Toxicity and Breakdown of ‘Hormone’ Herbicides

Abstract

DURING the course of an investigation of the effect of certain ‘hormone’ herbicides, when applied to the rooting medium, on the growth and nodulation of white clover (Trifolium repens), some interesting features emerged on the relative toxicities of these compounds. The substances utilized were the sodium salts of 24D (2 : 4-dichlorophenoxyacetic acid), MCPA (2-methyl-4.chlorophenoxyacetic acid), 245T (2 : 4 : 5-trichlorophenoxyacetic acid), 24DB (γ-(2 : 4-dichlorophenoxy)butyric acid) and MCPB (γ-(2-methyl-4-chlorophenoxy)butyric acid). Their action on the white clover was determined by growing the plants in (a) aseptic agar culture in test-tubes and (b) soil culture in pots. The agar rooting medium was the nitrogen-free formula described by Thornton¹ and the soil was John Innes compost which had been allowed to weather for a year in the open air. The plants were inoculated with a known effective strain of Rhizobium trifolii. The hormone substances were incorporated in the agar in the appropriate concentrations. In the case of the pot experiment, the soil was air-dried, and 100 c.c. of the chemical at the appropriate concentration was added to each pot containing 175 gm. of soil. Each pot was stood in a Petri dish of water to eliminate the effects of leaching. There were seven tubes and seven pots per treatment.