The phosphorus (P) balances and 32P kinetics of 15 Droughtmaster heifers were studied in mid and late pregnancy and early and late lactation to compare their P absorption coefficients and their P endogenous faecal losses with those of growing cattle. The heifers continuously grazed three paddocks of tropical pasture for a year to provide a range of P intakes.

The daily dry matter (DM) and P intakes of the heifers were generally much higher during lactation (17–26 g DM/kg LW and 11–40 mg P/kg LW) than during pregnancy (10–16 g DM/kg LW and 6–22 mg P/kg LW), due in part to the heifers calving at the beginning of the wet (pasture growing) season. Plasma inorganic P levels decreased as the experiment progressed and were very low (<30 mg/l) in all treatments in late lactation. Milk and milk P secretion were high in early lactation (4·0–5·5 l/day and 12–16 mg P/kg LW per day) so the heifers were in more negative balance during lactation than during pregnancy.

The coefficient of P absorption was similar to that previously observed in growing cattle consuming forage diets and it was concluded that 0·75 was the appropriate value to use in the estimation of requirements. Endogenous faecal losses ranged from 7 to 17 mg/kg LW and were related to P intake or DM intake plus plasma inorganic P. Comparison of the regressions with those from the larger data set for growing cattle showed that the endogenous faecal losses were similar and there were no special faecal endogenous losses in breeding cattle.