A review of the effects of long-term 2,4-D and MCPA applications on wheat production and selected biochemical properties of a Black Chernozem

Abstract

Rotational plots at the Agriculture Canada Experimental Farm at Indian Head, Saskatchewan, have been receiving annual applications of amine and ester formulations of 2,4-D since 1947, and MCPA amine since 1953. These research plots are unique world-wide for the length of time of application of these herbicides. Wheat yield data from 1973 to 1989 indicate that the continuous use of the herbicides significantly increased yields compared to untreated control plots. In both 2,4-D and MCPA treatments, susceptible weeds were greatly reduced, but not eliminated. No new major weed species became established. After 35 yr of 2,4-D application there were no ecologically significant effects on selected soil biochemical processes, nor had the herbicides interfered with the normal cycling of carbon, nitrogen, or phosphorus. Following 33 yr of MCPA treatments there were, except for yeasts, no significant changes in microbial biomass or microbial populations. After 40 successive treatments of 2,4-D amine and ester, and 35 annual applications of MCPA, herbicide residues in the 0- to 15-cm and 15- to 30-cm soil depths were less than detectable limits of < 0.03 kg acid ha−1. Under laboratory conditions, the breakdown of added 2,4-D and MCPA was faster in soils receiving continuous herbicide applications than in soil from untreated control plots, suggesting that some soil microbiological adaptation had occurred. It can be concluded that there have been no adverse effects on crop productivity of the soil from the long-term use of 2,4-D and MCPA. Key words: Phenoxies, continuous use, wheat production, weed populations, soil biochemical activities, microbial degradation