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Context: Annual ryegrass (Lolium rigidum) is a major weed of crop production in southern Australia that readily develops resistance to herbicides. Resistance increases both yield losses and control costs associated with this species. Aims: This study aimed to gauge the extent and distribution of resistance to herbicides in L. rigidum across south-eastern Australian grain production systems by collecting seed from randomly selected fields. Methods: A total of 1441 weed populations were collected through random surveys conducted over 5 years across 13 agricultural regions of four states with these samples then tested for resistance to eight herbicides from six modes of action. Key results: Resistance to diclofop-methyl and sulfometuron-methyl was most common, being present in 64% and 63% of populations respectively. Glyphosate resistance was present in 4% of populations collected. Only 15% of populations collected were susceptible to all herbicides tested. Large differences in resistance occurred between the 13 regions surveyed with resistance to diclofop-methyl ranging from 15% to 86% of populations and sulfometuron-methyl from 12% to 96%. Resistance to post-emergent herbicides tended to be higher than pre-emergent herbicides. Multiple resistance was common with 60% of populations collected having resistance to two or more herbicide modes of action. Conclusions: There were significant differences in the extent of multiple resistance in L. rigidum populations collected from individual regions suggesting that the rates of resistance evolution have differed between regions. Implications : The high incidence of herbicide resistance in L. rigidum populations randomly collected from south-eastern Australian cropping fields highlights the need for the adoption of additional weed control practices to mitigate the impact of this species on grain production systems. Herbicide resistant weeds are a major threat to crop production, with annual ryegrass being the major species of concern in Australia. Surveys of south-eastern Australia conducted over a 5 year period found over 60% of populations were resistant to two or more herbicide groups and only 15% of ryegrass populations were susceptible to all tested herbicides. The high level of resistance found across south-eastern Australia highlights the need for farmers to adopt non-herbicide practices, in conjunction with herbicides, to control this species. [ABSTRACT FROM AUTHOR]

Conyza bonariensis (L.) Cronq. (syn. of Erigeron bonariensis L.) is a difficult-to-control summer weed species in the cropping belt of south-eastern Australia. Herbicide resistance may be affecting the ability to control C. bonariensis in the agricultural region of north-eastern Victoria; therefore, a survey was conducted to evaluate resistance to herbicides from several mode-of-action groups. Of the populations collected, 40% were resistant to glyphosate at 1080 g ha–1. Chlorsulfuron failed to control any of the populations collected. Further research identified multiple herbicide resistance to glyphosate, chlorsulfuron, metsulfuron-methyl and sulfometuron-methyl in five of nine populations fully characterised. Resistance was not found to 2,4-D, clopyralid or paraquat. There was no correlation between prevailing land use and the frequency of glyphosate-resistant populations, suggesting that resistance had been selected in multiple production systems. The high frequency of resistance could explain the difficulty experienced in controlling C. bonariensis across north-eastern Victoria and demonstrates the importance of integrated weed management to manage this weed. Flax-leaf fleabane is a difficult-to-control summer weed in Australia. This work showed that one reason for this problem is that populations are becoming resistance to glyphosate, the most common herbicide used in summer fallows. Some of these populations have multiple resistance to other important herbicides used in crop production. [ABSTRACT FROM AUTHOR]

