S U M M A R YIn an experiment, carried out over 3 years, two groups of dairy cows offered ad libitum access to grass silage received 8 kg/day concentrate differing only in phosphorus content (low P concentrate: 4 0 -4-5 g/kg dry matter (D.M.); high P concentrate: 60-6-5 g/kg D.M.) during the winter feeding period of approximately 6 months. During the summer period all animals were grazed together on perennial ryegrass pastures. There were no consistent significant effects of P content on any of the variables measured. The mean calving indices were 371 (S.E. = 3-71, n = 122) and 379 (S.E. = 4-28, n = 95) days and overall conception rates were 0-70 and 0-68 for the low and high P treatments, respectively. Mean plasma P concentrations during the winter period were significantly reduced (P < 001 or greater) in years 2 and 3 of the study on the low P treatment. In 1 year milk yield was significantly higher (P < 001) on the low P treatment although the fat content was significantly reduced (P < 005) but these effects were not recorded in the other 2 years. e s t ' m a t e s f°r sheep were reduced from 43-5 mg (ARC, 1965) to 140 mg P/kg live weight (LW) per The Technical Review of the nutrient requirements of day. The estimates for cattle were reduced from farm livestock by the Agricultural Research Council between 12 and 28 mg to 12 mg P/kg LW per day. (1965), contained estimates of the requirements of More recent information (Braithwaite, 1984) sugvarious classes of livestock for essential minerals.