EFFECTS OF THE CONTINUOUS USE OF 2,4-D AND MCPA ON SPRING WHEAT PRODUCTION AND WEED POPULATIONS

An experiment was conducted to determine whether annual applications of herbicides have any harmful residual effects on the growth of spring wheat (Triticum aestivum L.). Amine and ester formulations of (2,4-dichlorophenoxy) acetic acid (2,4-D) were applied to plots on Indian Head clay soil each year from 1947 to 1972. On other plots in the same experiment, the sodium salt of 2,4-D was applied each year from 1947 to 1952, followed by amine or ester formulations of [(4-chloro-o-tolyl)oxy]acetic acid (MCPA) from 1953 to 1972. Two rates of application were used, a low rate suitable for controlling susceptible annual weeds and a high rate suitable for controlling perennial weeds. Check plots receiving no herbicide were arranged to be adjacent to each treated plot. A 3-yr rotation of wheat, wheat, summer fallow was followed. The effects of the herbicides on grain yields and weed growth were observed. Some reductions in grain yields were caused by the high rates of ester formulations but there was no evidence o...