Pastoral weeds in New Zealand: status and potential solutions

Currently there are some 187 plant species, almost all exotic in origin, occurring as weeds in pastures in New Zealand. Judging from their occurrence in scientific papers published in the proceedings of the New Zealand Plant Protection Society, 65 of these species are, or have been considered historically, to be significant pastoral weeds. While 34 of these pastoral weeds are currently being managed under Regional Pest Management Strategies, 15 of these regionally managed species are apparently un-researched in New Zealand, implying that their management does not have a scientific basis. The aggregate cost of pastoral weeds to the New Zealand economy is estimated to be NZ$1.2 billion per annum, based on an analysis made in 1984, but this analysis is hampered by a lack of both objective data on the impacts of weeds on pastoral production and an accurate and comprehensive national census of the main problem species. Ongoing naturalisations, from an existing pool of exotic plant species estimated currently at 25 000, and new arrivals through international trade in plant species and germplasm, can be expected to steadily add to, the list of pastoral weeds. Prioritisation of these weed species will be necessary if New Zealand's pastoral agricultural industries are to make longterm, economically optimal decisions about their management. Such prioritisation will require robust models of their current and potential distributions, their rates of population increase and spread, a full understanding of the merits and pitfalls of alternative control options, and robust models of their impacts on pastoral productivity. Currently there are some 187 plant species, almost all exotic in origin, occurring as weeds in pastures in New Zealand. Judging from