The relative toxicity of 2:4‐dichlorophenoxyacetic acid and 2‐methyl‐4‐chlorophenoxyacetic acid to annual weeds

Abstract

Results are presented showing differences in the rclativc toxicity of sodium MCPA (the sodium salt of 2-methyl-4-chlorophenoxyacetic acid) and sodium 2.4-D (the sodium salt of z : 4-dichlorophenoxyacctic acid) t o Bvassica alba spravcd in the field. The apparent toxicity ratio is shown t o vary with : (a) the interval after spraying a t which observations are made ; (6) the dosage applied ; (c) the incidence of rain after spraying ; ( d ) the particular salt used.Spray droplets of sodium z,.+-D dry out relatively completely. Icaving a crystalline deposit. whereas those of sodium MCPA tend t o dry t o a non-crystalline film. I t is suggested t h a t the observed variation in rrlativc toxicity may be due to differcnces between t h e two materials in t h e facility with which thcy cntcr the leaf ; sodium MCFA pcnctrates t h e mnrc readily owing t o t h e non-crystalline nature of tho spray deposit, and thus gives better results when rain follows after an interval which much dccrcases the efficacy of sodium 2.4-D. IntroductionA comparison of the relative toxicities of XCI'A (a-methyl-4-chlorophenoxyacctic acid) and 2,4-L) (z : 4-dichlorophenoxyacetic acid) is now being made in many countries of Europe, the U.S.A. and Canada, but the arrival of two types of hormone weedkiller has been achieved by rather different routes in these areas.In the early stages of development of selective weedkillers MCPA was favoured in Great Britain and 2,4-L) in the U.S.A. In the first place the sodium salts were used, but the need for something more soluble.than sodium 2,4-D led to the development of the amine and ester formulations.With MCPA the solubility of the sodium salt is sufficient to allow its use in low-volume application, and therefore there is not so great a need for other formulations from this point of view. Subsequently, 2,4-D in all its formulations was introduced commercially into Great Britain, and more recently MCPA was introduced in the U.S.A. Furthermore, formulations of MCI'A other than the sodium salt, namely amine and ester, have been tried in the U.S.A. and in this country.A comparison of the toxicity of 2,4-D and MCPA is therefore possible in a number of ways. If the sodium salts are pound-for-pound comparable, are the amincs and esters of the respective