Evidence for cross‐adaptation between s ‐triazine herbicides resulting in reduced efficacy under field conditions

BACKGROUND: Enhanced atrazine degradation has been observed in agricultural soils from around the globe. Soils exhibiting enhanced atrazine degradation may be cross‐adapted with other s ‐triazine herbicides, thereby reducing their control of sensitive weed species. The aims of this study were (1) to determine the field persistence of simazine in atrazine‐adapted and non‐adapted soils, (2) to compare mineralization of ring‐labeled 14 C‐simazine and 14 C‐atrazine between atrazine‐adapted and non‐adapted soils and (3) to evaluate prickly sida control with simazine in atrazine‐adapted and non‐adapted soils. RESULTS: Pooled over two pre‐emergent (PRE) application dates, simazine field persistence was 1.4‐fold lower in atrazine‐adapted than in non‐adapted soils. For both simazine and atrazine, the mineralization lag phase was 4.3‐fold shorter and the mineralization rate constant was 3.5‐fold higher in atrazine‐adapted than in non‐adapted soils. Collectively, the persistence and mineralization data confirm cross‐adaptation between these s ‐triazine herbicides. In non‐adapted soils, simazine PRE at the 15 March and 17 April planting dates reduced prickly sida density at least 5.4‐fold compared with the no simazine PRE treatment. Conversely, in atrazine‐adapted soils, prickly sida densities were not statistically different between simazine PRE and no simazine PRE at either planting date, thereby indicating reduced simazine efficacy in atrazine‐adapted soils. CONCLUSIONS: Results demonstrate the potential for cross‐adaptation among s ‐triazine herbicides and the subsequent reduction in the control of otherwise sensitive weed species. Copyright © 2008 Society of Chemical Industry