THE EFFECTS OF ONCE OR TWICE-DAILY INJECTIONS OF PFSH ON SUPEROVULATORY RESPONSE IN HEIFERS

Follicle stimulating hormone (FSH) is a glycoprotein hormone with a sh ort half-life and has to be given twice daily for 3-4 days to induce s uperovulation in heifers. Since such a regimen is time consuming we co mpared the ovulatory response and yield of embryos in heifers followin g superovulation with either once or twice daily injections of pFSH fo r 4 days during the mid-luteal phase of a synchronized estrous cycle o r during a prolonged luteal phase in heifers which had been immunized against prostaglandin F2alpha (PG). In Experiment 1, crossbred heifers (n = 42) previously actively immunized against a PG immunogen were su perovulated in a 2 (cyclic or persistent corpus luteum) x 2 (once or t wice daily injection) factorial plan. The heifers were superovulated w ith 75 units pFSH, which was injected subcutaneously once (22.5, 22.5, 15 and 15 units per day) or twice daily (9.3 units per injection) for 4 days. In Experiment 2, cyclic crossbred beef heifers (n = 80) were superovulated using pFSH which was given randomly to heifers once dail y subcutaneously (T1) or twice daily intramuscularly (T2) using the sa me daily dose of 9, 7, 5, and 3 mg per day. Estrus was induced in all heifers in both experiments using 500 mug and 250 mug Cloprostenol 12 hours apart on the third day of pFSH injections. All heifers were inse minated twice with frozen-thawed semen at 12 and 24 hours after the on set of standing estrus or at 56 and 72 hours after the first PG if est rus was not observed. Embryos were recovered at slaughter and graded o n a scale of 1 to 5 (1 = excellent, 5 = degenerated). Data were record ed for the number of corpora lutea (CL), large (greater-than-or-equal- to 10 mm) and medium (5-9 mm) follicles, number of embryos recovered a nd embryo morphology. Data were analyzed by least squares analysis of variance procedures. In Experiment 1, there was no difference in ovula tion rate between main effects. Fewer embryos were recovered from heif ers with a persistent corpus luteum (pCL) and injected once daily (1.7 1 +/- .75 vs 5.75 +/- 1.27) than from any other group. Heifers with pC L yielded lower (P < 0.05) numbers of freezable embryos than cyclic an imals, regardless of injection regimen. In Experiment 2, T2 heifers ha d a significantly higher number of CL (16.4 +/- 1.7 vs 7.7 +/- 1.7; P = 0.0003), large follicles (4.1 +/- 0.5 vs 2.8 +/- 0.5; P = 0.04), med ium follicles (6.4 +/- 0.7 vs 4.4 +/- 0.7; P = 0.04), embryos recovere d (9.6 +/- 1.1 vs 4.9 +/- 1.1; P = 0.0025) and freezable embryos (4.7 +/- 0.7 vs 2.1 +/- 0.7; P = 0.014) than T1 heifers. It is concluded th at a single daily subcutaneous injection of pFSH results in a lower su perovulatory response than the twice daily regimen in heifers.