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Effects of Long-Term 2,4-D and MCPA Field Applications on the Soil Breakdown of 2,4-D, MCPA, Mecoprop, and 2,4,5-T

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

^* Corresponding author.

ABSTRACT

Under laboratory conditions, the rates of breakdown of (¹⁴C)2,4-D (2,4-dichlorophenoxyacetic acid), (¹⁴C)MCPA (4-chloro-2-methylphenoxyacetic acid), (¹⁴4C)mecoprop (2-[4-chloro-2-methylphenoxy]propionic acid), and 2,4,5-T (2,4-5-trichlorophenoxyacetic acid) in soils from Canadian prairie field plots, 6 wk after receiving the 43rd annual application of 2,4-D formulations or the 37th annual treatment with MCPA, were compared with those in soils from untreated plots. Loss of 2,4-D and MCPA was faster in soils that had received continuous applications with the appropriate herbicide than in soil from the control plots. There was some indication that (¹⁴4C)2,4-D was dissipated more rapidly from soils treated annually with MCPA than from the control soils, though breakdown was slower than in soil from the 2,4-D-treated plots. In contrast, (¹⁴C)MCPA breakdown in the 2,4-D-treated plots was similar to that in untreated soils. Breakdown of (¹⁴C)mecoprop and 2,4,5-T in the 2,4-D and MCPA-treated soils was very similar to that in the control soils. Thus, there was no support for the phenomenon of cross-enhancement. Forty-eight weeks after the last herbicide applications, the field soils still maintained their ability to degrade 2,4-D or MCPA more rapidly than soil from untreated control plots.

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