A survey of southern New South Wales to determine the level of herbicide resistance in brome grass and barley grass populations

Herbicide resistance has developed in many populations of the two major annual grass weed species, annual ryegrass and wild oats, found in the cropping region of southern New South Wales. In those weed species resistance is a major factor in weed management. Management practices for control of resistant annual ryegrass or wild oats may encourage the growth of brome or barley grass. Approximately 4 million ha of southern New South Wales were surveyed for herbicide resistance in 2007. Where present, samples of brome and barley grass were collected resulting in 10 brome grass and 13 barley grass samples from 22 locations. Each accession was screened with five herbicides (Groups A 'fop', A 'dim', C, L and M) with none found to be resistant to any of the tested herbicides. This was despite high levels of herbicide resistance in both the annual ryegrass and wild oat populations collected from the same paddocks. Resistance to a Group A 'fop' herbicide was recorded in 82% of annual ryegrass and 57% of wild oat accessions from these sites, while 21% of the annual ryegrass accessions tested were also resistant to the Group A 'dim' herbicide clethodim. While not all herbicides used for the control of annual ryegrass or wild oats are also used for brome grass and barley grass, many are. That no brome or barley grass populations were resistant, despite the selection pressure placed on them when targeting annual ryegrass and wild oats, shows that herbicide resistance is yet to develop in these two species in the surveyed area.