Theoretical Leaching Potential vs. Lab-Determined Leaching Patterns of Phenoxy Herbicides in Alberta Soils

Susan Byers, Dan Inaba, Bernie Hill, Jim Miller, and Neil Harker
Agriculture and Agri- Food Canada Research Centre, Lethbridge, Alberta
P.O. Box 3000 Main, T1J 4B1; (403) 317-2267; byerssu@em.agr.ca

Abstract:
We used a model to calculate the leaching potential (LP) of herbicides listed in the "Crop Protection with Chemicals" handbook, which is often referred to as the ‘Blue Book' (Alberta Agriculture, Food and Rural Development, 1998). We also measured the leaching of nine herbicides (bromoxynil, 2,4-D, dicamba, diclofop, fenoxaprop, MCPA, quinclorac, triallate and trifluralin) on soils from five major soil zones of Alberta (Brown, Dark Brown, Black, Dark Grey, Grey) using 50-g packed soil columns [14 x 2.06-cm i.d.].

Dicamba leached very rapidly in all five soil types; whereas 2,4-D, MCPA and quinclorac leached very rapidly or rapidly in all soils tested. Bromoxynil was found to leach very rapidly to steadily, and diclofop was found to leach rapidly to very slowly in the five soils. Fenoxaprop leached rapidly to very slowly and triallate leached slowly or not at all. Trifluralin did not leach in any of the soils. In general, the herbicides leached more rapidly in Lethbridge soil than in the other four soils and the leaching rates were lower in the Lacombe soil than the other four soils tested.

The LP model predicted the relative leaching potentials of the nine compounds quite well for the soils tested, although the leaching of diclofop was generally overestimated. Overall, leaching rates varied greatly among soil types but the relative order of leaching among the herbicides was generally not affected by soil type. Because the relative order of leaching was similar in all soil types, the LP model provides a useful tool for comparing herbicides and determining those with lower leaching potential regardless of soil type. A list was prepared containing the relative ranking of leaching potentials for all herbicides recommended in Alberta. The inclusion of this list in the Blue Book will provide producers with the information needed to choose herbicides with lower leaching potential.