Chapter 79 - Symmetrical Triazine Herbicides: A Review of Regulatory Toxicity Endpoints

Abstract

This chapter explores the chemistry, usage and hazard identification of triazine herbicides. Symmetrical triazines (s-triazine) have been used as selective herbicides in agriculture for more than 50 years. Certain of these s-triazine herbicides remain agronomically and commercially important in the food production system for the pre-emergent and post-emergent control of broadleaf weeds and certain grasses. The major commercial symmetrical triazines are further divided into chloro-s-triazines: simazine, atrazine, propazine, terbuthylazine, cyanazine; the thiomethyl-s-triazines: ametryn, prometryn, terbutryn; and the methoxy-s-triazine prometon. Symmetrical triazines (s-triazines) have a chlorine, sulfur, or oxygen atom at the 2-position of the ring and are usually substituted in the 4- and 6-positions with alkylamino-groups. S-triazines block photosynthetic electron transport by displacing plastoquinone from a specific-binding site on the D1 protein subunit of photosystem II. Hazard endpoints were identified for the s-triazine herbicides based upon protocols established under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). Evaluation of hazard profiles for the currently commercial triazine herbicide products reveals that these products are generally not acutely toxic, are well-tolerated when administered to animals over a long duration of time, are generally not developmental or reproductive toxins, and are not mutagenic or carcinogenic in mice or male rats. The chloro-s-triazines appear to produce an earlier onset or an increased evidence of mammary tumors in female SD rats at high doses.