Soil properties and agri-environmental conditions affect imazamox : imazethapyr (1:1) and flucarbazone-sodium phytotoxicity and dissipation

In 2002, approximately one out of four farmers in Manitoba used a herbicide product containing the combined active ingredients imazamox and imazelhapyr.The active ingredient flucarbazone-sodium is equally popular, with 29 o/o of producers surveyed (2002) in Manitoba using herbicide products containing this active ingredient.Imazamox, inazethapyr and flucarbazone-sodium, classified as Group 2 (ALS inhibitor) herbicides, are relatively persistent in soil (with reported half-lives of 20-30, 60-90 and 17 days, respectively), and hence herbicide residues may damage subsequent sensitive crops when herbicide residues persist and are bioavailable to the plant by root uptake.In addition, herbicide residues may persist into years when other Group 2 herbicides are applied.In 2002, 37 o/o ofManitoba respondents surveyed applied soil residual ALS inhibitors in successive years.Concerns have been raised about these repeated applications after f,reld agronomists reported increased incidence of field pea injury when fields were treated with imazamox:imazethapyr (1:1) following flucarbazone-sodium applications in the previous year.No published research was found on the phytotoxicity of imazamox:imazethapyr (1:1) in Manitoba soils, and only one study was found for flucarbazone-sodium phytotoxicity in Manitoba soils.Tliis M.Sc.project utilized an oriental mustard root bioassay applied to four Manitoba soils to determine the irnpact of soil properties, nitrogen applications, herbicide co-applications, soil moisture conditions and soil temperature on herbicide dissipation, particularly phytotoxicity.Root length, as a percent of control, was the response measured in the bioassay that has been shown an effective indicator of flucarbazone-sodium phytotoxicity.Results of the phytotoxicity experiments described in Chapters 2 and 3 demonstrated that oriental mustard was generally more sensitive to imazamox: imazethapyr (1:1) than to flucarbazone-sodium residues in soil.For both herbicides, phytotoxicity showed an inverse correlation with soil organic carbon content, suggesting that herbicide sorption by soil decreased the bioavailability of herbicide residues to plants.Quantification of the sorption of imazamox and imazelhapyr by each of the four soils conhrmed this, as a negative correlation between sorption and phytotoxicity was observed.The effect of nitrogen on herbicide phytotoxicity was dependent on soil characteristics, the concentration of nitrogen applied, and the concentration of herbicide applied.The effects of herbicide co-application were additive or synergistic (i.e.stacking) or antagonistic depending on soil characteristics and the amounts of herbicide residues in soil.For the dissipation experiments described in Chapter 4, soils were incubated with lrerbicides at a range of moisture contents (50, 75 or 100 o/o field capacity), a range of temperatures (5, 15 or 25"C), or a range of soil nitrogen concentrations (0, 75 or 150 kg N ha-r).Results indicated that the phytotoxicity throughout incubation of both imazamox:imazethapyr (1:1) and flucarbazone-sodium was smallest at 100 o/o fteld capacity and at 25oC and that herbicide phytotoxicity increased with decreasing soil iii moisture contents or soil temperatures because of the lesser herbicide degradation in drier and cooler soils.Soil moisture had a greater effect on the dissipation of imazamox: imazethapyr (1:1), while root length response in flucarbazone-sodium-treated soils was more affected by declining temperature.Effects of soil nitrogen treatments on herbicide dissipation were rninimal for flucarbazone-sodium, but pronounced for imazamox: imazethapyr (1:l), where phytotoxicity increased with increasing soil nitrogen level, suggesting that the addition of nitrogen to soil increases herbicide sensitivity.This research supports the notion that weed control or crop injury is not determined by the total chemical concentration of the herbicide in soil, but by the bioavailability of the herbicide residues to the plant.As demonstrated, herbicide bioavailability and hence phytotoxicity is influenced by many factors, some of which interact.In order to minimize the potential for crop damage following the use of imazamox:imazetltapyr (1:1) or flucarbazone-sodium, well-planned rotations must be devised, particularly for soils that are of coarse texture, with low organic carbon contents and that are dry and cool throughout the growing season.

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