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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.

The repeated use of glyphosate to control annual ryegrass along fence lines and crop margins has resulted in the evolution of resistance to this herbicide in populations of annual ryegrass (Lolium rigidum) in cropping regions of Australia. Field trials were conducted between 2009 and 2011 at four fence-line sites in South Australia to identify suitable herbicide treatments for controlling glyphosate-resistant annual ryegrass. Annual ryegrass populations growing at all four sites were found to have 12–24-fold resistance to glyphosate compared with a standard susceptible population in dose-response experiments. Glyphosate alone (1080g ha-1) did not effectively control glyphosate-resistant annual ryegrass at any location. Single applications of paraquat+diquat and paraquat+amitrole were effective where weed populations were low, with up to 99% reduction in seed-head production. Mixtures of paraquat+diquat+diuron, glufosinate ammonium+diuron and two applications of paraquat+diquat 14 days apart consistently provided high levels of control of glyphosate-resistant annual ryegrass at all locations, with >90% reduction in seed-head production. In 2011, glyphosate resistant individuals of annual ryegrass were identified up to 50m inside the fields adjacent to the fence. Therefore, failure to control glyphosate-resistant annual ryegrass along crop margins risks movement of resistance into adjacent cropped fields.

Herbicide-resistant populations of annual ryegrass (Lolium rigidum) are estimated to affect crop production on about 5000 farms in southern Australia. In order to manage resistant populations, some farmers have adopted a two-to-three-year pasture phase which allows use of grazing by sheep, and non-selective herbicides to deplete the weed seed-bank. However, in low-to-medium rainfall zones, where financial returns from pastures are relatively low, farmers have generally combined cultural practices for weed management with the use of alternative herbicides, mainly trifluralin. Used singly, none of the currently available cultural techniques provides an adequate level of weed control. However, when used in carefully planned combinations, extremely effective ryegrass control can be achieved. Some of the important cultural practices for ryegrass control include delayed sowing (sometimes in conjunction with a shallow autumn cultivation); stubble burning; cutting the crop for hay or green manure, increased crop density and capture of weed seeds at harvest. Selection of crop species and cultivars with superior weed suppression potential is also receiving considerable attention. ©1997 SCI