Phalaris minor Retz. is the major grass weed of wheat, which has developed resistance to multiple herbicides working through three modes of action. The identification of suitable herbicides with different modes of action will assist in the control of this weed in resistance affected areas. Keeping this in view, the efficacy of flufenacet, an oxyacetanilide herbicide, was investigated in 2013 and 2014 at Ludhiana, India. Three doses of flufenacet (200, 250 and 300 g ha−1) were tested at three application timings on Zadoks wheat stages (Z00, Z13 and Z21) and compared with clodinafop application and weedy check for the control of resistant P. minor biotype. Flufenacet 250 and 300 g ha−1, applied at Z00 and Z13 wheat stages, had more than 85% mortality and more than 90% growth reduction of P. minor compared to the unsprayed check (76–109 plants m−2; 60–69 g m−2) at 45 days after sowing in both years, whereas clodinafop just provided 26–37% reduction in density and 39–40% reduction in biomass of this weed. Flufenacet 250 g ha−1 at Z00 and Z13 produced better wheat grain yield (4.2–4.5 t ha−1) in both years, while weed competition in the unsprayed check reduced wheat grain yield by 54–67% (1.75–1.97 t ha-1). This study suggests that flufenacet could be used to control multiple herbicide resistant P. minor in northern Indian wheat production systems. • Flufenacet 250 – 300 g ha−1 applied at Z00 and Z13, provided >85% mortality and >90% growth reduction of P. minor. • Clodinafop @ 60 g ha−1 provided only 26–37% reduction in density and 39–40% reduction in biomass of P. minor. • Flufenacet efficacy was higher in 2014 over 2013, owing to higher rainfall events in second year. • Flufenacet 250 g ha−1 applied at Z00 and Z13 produced better wheat grain yield (4.2–4.5 t ha−1) in both years. • Flufenacet provided better control of resistant P. minor population as compared to clodinafop. [ABSTRACT FROM AUTHOR]

The amino acid substitutions that confer herbicide resistance may also involve fitness costs leaving mutant plants at a competitive disadvantage compared with wildtypes. This research investigated the effect of two target–site point mutations of ACCase gene: Ile-1781-Leu/Val and Ile-2041-Asn on plant growth and fecundity of Hordeum glaucum biotypes grown in monoculture and/or under interspecific competition in the field with Triticum aestivum L. and Lens culinaris M. The amino acid substitutions at 1781 position of ACCase gene did not impose any negative pleiotropic effects on relative growth rate (RGR), panicle emergence, plant height, total biomass, and seed production in H . glaucum mutant plants. There was some evidence for fitness cost associated with Ile-2041-Asn mutation in terms of reduced RGR and reduced vegetative biomass and seed production when grown in competition with L . culinaris . The absence of measurable negative pleiotropic effects on plant growth and fecundity associated with Ile-1781-Leu/Val ACCase mutations in H. glaucum suggest that the frequency of these alleles will not decline in the absence of herbicide selection pressure. However, the 2041-Asn allele could decrease in frequency in the absence of herbicide selection pressure. [ABSTRACT FROM AUTHOR]

Abstract: Five field experiments were undertaken over a three-year period in South Australia to investigate the efficacy of imidazolinone herbicides for the control of rigid brome in imidazolinone-tolerant wheat (Clearfield™, CLF). Imidazolinone herbicides, imazapyr, imazapyr plus imazapic and imazapyr plus imazamox applied post-emergence to imi-wheat (cvs. CLF-Janz and CLF-Stiletto) at the four-leaf stage, provided consistent and high levels of rigid brome control (≥87%). In contrast, applications of the alternative herbicide mesosulfuron-methyl (post-emergence) provided limited and variable control of rigid brome (11–67%). Furthermore, imidazolinone treatments caused a large reduction (P <0.05) in rigid brome seed production (≤6 seedsm−2) as compared to mesosulfuron-methyl (461–3983 seedsm−2) and the non-treated control (2257–11 865 seedsm−2). Imazapyr plus imazapic provided consistent control of rigid brome and resulted in higher grain yields (28–45%) than wheat treated with mesosulfuron-methyl. Based on these results post-emergence applications of imazapyr, imazapyr plus imazapic and imazapyr plus imazamox to imi-wheat could play an important role in the management of rigid brome in southern Australia. However, the effect of these herbicides on existing and new cultivars of imi-wheat (multiple-gene resistance) and issues concerning persistence in low rainfall environments and crop damage on sandy soils require attention. [Copyright &y& Elsevier]