The rapid evolution of resistance to post-emergence herbicides in Lolium
spp. (ryegrass species) has complicated weed management in cereals. Ensuring
good pre-emergence control is therefore increasingly important to
protect yields. Prosulfocarb is a broad spectrum thiocarbamate herbicide
that kills plants by disrupting the biosynthesis of very long chain fatty
acids. This is the first report of resistance to prosulfocarb in Lolium species
from farm sites and glasshouse selection. The recurrent selection process
with prosulfocarb applied pre-emergence at its maximum labelled rate
(4,000 gai/ha) on a susceptible Lolium multiflorum Lam. population showed
that the evolution of resistance due to repetitive use was possible but slow.
Over three generations, it resulted in modicum levels of resistance which
were not significant at practical field rates. For the last progeny obtained,
there was no evidence of either cross-resistance or increased sensitivity to
an herbicide under-development with a mode of action similar to prosulfocarb
(RF50 = 0.99, 95% CI = 0.77 - 1.27). Likewise, high and increasing
levels of non-target-site based resistance to clodinafop-propargyl did not
result in cross-resistance to prosulfocarb. Prosulfocarb exerted low levels
of negative cross-resistance on LL1781, SS1999, GG2078 and RR2088, four
different ACCase mutant L. multiflorum sublines (average RF50 = 0.67).
In order to control susceptible and resistant populations in winter cereals,
prosulfocarb was mixed with diflufenican, metribuzin, pyroxasulfone and
the formulated mixture {iodosulfuron:mesosulfuron}. Over the 86 mixtures
tested, 11 showed a high potential. Prosulfocarb + diflufenican at 2,400
+ 32 gai/ha presented interesting levels of synergism (+20 points on average).
A non-random survey in 34 farm sites from England showed that
most Lolium samples (80%) were sensitive or only partially resistant to prosulfocarb.
However, prosulfocarb efficacy was lower where other herbicides
had been used intensively. Future research may now concentrate on the
determination of prosulfocarb resistance pathways in Lolium spp.