SUPEROVULATION IN HEIFERS USING EITHER PREGNANT MARES SERUM GONADOTROPIN OR FOLLICLE-STIMULATING-HORMONE DURING THE MID LUTEAL STAGE OF THE ESTROUS-CYCLE

Techniques for the superovulation of cattle involve administration of gonadotrophins at the mid-luteal phase of the estrous cycle, followed by prostaglandin F2-alpha 48 hours later to induce luteolysis, estrus and ovulation. Two of the most common gonadotrophins used in cattle are pregnant mares serum gonadotrophin (PMSG) and follicle stimulating hormone (FSH). The objective of this experiment was to compare the superovulatory response and embryo yield in heifers following either gonadotrophin. Estrus was synchronized in 200 beef heifers using 500-mu-g cloprostenol (PG). These heifers were randomly allocated to receive either 24 mg pFSH (Folltropin, n = 105) with 4.5, 3.5, 2.5 and 1.5 mg given twice daily for four days, or a single injection of 2000 IU PMSG (n = 95) at mid-cycle. The PG was given at 48 (500-mu-g) and 60 hours (250-mu-g) post gonadotrophin initiation. Heifers were artificially inseminated at estrus and slaughtered 7 days later, and their reproductive tracts were recovered. The numbers of corpora lutea (CL) and large follicles (greater-than-or-equal-to 10 mm) were determined. Following embryo recovery with phosphate buffered saline and 3% heat treated steer serum, the number and quality of embryos and the fertilization rate were determined. Significant differences between treatments were determined using t-tests and Chi-square tests. The mean (+/- SEM) number of CL for pFSH-treated heifers was 19.0 +/- 0.99, with 12.3 +/- 0.81 embryos collected. Both the numbers of CL (13.9 +/- 0.84) and embryos collected (8.1 +/- 0.60) were lower (P = 0.001 and P = 0.0001, respectively) in PMSG than in pFSH-treated heifers. There was a greater number (P = 0.01) of freezable embryos (Grades 1 and 2; 4.4 +/- 0.45) in the pFSH-treated heifers than in PMSG-treated heifers (3.0 +/- 0.32). There was a greater number (P = 0.009) of degenerated embryos (Grades 4 and 5; 5.7 +/- 0.70) in the pFSH-treated heifers than in the PMSG-treated heifers (3.4 +/- 0.46O). The number of transferable embryos (Grade 3; FSH-p, 2.1 +/- 0.24; PMSG, 1.5 +/- 0.23) and the number of unfertilized eggs (pFSH, 2.1 +/- 0.50; PMSG, 1.0 +/- 0.25) did not differ (P = 0.09 and P = 0.055, respectively) between the two treatments. Results of this study indicate that the numbers of CL, embryos recovered, and Grade 1 and 2 embryos were higher in the pFSH-treated heifers than the PMSG-treated heifers.