Calcium and Phosphorus Supplementation of Rations for Lactating Cows

Forty Holstein cows were assigned among four dietary treatments that resulted in dietary calcium and phosphorus: 1) 1.0%, .31%;2) 1.0%, .54%;3) 1.8%, .30%;and 4) 1.7%, .54%.Calcium was supplemented as ground limestone and phosphorus as sodium tripolyphosphate.Treatment diets were fed from parturition until 10 mo postpartum.Cows fed diets low in phosphorus had lower milk yields, feed intakes, and poorer efficiencies of milk production.Inorganic phosphorus in blood plasma was reduced by low phosphorus intake.Overall, calcium of the diet did not affect milk production.However, there was a significant calcium-phosphorus interaction with increased milk production and greater feed intake when calcium was added to low phosphorus diets but not to diets containing excess phosphorus.Added calcium did not affect inorganic phosphorus, calcium, or magnesium of blood plasma.Fecal pH was increased and percent starch in the feces decreased with added dietary limestone.Efficiency of milk production was not improved by added limestone.Maximum milk yields and performance cannot be obtained with .3%dietary phosphorus.