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Effects of Long-Term 2,4-D and MCPA Field Applications on Soil Residues and Their Rates of Breakdown

Research Station, Agriculture Canada, 5000 Wascana Pkwy., Box 440, Regina, Sask. S4P 3A2;

V. O. Biederbeck

Research Station, Agriculture Canada, Swift Current, Sask. S9H 3X2.

^* Corresponding author.

ABSTRACT

A 3-yr rotation of wheat (Triticum aestivum L.), wheat, and summerfallow in field plots on a clay soil at the Indian Head Experimental Farm, Saskatchewan, has been receiving annual applications of ester and amine formulations of 2,4-D (2,4-dichlorophenoxyacetic acid) since 1947, and MCPA (4-chloro-2-methylphenoxyacetic acid) since 1953. In the fall of 1987, after 40 successive applications of 2,4-D and 34 annual treatments of MCPA, soil samples were taken from the 0- to 15-cm and 15- to 30-cm depths of replicate treatments. Gas chromatographic analysis revealed that residual amounts of 2,4-D and MCPA were less than 0.02 mg kg^–1, indicating that there had been complete degradation of the herbicides. Under laboratory conditions, the breakdown of 2.0 mg kg^–1 (¹⁴C)2,4-D and (¹⁴C)MCPA was slightly faster in soils that had received continuous applications with the appropriate herbicide, than in soil from the untreated control plots, suggesting some soil microbial adaptation in response to long-term use of these herbicides.

Joint contribution from the Agric. Canada Research Stations at Regina, Sask., and at Swift Current, Sask.

Received for publication November 15, 1988.