Simazine is an important management tool for weed control in vineyards because of its relatively low price, reliable control of several problem weeds, and long residual activity. After repeated and extensive use of simazine, several growers in the Central Valley of California expressed concerns about reduced, residual weed control with this herbicide. Experiments were conducted to evaluate the rate of simazine dissipation in soils with differing simazine-use histories and to determine whether residual weed control differed among sites. Two raisin vineyards were used in all studies, one with extensive simazine-use history (adapted) and one with no recent simazine-use history (nonadapted). Results indicated that simazine dissipation from biotic processes was fourfold greater in soil with a long simazine-use history relative to soil with no recent simazine applications. In the field, simazine persisted longer at the nonadapted site, and weed-control duration was affected by dissipation rate. Central Valley vineyard soils that have had repeated simazine applications can develop enhanced, microbial degradation, and reduced, residual weed control is possible; however, weed control is also affected by environmental conditions and other crop management practices.

Nomenclature: Simazine, 6-chloro-N,N′-diethyl-1,3,5-triazine-2,4-diamine; raisin grapes, Vitis vinifera L.