Comparing herbicide resistance in New Zealand and Australia.

Although New Zealand is developing an increasing number of herbicide-resistant weed populations, it has a much lower incidence of herbicide resistance than Australia. Understanding the reasons for these differences may help with future management of herbicide resistance in both countries. Australia is much larger than New Zealand so greater areas of weeds are exposed to herbicides annually in Australia. Another difference is the high frequency of Lolium rigidum in Australian agricultural systems, a species almost absent in New Zealand, and many of the cases of resistance in Australia involve this species. However, species closely related to L. rigidum are increasingly being found resistant to herbicides in New Zealand. Higher rainfall in most parts of New Zealand allows much more crop rotation than is possible in Australia, and greater crop yields give New Zealand farmers more flexibility to use higher application rates and more expensive herbicides in rotation than is feasible in Australia. Dry conditions result in more use of summer fallows in Australia using glyphosate which has caused some of the problems. Selection pressure for resistance still occurs in New Zealand, so herbicide and crop rotation may just be delaying the appearance of resistance in this country. [ABSTRACT FROM AUTHOR]