EXTENDED FUNCTION OF THE CORPUS-LUTEUM AND EARLIER DEVELOPMENT OF THE2ND FOLLICULAR WAVE IN HEIFERS TREATED WITH BOVINE SOMATOTROPIN

Effects of recombinant bovine somatotropin (bST) on growth of the corp us luteum (CL) and development of ovarian follicles were tested. Start ing at estrus (Day=0), the following treatments were administered: con trol (saline injected Days 0 to 19, n=5); bST[0-9] (25 mg bST injected Days 0 to 9, saline injected Days 10 to 19, n=5); bST[10-19] (saline injected Days 0 to 9, 25 mg bST injected Days 10 to 19, n=5); and bST[ 0-19] (25 mg bST injected Days 0 to 19, n=6). Blood was collected dail y for progesterone analysis, and ultrasound examinations were performe d daily for measurement of follicles and CL. Compared with the heifers treated with saline, those treated with bST had larger CL and more pr ogesterone during the early (less than or equal to Day 10) and late (D ays 15 to 21) phases of the estrous cycle. At mid-cycle (Days 10 to 15 ), luteal tissue area and progesterone levels were similar for bST- an d saline-treated heifers. Ovarian follicular growth was similar for bS T- and saline-treated heifers during the first follicular wave. Howeve r, during recruitment for the second follicular wave (Days 10 to 14), second largest follicles emerged sooner (P<0.001), the number of Class -3 (greater than or equal to 10 mm) follicles was greater (P<0.01) and largest follicles were smaller (P<0.001) in bST than in saline-treate d heifers. Estrous cycle length and ovulation rate were similar for ea ch group. In conclusion, bST increased initial development of the CL a nd extended its function. Furthermore, the second follicular wave was earlier with bST.