Citrus Herbicide Mitigation Practices: Demonstration and Evaluation

SUMMARY Simazine loss from soil to surface runoff water produce both a decrease in the effectiveness of the soil chemical treatment and a water quality hazard in receiving waters. Simazine has been found in 30% of wells sampled in Fresno and Tulare counties, California and some contamination has been attributed to simazine use in citrus orchards. Contamination of receiving water from herbicides could put important weed control tools at risk and potentially increase risks to human health. In order to better understand the off-site movement of herbicides, studies were conducted in citrus orchards to evaluate weed control efficacy under selected weed management practices and to measure selected management practices for mitigating preemergent herbicide movement in rainfall runoff from citrus orchard middles. The results showed weed management practices that avoid broadcast application of preemergent herbicides during the rainy season were as effective as the standard rate at 2.0 lb ai /acre for weed control by using spray application to the entire orchard floor. The most common weed species included: spotted spurge, common groundsel, horseweed, annual sowthistle, and purple cudweed. Common groundsel is found to be resistant to preemergence application. Modifying how preemergence herbicides are used to control weeds is becoming increasingly important to protect the environment and to continue use of these important weed control tools. Runoff water data showed that the first runoff events following application of simazine produced peak concentrations. Data also showed that shallow mechanical incorporation and surfactant application did not consistently reduce mean concentration of simazine in runoff water. However, shallow mechanical incorporation and surfactant application reduced mass loss in runoff water. Soil simazine concentration was much higher in the middle than in the furrow after post-rainfall. Plots with shallow mechanical incorporation had higher recovered simazine than the plots without mechanical incorporation.