Influences of Calcium and Phosphorus Intakes, Vitamin D Supplement, and Lactation on Calcium and Phosphorus Balances

Calcium and phosphorus balances were measured on 45 Holstein cows during the second and fifth months of lactation and during the dry period.C'ows were conditioned with and continuously fed alfalfa hay-sorghum grain-soybean oil meal rations with mineral supplement to provide Ca:P ratios near 1.1:1 or 2.3:1, either with or without 300,000 IU vitamin D a weekly by capsule.Supplemental vitamin D improved predictability of Ca balance from Ca and P intakes and milk Ca.Calcium requirement for lactation, estimated from balances with vitamin D supplemented cows, was 5 g Ca per kilogram milk.Phosphorus requirement, estimated from P balances associated with positive Ca balances, was 2.3 g per kilogram milk.These requirements were estimated from the intakes at which equilibrium was probable for the respective element.