Calcium and Phosphorus Supplementation of Rations for Lactating Cows

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Abstract
Forty Holstein cows were assigned among 4 dietary treatments of Ca and P: 1.0%, 0.31%; 1.0%, 0.54%; 1.8%, 0.30%; and 1.7%, 0.54%, respectively. Ca was supplemented as ground limestone and P as sodium tripolyphosphate. Treatment diets were fed from parturition until 10 mo. postpartum. Cows fed diets low in P had lower milk yields, feed intakes and poorer efficiencies of milk production. Inorganic P in blood plasma was reduced by low P intake. Ca in the diet did not affect milk production. There was a significant Ca-P interaction with increased milk production and greater feed intake when Ca was added to low P diets but not to diets containing excess P. Added Ca did not affect inorganic P, Ca or Mg 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 0.3% dietary P.