Pituitary and ovarian responses to gonadotropin releasing hormone, calf removal and progestogen in anestrous beef cows.

Effects of Syncro-Mate B (SMB) and(or) short-term calf removal (CR) on pituitary and ovarian responses after gonadotropin releasing hormone (GnRH) were studied in two experiments. In Exp. 1, 24 thin, 2-yr-old, anestrous, Brahman crossbred cows were allotted randomly to a 2 x 2 factorial design involving 27-h CR and SMB. All cows were injected (iv) with 150 /ag GnRH at 24 h after removal of implants and(or) calves. Blood samples were collected at 3 0 , 0, 15, 30, 45, 60, 90, 120 and 180 min post-GnRH. The GnRH-induced release of luteinizing hormone (LH) differed among treatments (P<.05). The release of LH was higher in cows with CR than in control cows (P<.05). Although pretreatment with SMB increased the quanti ty of LH release compared with the CR and control cows, SMB + CR did not show an additive effect over SMB alone. In Exp. 2, 49 mature Charolais x Hereford cows were allotted randomly to one of the following groups: control, 24-h CR or 32-h CR. Follicular size was measured by palpation per rectum at