RESUMEN
Diurnal changes in percents of inorganic phosphorus in plasma were measured in three Holstein cows fitted with indwelling jugular catheters. Blood was sampled 34 times over 48 h. Changes of inorganic phosphorus of blood plasma appeared to be related to patterns of feed consumption. In a second experiment, effects of diet, season of calving, stage of lactation, lactation number, and milk yield on inorganic phosphorus of plasma and milk were measured with 40 Holstein cows. Dietary treatments were 1.0% calcium, .31% phosphorus; 1.0% and .54%; 1.8% and .30%; 1.7% and .54%. Blood and milk samples were taken at wk 6 postpartum and every 5th wk thereafter. Inorganic phosphorus in plasma and milk was higher for cows in first lactation than multiparous cows. Cows which calved in November to December had the highest inorganic phosphorus in plasma but the lowest in milk. Month of year affected inorganic phosphorus in milk but not in plasma. Inorganic phosphorus in plasma increased as milk yields decreased and as inorganic phosphorus in milk decreased. Dietary phosphorus affected inorganic phosphorus in plasma but not in milk. The correlation between inorganic phosphorus in milk and plasma was -.15. These data indicate the limitations of using inorganic phosphorus in plasma or milk as sole or primary means of determining nutritional phosphorus status of lactating cows.