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Abstract
Nine field experiments were conducted in 2011 and 2012 at various locations in southern Ontario, Canada to determine the tolerance of soybean (Glycine max (L.) Merr.) to herbicides inhibiting protoporphyrinogen oxidase (Protox) and very long chain fatty acid (VLCFA) synthesis applied alone and in combination. Preemergence applications were evaluated for soybean injury, plant height, shoot dry weight, and yield in the absence of weed competition. Early-season soybean injury from the Protox inhibitors persisted 4 weeks after soybean emergence (WAE) with 3%, 5%, and 18% injury for flumioxazin, saflufenacil, and sulfentrazone, respectively. When Protox inhibitors were tank mixed with VLCFA inhibitors (i.e., dimethenamid-P, S-metolachlor, and pyroxasulfone), additive interactions were observed for injury with saflufenacil and sulfentrazone; whereas synergistic interactions were observed with flumioxazin. However, injury subsided over time decreasing from as much as 34% injury 1 WAE for the flumioxazin + S-metolachlor tank mix down to 9% injury 4 WAE. In general, when saflufenacil or flumioxazin were tank mixed with VLCFA inhibitors, greater than expected reductions in height and dry weight were observed indicating synergistic responses; while no interactive effects were detected with sulfentrazone and VLCFA inhibitor tank mixes. For the flumioxazin tank mixes that contained dimethenamid-P or S-metolachlor, the reduction in yield was greater than expected indicating synergistic interactive effects. Yet, all the demonstrated impacts were transient as the yield for soybean treated with any of the Protox inhibitor and VLCFA inhibitor tank mixes tested were similar to the untreated control. Therefore, usage restriction on these mixtures, based on perceived negative yield impact, should be lifted so the herbicides could be combined to expand weed control options.
Highlights
Herbicide premixes and tank mixes are commonly used in weed management to increase the spectrum of weeds controlled and, in glyphosate-resistant crops, increase the diversity of modes of action used in production systems often dominated by a single herbicide [1]-[3]
Similar levels of soybean injury have been reported for saflufenacil [21] [24]; while flumioxazin and sulfentrazone injury in soybean has been shown to vary between years [22] [23] and substantially among cultivars [25]
For sulfentrazone and saflufenacil, paired t-tests indicated that tank mixes with very long chain fatty acids (VLCFA) inhibitors resulted in additive interactions
Summary
Herbicide premixes and tank mixes are commonly used in weed management to increase the spectrum of weeds controlled and, in glyphosate-resistant crops, increase the diversity of modes of action used in production systems often dominated by a single herbicide [1]-[3]. Additive responses of herbicide combinations occur when the observed and expected responses are similar, whereas antagonistic and synergistic responses are characterized by the observed response being less than and greater than expected, respectively [5]. The herbicide label for flumioxazin, primarily a broadleaf weed herbicide that inhibits the protoporphyrinogen oxidase (Protox) enzyme [6] [7], warns users about the potential for greater than expected soybean (Glycine max (L.) Merr.) injury if tank mixed with chloroacetamide herbicides (e.g., dimethenamid-P and S-metolachlor) for additional grass weed control [8]. Significant earlyseason soybean injury has been reported when using this flumioxazin/pyroxasulfone premix; the injury was transient and did not affect yield [16]-[20]
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