Enhanced Atrazine Degradation: Evidence for Reduced Residual Weed Control and A Method for Identifying Adapted Soils and Predicting Herbicide Persistence

Authors: Krutz, Larry; BURKE, IAN - WASHINGTON STATE UNIV; Reddy, Krishna; Zablotowicz, Robert; Price, Andrew

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2009
Publication Date: 7/10/2009
Citation: Krutz, L.J., Burke, I.C., Reddy, K.N., Zablotowicz, R.M., Price, A.J. 2009. Enhanced atrazine degradation: Evidence for reduced residual weed control and a method for identifying adapted soils and predicting herbicide persistence. Weed Science. 57:427-434.

Interpretive Summary: Rapid dissipation of the herbicide atrazine by soilborn bacteria may reduce the herbicides residual control of sensitive weed species. The objectives of this study were to 1) compare the effects of planting date on atrazine persistence and prickly sida density in s-triazine adapted and non-adapted soils; 2) identify a diagnostic test for determining s-triazine adapted soils; and 3) develop and validate an s-triazine persistence model based on data generated from the proposed diagnostic test. Atrazine half-life values in s-triazine adapted soils were at least 10-fold lower than USEPA estimate (120 d). As expected, atrazine controlled prickly sida in the non-adapted soils. Conversely, in the s-triazine adapted soil, residual control of prickly sida by atrazine was reduced. These data indicate that enhanced atrazine degradation can reduce the herbicides residual control of sensitive weeds, and that a diagnostic tool for identifying s-triazine adapted soils is required. Our analysis indicates that cumulative mineralization of ring-labeled 14C-atrazine in excess of 50% of the initial 14C applied after 30 d is diagnostic for enhanced s-triazine degradation. Moreover, a model was developed that accurately predicted atrazine persistence under field conditions, regardless planting date or s-triazine use history. These data suggest that coupling the diagnostic test with the persistence model will allow weed scientists to identify s-triazine adapted soils, predict herbicide persistence under field conditions, and implement alternative weed control strategies in affected areas if warranted.

Technical Abstract: Soilborn bacteria with novel metabolic abilities have been linked with enhanced atrazine degradation and complaints of reduced residual weed control in soils with an s-triazine use history. However, no field study has verified that enhanced degradation reduces atrazine’s residual weed control. The objectives of this study were to 1) compare the effects of planting date on atrazine persistence and prickly sida density in s-triazine adapted and non-adapted soils; 2) identify a diagnostic test for determining s-triazine adapted soils; and 3) develop and validate an s-triazine persistence model based on ring-labeled 14C-s-triazine mineralization kinetics. Atrazine half-life values in s-triazine adapted soils were at least 5-fold lower than historic estimates (60 d) and 10-fold lower than the USEPA’s estimate (120 d). Atrazine reduced prickly sida density in the non-adapted soils, regardless of planting date (P = 0.0091). Conversely, in the s-triazine adapted soil, prickly sida density was not different between no atrazine PRE and atrazine PRE at the 15 March planting data (P = 0.1397). A lack of significance in this contrast signifies that enhanced degradation can reduce atrazine’s residual control of prickly sida, and that a diagnostic tool for identifying s-triazine adapted soils is required. Our analysis indicates that cumulative mineralization of ring-labeled 14C-atrazine in excess of 50% of C0 after 30 d of incubation under optimal temperature and moisture levels is diagnostic for enhanced s-triazine degradation. An s-triazine persistence model was developed and subsequently validated with independent data. Model predictions for both atrazine and simazine were within the 95% confidence intervals of observed values, regardless of planting date or s-triazine use history. These data suggest that coupling the diagnostic test with the persistence model will allow weed scientists to identify s-triazine adapted soils, predict herbicide persistence under field conditions, and implement alternative weed control strategies in affected areas if warranted.