History and status of weed science in New Zealand

New Zealand has one of the highest percentages of introduced plants in the world. The early European visitors in the 1800s introduced most of our weed problems as few native species have become weedy. With expanding trade and travel, new plant species have continued to become established as until the 1990s limited controls existed on their importation. Inspection at the border has been considerably strengthened in recent years by the introduction of x-ray machines and sniffer dogs to detect undeclared fruit and vegetable matter, although weed seeds remain a problem. New Zealand economy is heavily reliant on primary industry – notably dairy, beef, sheep and timber. Farming is overwhelmingly pastoral, with less than 5% of the area under cultivation and very few farms used solely for cropping. Weeds in permanent pastures are controlled through a combination of herbicides, grazing management and mowing/topping, supplemented by biological control for a few species. Weeds in the major crops (cereals, maize, herbage seeds and vegetables) are managed mostly by herbicides. The weed seed bank is very large in cropped soils, with viable seed populations of up to 280 x 106 and up to 40 x 106 seedlings/ha recorded after cultivation in early summer. Although only a few species dominate at any one time, our maritime climate with its well distributed rainfall encourages sequential emergence of different species. Growers of permanent horticultural crops have mostly opted out of residual herbicides due to trade and environmental concerns. Organic farming is very limited in both pastoral and cropping sectors. Forestry, another land based sector important to New Zealand, is also a major user of herbicides, although it is now declining under the recently signed Forest Stewardship Council standards. Such weeds are sometimes species which ‘escaped’ from garden cultivation. A number of aquatic weeds, many of which have arrived in New Zealand as aquarium or pond plants, have major impacts on commercial and recreational activities. Relatively limited control options are available for these compared with terrestrial weeds. Herbicides have been and continue to be an important component of primary production in New Zealand. Over 3,500 tonnes of pesticide active ingredients are used every year, with herbicides amounting to nearly two thirds of this total. Herbicides constitute nearly 80% of total pesticide use in the pastoral sector, with phenoxy hormones and the non-selective (non-residuals) being the largest contributors. New Zealand soils have unusually high organic matter (>5%) levels, are acidic and some are derived from volcanic materials containing a high proportion of allophane clay. This has necessitated considerable research on improving the performance and reliability of soil-applied herbicides, while avoiding residue carry-over and environmental contamination problems. While the widely known resistance to atrazine and chlorsulfuron has appeared in some cropping weeds, some of the interesting and novel herbicide resistance problems in New Zealand are associated with applications to perennial pasture. Resistance to glyphosate in perennial and annual ryegrass has been detected very recently.