Rib bone samples were taken from free-grazing, crossbred, beef-type cows at different stages of their reproductive cycles to monitor the P status of supplemented and unsupplemented groups. Two experimental sites, Glen and Armoedsvlakte were used, the latter notorious for its Pdeficient pastures. P content of bone samples (expressed as mg P/cm3 fresh bone) proved a reliable and sensitive indicator of the P status of grazing cattle. Concentrations suggesting adequacy (140 -150 mg P/cm3) agree with those pUblished elsewhere. During lactation, concentrations below 100 mg/cm3 were observed in the severely P-deficient (- P) group at Armoedsvlakte. There was no suggestion of even a sub-clinical deficiency at Glen; the only advantage of supplementation was maintenance of higher bone mineral reserves during late lactation, especially in the young heifer.
