Submitted to: Western Society of Weed Science Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 2/1/2009
Publication Date: 3/14/2009
Citation: Rainbolt, C.M. B.D. Hanson, A. Shrestha, D.L. Shaner, L.J. Krutz. Proceedings Western Society of Weed Science. 62:15 Interpretive Summary: This report presents research data on degradation of the herbicide simazine in soil collected from orchard and vineyards with varying simazine use history. It is reported that, in some cases, simazine degrades nearly 10 times faster in soil with several years of annual exposure to the herbicide compared to soil with no recent exposure. This rapid biological degradation presumably occurs due to a buildup of soil microorganisms able to metabolize simazine as a source of carbon.
Technical Abstract: Simazine is a preemergent herbicide commonly used in California vineyards and orchards. It is valued for its relatively low cost and long residual activity. Simazine may be subject to enhanced biodegradation in some areas which can result in decreased herbicide half-life and reduced residual weed control. Laboratory studies were conducted to compare simazine degradation and mineralization rates in two vineyard soils, one with annual simazine use (adapted) and one with no recent simazine use (non-adapted) and in nine citrus orchard soils with varying simazine use histories. To compare simazine degradation rates, soils were treated with simazine, samples were collected over 35 days, and simazine concentration was quantified using HPLC. The data were fitted to a sigmoid curve, and the simazine half-life was calculated. In the vineyard soils, simazine half-life was 5.6 and 26.2 days in the adapted and non-adapted soil, respectively. Preliminary data from orchard soils indicate that simazine half-life ranges from 3.1 days to 29.9 days and is loosely correlated with simazine use history. In separate studies, simazine mineralization rates were assessed in both vineyard and orchard soils using ring-labeled 14C-simazine. Evolved 14C-CO2 was evaluated at regular intervals, cumulative mineralization was calculated over the course of 35 days, and data were fitted to a sigmoid curve. Maximum cumulative mineralization (% 14C-simazine applied) ranged from 41.9 to 79.1% and clearly indicates that simazine dissipation is due to biological degradation and that degradation rates varied considerably among soils with various simazine exposure histories.