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Rigid ryegrass is the most-troublesome, herbicide-resistant weed in cropping systems of southern Australia. Field experiments were undertaken at Roseworthy, South Australia, in 2013 and 2014, to identify effective herbicide options for the control of clethodim-resistant rigid ryegrass in Clearfield canola. PPI trifluralin + triallate followed by (fb) POST imazamox + imazapyr + clethodim + butroxydim had the lowest plant density of rigid ryegrass in 2014 and provided superior control compared with the standard grower practice of PPI trifluralin + triallate fb POST imazamox + imazapyr + clethodim in 1 of 2 yr. Propyzamide either alone or as a split application (PPI fb POST) or in combination with clethodim provided similar rigid ryegrass control to that of the standard grower practice (38 to 553 plants m−2). Rigid ryegrass treated with PPI dimethenamid-P, pethoxamid, pethoxamid + triallate, and PPI trifluralin fb carbetamide POST produced significantly more seeds than the standard grower practice, which would lead to reinfestation of subsequent crops. Canola yield responded positively to effective herbicide treatments, especially in 2014, when rigid ryegrass density was greater. PPI dimethenamid-P and pethoxamid alone or in combination with triallate and propyzamide were ineffective in reducing rigid ryegrass density and seed production to levels acceptable for continuous cropping systems.
Rigid ryegrass, an important annual weed species in cropping regions of
southern Australia, has evolved resistance to 11 major groups of herbicides.
Dose–response studies were conducted to determine response of three
clethodim-resistant populations and one clethodim-susceptible population of
rigid ryegrass to three different frost treatments (−2 C).
Clethodim-resistant and -susceptible plants were exposed to frost in a frost
chamber from 4:00 P.M. to 8:00 A.M. for three nights before or after
clethodim application and were compared with plants not exposed to frost. A
reduction in the level of clethodim efficacy was observed in resistant
populations when plants were exposed to frost for three nights before or
after clethodim application. In the highly resistant populations, the
survival percentage and LD50 were higher when plants were exposed
to frost before clethodim application compared with frost after clethodim
application. However, frost treatment did not influence clethodim efficacy
of the susceptible population. Sequencing of the acetyl coenzyme A
carboxylase (ACCase) gene of the three resistant populations identified
three known mutations at positions 1781, 2041, and 2078. However, most
individuals in the highly resistant populations did not contain any known
mutation in ACCase, suggesting the resistance mechanism was a nontarget
site. The effect of frost on clethodim efficacy in resistant plants may be
an outcome of the interaction between frost and the clethodim resistance
Two field experiments were conducted during 2012 and 2013 at Roseworthy, South Australia to identify effective herbicide options for the management of clethodim-resistant rigid ryegrass in faba bean. Dose–response experiments confirmed resistance in both field populations (B3, 2012 and E2, 2013) to clethodim and butroxydim. Sequencing of the target site of acetyl coenzyme A carboxylase gene in both populations identified an aspartate-2078-glycine mutation. Although resistance of B3 and E2 populations to clethodim was similar (16.5- and 21.4-fold more resistant than the susceptible control SLR4), the B3 population was much more resistant to butroxydim (7.13-fold) than E2 (2.24-fold). Addition of butroxydim to clethodim reduced rigid ryegrass plant density 60 to 80% and seed production 71 to 88% compared with the standard grower practice of simazine PPI plus clethodim POST. Clethodim + butroxydim combination had the highest grain yield of faba bean (980 to 2,400 kg ha−1). Although propyzamide and pyroxasulfone plus triallate PPI provided the next highest levels of rigid ryegrass control (< 60%), these treatments were more variable and unable to reduce seed production (6,354 to 13,570 seeds m−2) to levels acceptable for continuous cropping systems.
Clethodim resistance was identified in 12 rigid ryegrass populations from
winter cropping regions in four different states of Australia. Clethodim had
failed to provide effective control of these populations in the field and
resistance was suspected. Dose–response experiments confirmed resistance to
clethodim and butroxydim in all populations. During 2012, the
LD50 of resistant populations ranged from 10.2 to 89.3 g
ha−1, making them 3 to 34–fold more resistant to clethodim
than the susceptible population. Similarly, GR50 of resistant
population varied from 8 to 37.1 g ha−1, which is 3 to 13.9–fold
higher than the susceptible population. In 2013, clethodim-resistant
populations were 7.8 to 35.3–fold more resistant to clethodim than the
susceptible population. The higher resistance factor in 2013, especially in
moderately resistant populations, could have been associated with lower
ambient temperatures during the winter of 2013. These resistant populations
had also evolved cross-resistance to butroxydim. The resistant populations
required 1.3 to 6.6–fold higher butroxydim dose to achieve 50% mortality and
3 to 27–fold more butroxydim for 50% biomass reduction compared to the
standard susceptible population. Sequencing of the target-site ACCase gene
identified five known ACCase substitutions (isoleucine-1781-leucine,
isoleucine-2041-asparagine, aspartate-2078-glycine, and
cysteine-2088-arginine, and glycine-2096-alanine) in these populations. In
nine populations, multiple ACCase mutations were present in different
individuals. Furthermore, two alleles with different mutations were present
in a single plant of rigid ryegrass in two populations.
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