The First East African Herbicide Conference III-Selectivity of Herbicides

Abstract

Chemical methods of killing weeds in cereals began about 60 years ago in France when Bonnet in 1895 showed that a solution of copper sulphate would kill charlock plants growing with cereals. Sixteen years later, Rabate demonstrated that dilute sulphuric acid could also be used for this purpose and, in 1932, the use of DNC (dinitro-ortho cresol) to destroy annual weeds in cereals was discovered, again in France. The big development in selective weed control, however, arose in 1941 from the observations of Templeman in England that alpha-naphthylacetic acid applied to charlock plants growing with wheat caused distortion and final destruction of the charlock without damage to the cereal. Arising from these developments, to which prominent contributions were also made in America, the phenoxy acids became established as selective weedkillers. The discovery and application of these so-called hormone weedkillers has made a tremendous impact on agriculture and more than 100,000,000 acres are now sprayed annually with these chemicals. Nevertheless, it must be remembered that they can only be safely used on a limited number of crops and, furthermore, they cannot be used on cereals at the early stages of growth. In addition to the phenoxy acids, however, a fairly wide range of unrelated organic chemicals are finding use in selective weed control, each having its own specific uses. Most of these chemicals were discovered as weedkillers as a result of the routine screening of many hundreds of compounds.