INFLUENCE OF BODY SIZE DURING LACTATION ON LEVEL OF MILK PRODUCTION

The influence of body weight on milk fat and solids-not-fat yield declined with increasing parity. The decline was more evident in Holsteins than in Ayrshires and Jerseys. Standard partial regression coefficients demonstrated that liveweight 180 days after freshening had a negative relationship with yield, when age at freshening and freshening weight were held constant. Also, wither height at first freshening had a negative relationship with yield when freshening weight and age were constant. These results show that body condition at freshening and weight loss during lactation are as important as actual weight in determining yield. Since body condition at first freshening affects yield-age relationships within and between herds, it has important implications in the evaluation of sires from first-lactation daughter performance. An objective and practical measure of optimum body condition is needed, particularly if maximum growth and minimum breeding age are to be achieved.