The relative toxicity of 2 : 4‐dichlorophenoxy‐acetic acid and 2‐methyl‐4‐chlorophenoxyacetic acid to wheat, oats and barley

Abstract

The herbicides 2‐methyl‐4‐chlorophenoxyacetic acid (MCPA) and 2:4‐dichlorophenoxy‐acetic acid (2,4‐D) are compared, as regards their relative toxicity to the cereals wheat, oats and barley. Reference is made to the main factors governing toxicity, in particular nature of derivative, which affects not only toxicity of each chemical but the relative toxicity of one as compared with the other. The influence of concentration on toxicity is dealt with, it is shown that the so‐called concentration effect is bound up with derivative, crop and even variety of crop to a certain extent. Again, stage of development of crop at time of application of chemical appears to have a profound effect on the relative toxicity of the two growth substances. The cereals are dealt with in turn; and the general impression arising from evidence both from this country and abroad is that the wheat crop is fairly resistant to both MCPA and 2,4‐D, provided that dosage rate of chemical and stage of crop are correct for the circumstances prevailing. Oats is the most susceptible of the three cereal crops to a lowering of yield and grain quality as a result of hormone treatment; experimental results suggest that, with spring oats at least, 2,4‐D is more toxic than MCPA. Finally, barley seems more liable to abnormalities than either oats or wheat, and more abnormalities on this crop are caused by 2,4‐D than by MCPA. Though not necessarily related to loss of yield, abnormalities are definite manifestations of toxicity and on this basis 2,4‐D is the more toxic to barley of the two growth substances under discussion.

Response of Spring Wheat to the Application of 2,4‐d at Various Growth Stages*

“…Spike malformations, found exclusively in some aestivum plants treated at spike initiation, indicate that direct damage was suffered by the embryonic spike and provide additional evidence for the special sensitivity of this stage. However, the present results support the conclusion of Allen (1952) that yield reduction due to the effect of 2,4-D on the embryonic spike is not dependent on visible malformations. The damage caused to the embryonic spike was evident, in this experiment, mainly from the decrease in number of kernels per spikelet of the treated plants (Table 3).…”

“…The period close to earing, at which plants were again susceptible to 2,4-D seems to correspond with the second susceptible stage noted by Olson et al (1951), Allen (1952) and Karchi & Salomon (1963). At this period sporogenesis takes place and this explains the sensitivity of the plants and the decrease in kernels per spikelet following the application of 2,4-D.…”

“…thesis submitted by the junior author. Allen (1952), reviewing the results of experiments with different cereals from U.S.A., Scandinavia and Great Britain, concluded that the seedling and flower stages appeared to be the most sensitive to 2,4-D whereas the period 'from the commencement of tillering to a reasonable time before heading' was 'safe'. The conclusions reached in more reeent reports concerning spring wheat are summarized below (the definitions ofthe growth stages are verbatim quotations from the respective authors).…”