The ecology of giant buttercup in Golden Bay dairy pasture : A thesis submitted in fulfilment of the requirements for the Degree of Master of Applied Science at Lincoln University

Abstract

Giant buttercup (Ranunculis acris L.) is a major weed of Golden Bay dairy pastures but is controlled by sheep and deer. Phenoxy herbicide resistance has developed in on Golden Bay dairy farms as a result of regular application of MCPA and MCPB over 25 years. The objectives of this research were to study the seasonal growth of giant buttercup in Golden Bay dairy pasture and to survey farmer experience in attempting to control It. The effect of increasing soil fertility on giant buttercup was investigated. Some management strategies for controlling the weed are discussed. Seasonality observations showed that seedlings took four weeks from germination to grow their first leaf. Rhizome growth occurred in midsummer, immediately following the peak of flowering. Vegetative reproduction also occurred by axillary bud development. Other aspects of phenology were similar to previous ecological studies. Farmers with high giant buttercup populations used a wider range of control strategies than herbicides only. Herbicide usage did not correlate with giant buttercup populations. Pugging increased the amount of giant buttercup in pasture. Survey results suggested that some other factors were more limiting on milk production than giant buttercup populations. Three field experiments to study the influence of soil fertility on giant buttercup in dairy pasture were set up in September 1990. Four rates of lime (0, 1.0., 2.5 and 5.0t/ha},two rates of 30% potassic super phosphate (0 and 400kg/ha) and two rates of urea (0 and 100kg/ha) were applied on two sites with contrasting soil types on one dairy farm. On another soil type the 30% potassic super phosphate and urea rates were doubled. This site was a runoff usually grazed by calves or heifers and was less intensively farmed. Within dairy pasture giant buttercup crown leaves were typically taller than grass. During giant buttercup flowering this was by 4.3 cm on average. On the less frequently grazed runoff giant buttercup was usually 2.1cm shorter than grass but during flowering was taller by 3.3 cm. Giant buttercup cover was decreased up to 3.6% (p=0.036) with 30% potassic super phosphate application on the lower terrace only. Other fertilizers had no significant influence on giant buttercup cover. Grass and clover cover was increased on the lower terrace and the runoff with 30% potassic super phosphate application. The same treatment caused a decrease in the weed proportion on the runoff. Urea application increased grass cover and decreased clover cover on each site. Giant buttercup and grass height increased with both 30% potassic super phosphate and urea application on the upper terrace and on the runoff. On the lower terrace only grass height responded to urea. There was no height response to 30% potassic super phosphate on the lower terrace. Urea application resulted in smaller giant buttercup crown diameters on the upper terrace. A decrease of up to 58mm was observed (p=0.018). While Golden Bay dairy farmers have potential to control giant buttercup with herbicides if current recommendations are closely followed, integration of sheep onto dairy farms is likely to improve giant buttercup control. Soil fertility alteration is unlikely to assist giant buttercup control however the impact of increased available nitrogen requires further study.