Variable Toxicity of Triazine Herbicides

Abstract

WHEN applied to the soil surface several triazine herbicides have been shown to be toxic to gymnosperm seedlings, with older seedlings much less susceptible than young ones^1,2. The method of herbicide application also greatly influences the toxicity of triazines. For example, no injury occurred to 2-year-old Pinus resinosa plants, when simazine was applied to the soil surface at 4 or 8 lb./ acre or when applied to the foliage only. However, incorporation in the soil of simazine applied to the soil surface caused severe injury and high seedling mortality². The triazine herbicides apparently are less toxic when applied to the soil surface than when incorporated in the soil because they do not leach readily and, in the former case, roots of trees often are below the soil layers containing phytotoxic amounts of these chemicals. For these reasons differences in actual toxicity of different triazines may not be readily apparent when the herbicides are applied to the soil surface. Experiments have now been extended to evaluate the effects of several soil-incorporated triazine herbicides on development of young Pinus resinosa seedlings.