SELECTIVITY OF HERBICIDES

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Abstract
Selectivity of herbicides depends upon differences in the chemical and physical nature of plant surfaces and in the reactivity of their protoplasm. Selectivity is relative depending upon the conc. or dosage of chemical applied. Many salts will kill weeds selectively in grains if applied in proper conc.; they act as general contact herbicides if applied in higher conc. and with a wetting agent added. Dinitro compounds in aqueous soln. are selective; toxicity increases with increasing length of the aliphatic chain in substituted dinitro phenols through the butyl substitution; dinitro amyl phenol is somewhat less toxic. Stove oil, Stoddard Solvent and similar oil fractions will kill weeds in carrots, celery, and other members of the Umbelliferae. Selectivity is due to the content of aromatic and olefinic compounds in the oil. Differences in root distribution and salt tolerance make possible the selective killing of weeds in deep-rooted perennial crops. The 2,4-D weed killers act selectively presumably because of differences in protoplasmic susceptibility to their toxic action. Grasses are generally tolerant; most broad-leaved plants are susceptible; certain broad-leaved weeds seem to be tolerant. (A list of susceptible, intermediate, and tolerant spp. is included.).