Growth-Regulating Substances as Herbicides (1947)

1. On the basis of the records of many experiments which demonstrated the relative toxicity of some of the growth-regulating substances and the differential sensitivity shown by plants in relation to them, the suggestion was made that greater attention be directed toward more critical study of these effects and their possible significance to agricultural and industrial practice. Following out this idea, it was specifically suggested that attention be given to the possible significance of these compounds in a program for national defense. 2. Experiments, in addition to work in progress, looking to both ends were carried out for a number of years. Summary results of certain of the specific experiments carried out from August, 1941, to May, 1944, are given. Others may be presented at some future time. 3. When attention was called to some of the chlorophenoxyacetic and iodobenzoic acids, additional critical experimental work was at once centered around some of them. 4. The 2,4-dichlorophenoxyacetic, 2,4,5-trichlorophenoxyacetic, and 4-chlorophenoxyacetic acids proved extremely toxic when applied under greenhouse conditions to the aerial parts of sugarbeet, radish, rape, kidney bean, red clover, and buckwheat plants. Emulsions of these compounds, when applied to the foliage of rice plants and to the water over the surface of the soil in which they were growing, were highly toxic. 5. Various other substances were effective as plant toxicants or in modifying growth when sprayed on the plants as lanolin or oil emulsions or when used as aerosols. 6. The concentrations of these agents required to kill or markedly to reduce the rate of growth of the plants ranged between 0.13% and 1.0% when the acids were applied as emulsion sprays. Penta- and 2-chlorophenoxyacetic acids were less toxic than 2,4-dichlorophenoxyacetic, 2,4,5-trichlorophenoxyacetic, and 4-chlorophenoxyacetic acids. 7. Dusts containing growth-regulating substances were relatively ineffective as applied in these tests. It was suggested that, on the basis of these results, the addition of a hygroscopic agent to the mixture of talc plus a growth-regulating substance might allow for the absorption of moisture and thereby place a sufficient amount of the agent in a form to induce growth responses and even severe injury. 8. In general, cereals were relatively insensitive to growth-regulating compounds as used in these experiments. Rice was killed by the application of 2,4-dichlorophenoxyacetic acid to the water surrounding the plants as well as to their leaf surface. Barley also proved to be somewhat sensitive to some of the mixtures but less so than most of the dicotyledonous plants tested. 9. Aside from the toxic effects, it was established that the application of 2,4-dichlorophenoxyacetic acid, and some of the other substituted chlorophenoxyacetic acids, brought about responses by the dicotyledonous plants that were outwardly different in appearance from those that resulted from the application of 2,3,5-triiodobenzoic acid. On the basis of the results obtained, it appeared that 2,4-dichlorophenoxyacetic and 2,4,5-trichlorophenoxyacetic acids would be outstandingly useful as herbicides or as differential herbicides. 10. In connection with the experimental work then under way, in March, 1944, plots were laid out at Beltsville, Maryland, to study the use of growth-regulating substances as herbicides when applied to field-grown plants. 11. The results of the various experiments carried out, together with many photographs, some of which accompany this paper, were conveyed to those authorized to receive such reports (13). 12. The use of growth-regulating substances in the control of weeds and other vegetation has now become widely publicized and exploited. Some of the other results of these experiments also are finding their application in agricultural and industrial practice.