume from t he first simulated rain event by approximately 50% with simazine conce ntration decreased to 0.14 mg L-1; runoff water volume was unaffected at the second rainfall event but simazine concentration remained low a t 0.07 mg L-1. Total simazine mass removed from both events, which als o accounts for mass recovered in furrow soil, was estimated at 13.1% o f the amount applied to row middles in undisturbed plots compared to o nly 2.1% in mechanically disturbed middies. We conclude that ambient r ainfall is unreliable for incorporation of preemergence herbicides int o orchard soil with low infiltration, end that shallow mechanical inco rporation should be tested under commercial citrus growing conditions.