Bovine Somatotropin Dose Titration in Lactating Dairy Ewes. 2. Dose Determination and Factors Affecting the Response

Abstract

Seventy-four lactating dairy ewes were injected with recombinant bovine somatotropin (bst; sometribove) in a sustained-release formulation. Ewes received 0, 80, 160, or 240mg of bST/14 d from wk 3 to 8 of lactation (part 1) and 0, 80, or 160mg of bST/14 d from wk 11 to 18 of lactation (part 2). The optimal dose of bST was studied as well as the factors (lactation stage, lactation number, initial milk production, body weight, and body condition) possibly affecting the increase in milk production following bST injection. Using a quadratic regression model, the maximum theoretical dose was determined to be 181mg of bST/14 d during the first part of lactation. During the second part of lactation, 143mg of bST/14 d was the maximum theoretical dose. The increment of milk production did not vary with lactation number, but first lactation ewes, in general, responded better than did multiparous ewes. Relative to initial milk production, improvement was greatest for ewes with average milk production (1500 ml/d) that received a dose of 192.3mg of bST/14 d during the first part of lactation; improvement was also measured from the highest producers (2000 ml/d) during the second part of lactation. The best response was obtained from ewes with average body condition (score 3 on a five-point scale where 1 = thin to 5 = fat) and a dose of 200mg of bST/14 d during the first part of lactation; during the second part of lactation, body condition score had no effect. Body weight had no effect on the increment of production at any time.