Studies on the activation of herbicides

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Introduction

Early in 1940 (Chabrolin (1940))4 reported that sodium pentachlorophenate at a concentration of 1.5 per cent killed wild-radish plants. Used at a dosage corresponding to 20 kg per hectare, it was effective as a selective herbicide against certain annual dicotyledonous weeds in cereals. Later in the same year (Hance (1940)) claimed that sodium pentachlorophenate functions as an activator with common herbicides such as sodium arsenite and sodium chlorate.

Tests in 1941 by R. N. Raynor at this station failed to indicate activation when sodium pentachlorophenate was used with sodium arsenite and sodium chlorate. The solutions were sprayed on grasses (mostly Hordeum murinum L.) and mixed annual weeds. Under the semiarid conditions existing, no synergism could be noted. Sodium pentachlorophenate proved toxic and, in solution, spread well on the grasses. But whenever a given concentration of sodium chlorate or sodium arsenite was applied, with and without sodium pentachlorophenate, the total toxicity of each mixture was equal only to the sum of the toxicities of the separate ingredients. Our tests varied from those of (Hance (1940)). We compared sodium arsenite and sodium chlorate solutions with and without sodium pentachlorophenate—that is, the concentration of arsenite or chlorate was the same in the two solutions being compared. Hance, on the contrary, compared his “activated” solutions with a so-called “standard” formula containing a higher concentration of the herbicide being tested.

Controlled Experiments on Activation

Because many variables are difficult to control in the field, the results of the above-described field tests were checked in the greenhouse. The testing method used has since proved valuable in comparing the relative merits of many chemicals as contact herbicides. Indicator plants (both crop plants and weeds) are grown in soil in no. 10 cans. When they are large enough for testing (usually 4 to 10 inches high), each can is placed on a turntable, and the spray solution is applied with an atomizer5, using about 4 pounds of air pressure. The percentage injury is estimated daily until a constant condition is reached. By this method large numbers of cultures, seeded the same day and grown under the same conditions, can be treated in a short time, the type or concentration of spray solution constituting the chief variable.

In preliminary tests, sodium pentachlorophenate was about twice as toxic as sodium chlorate.