Ninety-six 7-month-old Hereford heifers (two replicates of 48, one year apart) were individually fed for 2 years a basal ration containing .14% phosphorus on an "as fed" basis, which approxiamted 66% NRC recommendations. Forty-eight of the animals were limited to this low phosphorus (P-) diet. The other 48 (P+) received sufficient monosodium phosphate, top dressed on the basal ration, to elevate phosphorus intake to .36% on an "as fed" basis (174% NRC recommendations). The average daily weight gain for all groups was .45 kg. Feed efficiency was similar for the two groups. There was no evidence of lack of appetite or depraved appetite in either group; nor was there a difference in age of puberty. The P-cattle had a 96% pregnancy rate with 91% live calves compared to 100% and 93%, respectively, for the P+ cattle. These differences were not significant. After 9 months on trial (approximately 16 months of age) no difference in rib bone morphology was discernable from microradiographs. Blood and serum levels of phosphorus temporarily increased in the P+ cattle, but at 16 months of age were comparable to the less fluctuating values in the P-cattle. The phosphorus levels for bone and muscle were similar each year and for each treatment. Urine and fecal phosphorus excretion was significantly higher in the P+ than in the P-cattle.

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