Pituitary and Ovarian Responses to Gonadotropin Releasing Hormone, Calf Removal and Progestogen in Anestrous Beef Cows2

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Abstract
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 × 2 factorial design involving 27-h CR and SMB. All cows were injected (iv) with 150 µg GnRH at 24 h after removal of implants and(or) calves. Blood samples were collected at −30, 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 quantity 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 × 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 the time of CR. All cows received 300 jug GnRH (im) at 24 h after CR, at which time calves were returned to cows in the 24-h CR group. The pattern of release of LH over time was similar to the control and 24-h CR groups; however, LH release was greater (P<.05) in the 32-h CR group. A linear effect of follicular size on the pattern of release of LH after GnRH injection was observed regardless of treatment. Plasma concentration of progesterone at 7 and 14 d after GnRH, indicated that function and lifespan of the induced corpora lutea were not altered by CR or follicular size. These results indicate that pretreatment with SMB increased the release of LH to exogenous GnRH. Furthermore, CR also increased the responsiveness of the pituitary to GnRH, provided that the calves were not returned during the period of induced LH release. Copyright © 1983. American Society of Animal Science . Copyright 1983 by American Society of Animal Science.

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