File identification only – 57 61 Abstract Giant buttercup (Ranunculus acris L.) is estimated to cost the New Zealand dairy industry $118 million annually in lost milksolid production opportunity through the exclusion of palatable pasture species. Historical reliance by dairy farmers on annual or biennial applications of the ‘phenoxy’ herbicides MCPA and MCPB, has led to the widespread evolution of phenoxy-herbicide-resistant populations of the weed. The newer herbicides, thifensulfuron methyl and flumetsulam are now being used against giant buttercup, although thifensuluron, like MCPA, can cause clover damage and as a result may promote the growth of giant buttercup populations. In this paper we review the history of giant buttercup management on New Zealand dairy farms and the science that led to the discovery of the herbicide resistance. We also attempt to define a path toward sustainable ecologically-based management of this weed based upon an improved understanding of its population dynamics and the use of a pasture-selective mycoherbicide utilizing the naturally-occurring fungus, Sclerotinia sclerotiorum. Keywords: dairy pasture, economics, giant buttercup, herbicide resistance, mycoherbicide, opportunity cost, population dynamics, Ranunculus acris, weed Introduction Giant buttercup (Ranunculus acris L.) occurs in waste land, roadsides, river flats, swamp margins and pastures in New Zealand (Webb et al. 1988). It is a common weed of dairy pastures in the Tasman district (Bourdôt & Hurrell 1990), in pastures in the South Auckland, Hawkes Bay and Taranaki regions, and in the districts of South Wairarapa and Horowhenua (Tuckett 1961). While it is absent from most dairy pastures in Canterbury and Southland, localized populations, mainly on non- agricultural land pose a threat to dairy pastures in these regions. Giant buttercup may also occur in pastures grazed by sheep and other livestock, but is not a serious weed in such pastures, usually persisting as stunted plants that fail to reach above the pasture canopy. Giant buttercup replaces pasture grasses and clovers in dairy pastures through competition and is toxic to dairy cattle when eaten due the production of an acrid- tasting glycoside, ranunculin (Conner 1977). Both giant buttercup and neighbouring pasture plants are avoided by grazing dairy cattle (Harper & Sagar 1953). Economic impact Since giant buttercup replaces useful pasture species and is essentially not palatable to dairy cows, the opportunity cost of its presence on a dairy farm can be considered equal to the potential production value of the land area that it covers. A key assumption is that the gain in pasture dry matter made available by removing giant buttercup on infested farms would be translated into a gain in milk solid production by improved per cow performance and/ or increase in herd size. This assumption is reasonable given the concern expressed about the production impacts of giant buttercup particularly in the Taranaki region and the Tasman district where the weed is included in Regional Pest Management Strategies. In a recent study applying this concept, the seasonal progression of the weed’s ground cover in dairy pastures in the Tasman district was measured. This was used in conjunction with monthly pasture dry matter production values (excluding pasture conserved as hay and silage and restricting calculations to the milking season) to estimate the mean annual percentage loss in pasture production due to giant buttercup in a paddock. This estimation assumed that the conversion of pasture dry matter to milk solids is constant throughout the milking season. Calculations using this weighted mean paddock-scale annual loss, 33.2%, an estimate of dairy- pasture land in the Tasman district infested, 30% (estimated by local regional council and dairy industry consultants), and the monetary value of milk solid production in the district, NZ$65 million (calculated from data in “Dairy Statistics 1999-2000” (Anonymous 2000)), revealed an opportunity cost for giant buttercup in the Tasman district of NZ$7.2 million p.a. at 2000- 2001 prices. The opportunity costs of giant buttercup in the five other regions/districts known to be infested were estimated assuming that the 33.2% annual loss in pasture dry matter production measured in Tasman applies equally to infested pastures in these other regions; data were not available for these regions. The areas of dairy pasture land infested in these districts were estimated by local regional council and dairy industry consultants to be 5, 10, 35, 10 and 10% for South Auckland, Hawkes Bay, Taranaki, South Wairarapa and Horowhenua respectively. The estimated opportunity costs were summed across the six regions/ districts giving a national opportunity cost of $118 million p.a. Giant buttercup ( Ranunculus acris L.) management in dairy pastures – current problems and future solutions G.W. BOURDÔT and S. LAMOUREAUX AgResearch, PO Box 60, Lincoln 8175, New Zealand graeme.bourdot@agresearch.co.nz