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.