There are limitations to the production and persistence of white clover in drought prone regions. We report on the population dynamics of white clover during the summer/autumn drought of 2000. Seventeen paddocks were selected from Marlborough, Mid Canterbury, South Canterbury and North Otago on the east coast of the South Island, and Wairarapa in the North Island. The pastures were sown in 1998/99 and are comprised of ryegrass, tall fescue, cocksfoot or combinations of these as the grass base, with white clover included in the mix. White clover populations were monitored pre- and post-drought using tiller cores. The percentage of each region's long-term mean rainfall received during the 6-month "drought period" had the largest influence on the presence of white clover in these pastures. Marlborough, with only 73 mm of rainfall during November 2000-April 2001 (23% of its expected rainfall) suffered a 95% reduction in white clover. Mid Canterbury, South Canterbury and North Otago, at 53-61% of expected rainfall had a 40-78% loss. In contrast, Wairarapa, with 303 mm of rainfall, lost only 8%. The rate of recovery was closely related to the magnitude of white clover loss during drought. In regions where large losses occurred, further sampling after 18 months showed little increase in white clover presence. Key words: drought, persistence, populations, seedling, Trifolium repens, white clover

Water Use Efficiency (WUE) is an important performance indicator of sustainable production, widely used overseas. WUE is defined in a number of ways, and it is important to know what definition is being used. Overseas data show that WUE on dairy farms depends on both the efficiency of applying water to the paddock, and the efficiency of feed utilisation off the paddock. Pasture WUE requires measurements of the amount of water applied and the amount of pasture produced, which are rarely concurrently measured on dairy farms. Estimates of pasture WUE from previous research experiments are highly variable because of uncertainties about how much water was applied. We have suggested a benchmark for WUE based on measurements on commercial dairy farms, and suggest some ways that farmers can improve their irrigation practices. Keywords: water use efficiency, pasture yield, irrigation, evapotranspiration

Influence of structure and composition of ryegrass and prairie grass-white clover swards on the grazed horizon and diet harvested hy sheep Abstract Diet selection from ryegass-and prairie grasswhite clover swards, vertically stratified into three horizons (A>6 cm, B 3-6 cm, C>3 cm), was studied using oesophageally fistulated sheep during summer and autumn. Animals grazed for 3-day periods. Apparent herbage intake was calculated from total herbage disappearance. The composition of each horizon and of the diet selected was measured daily.Herbage mass (DM ha"') and sward height (cm) prior to grazing were not significantly different between swards in each season, and were 20 and 20 in summer and 1-6 and 10 in autumn. In summer. 36% and 5% of the green grass leaf (GGL) for prairie grass and ryegrass, respyectively, was distributed in horizons A and B. In autumn 39% and 29% of GGL occurred above 3 cm for prairie grass and ryegrass, respectively. GGL distribution determined which sward horizons were grazed. Sheep grazed horizon C (0-3 cm) of summer ryegrass pasture, and the surface canopy (>3 cm) of all other swards.In summer, apparent intake achieved by sheep grazing prairie grass swards was 87% higher than that achieved on ryegrass swards. In autumn a greater GGL distribution above 3 cm with prairie Correspondence; P. J. L'Huillier, Ruakura Agricultural Research Centre, Private Bag, Hamilton, New Zealand. grass (39%) than with ryegrass (29%) was associated with 36% higher apparent intake.

New Zealand pastoral farming has benefited greatly from the application of phosphorus (P) and sulphur (S) fertilisers supplied in the main by superphosphate (SSP). The long-term fertiliser trial at Winchmore, mid-Canterbury, New Zealand was set up in 1952 and has yielded a wealth of data on the effect of fertiliser, grazing by sheep and flood irrigation on pasture production. The trial was initially (1952Á1958) designed to measure the effect of no fertiliser and SSP applied each year at 188, 376 and 564 kg ha . The cessation of fertiliser decreased clover content and increased the proportion of weeds and low-fertility grasses. The decline in production once fertiliser application ceased followed a curvilinear pattern, but never reached the low production of the no fertiliser treatments even after 20 years. In 1980, the residual treatments were changed to compare a reactive phosphate rock (RPR)/S treatment as well as an intermediate (250 kg (1 y (1 treatments have shown that without any nitrogen fertiliser, ryegrass and clover will persist in irrigated pastures and result in high levels of pasture production (11Á12 t ha

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