Giant buttercup (Ranunculus acris L. subsp. acris), a weed of European origin with a potential distribution embracing all of New Zealand, currently infests pastures in six of 17 dairying regions. It reduces the quantity of pasture consumed by deterring grazing, but its impact on whole-farm profitability is not well understood. To redress this, the effect of the weed and the impact of herbicides varying in efficacy were modelled with Farmax Dairy Pro®. On a dairy farm "typical" of the Golden Bay area, with the ground cover of giant buttercup peaking at 12% in November (the average per paddock cover measured on an infested farm), profit was reduced by $1040/ha ($1830 vs. $2870). Synthetic herbicides applied at label rates increased profitability, but only where the control was better than ca. 30% with MCPA or ca. 60% with flumetsulam. By contrast, a hypothetical biological herbicide giving 50% control had a break-even cost of $485/ha. The models show that giant buttercup reduces the profitability of a typical Golden Bay dairy farm by 36% and that its effective control can bring large financial gains. Keywords: dairy pasture, economics, Farmax Dairy Pro®, model, profitability, weed control
