A Theory of Herbicidal Action

Abstract

Modern herbicides, such as dinitro- and chlorophenols, phenoxy compds., and carbamates, are used at low concns.; their entry from sprays into plants depends on their compatibility with the plant cuticle. Some act in the dust form where solubility in cuticle is not complicated by the presence of water. The cuticle does not form a perfect covering, especially on young leaves. This is why arsenic and chlorate from 1-2% solns. penetrate young leaves and why the Na salt of 2, 4-D is so effective on young plants. Salts of 2, 4-D are more readily absorbed from soil than the esters; the salts cause more crippling of oats and corn than do esters. There are 2 possible pre-emergence methods: (1) pre-emergence contact spray to kill weed seedlings that have grown since prepn. of the seed bed; and (2) selective sterilization of the soil to kill weeds selectively. Pre-emergence contact sprays are safe provided a non-polar or water-insoluble toxicant is used. Selective sterilization depends on profound differences between the crop and weed in susceptibility, root location, or some other factor. To penetrate cuticle and be absorbed by foliage, non-polar compounds should be used; for absorption by roots, polar compounds.