Effect of exogenous progesterone on superovulatory response in heifers inseminated with fresh or frozen semen

Superovulation in cattle normally involves the administration of gonadotrophins at specific times of the oestrous cycle, followed by the induction of luteolysis and insemination with high quality semen. The first aim of this experiment was to examine the effect of supplementary progesterone when used in conjunction with porcine FSH (pFSH) to induce superovulation in heifers. The methods compared were PGF2\g=a\given at mid-cycle or a progesterone-releasing intravaginal device (PRID) inserted at different phases of the cycle. The second aim was to determine whether site of insemination or use of fresh or frozen semen affected embryo production. A factorial design was used involving 185 beef heifers. The main factors were (i) synchronization methods PGF2\g=a\or PRID); (ii) semen type (fresh or frozen); (iii) insemination regimens (involving two inseminations and variations in the sites) and number of straws used (one or two) at the second insemination. Eight injections of pFSH were given twice a day for 4 days starting either on days 9, 10 or 11 of the oestrous cycle or on the fourth day after insertion of a PRID. Heifers were checked for oestrus, inseminated twice and embryos were recovered on day 7 of the superovulated cycle. There was no difference between heifers given either PRID or PGF2\g=a\ in the oestrous response (93% versus 96%), number of ovulations (15.9 \m=+-\1.11 versus 13.4 \m=+-\ 1.06), large follicles (2.5 \m=+-\0.24 versus 2.3 \m=+-\0.23) or embryos recovered (9.1 \m=+-\0.77 versus 9.1 \m=+-\0.74). The number of embryos that could be frozen was lower (P = 0.05) in heifers given PRID. The stage of the cycle at which the PRID was inserted affected the number of ovulations, large follicles and embryos recovered (P < 0.04). The use of fresh or frozen semen had no effect on the number of embryos recovered, but the use of frozen semen resulted in fewer grade 1 and 2 embryos and more grade 4 and 5 embryos in PRID-treated heifers. The number of straws did not affect the number or quality of embryos recovered. In conclusion, the use of a PRID, inserted at different stages of the cycle, in conjunction with PGF2\g=a\and pFSH resulted in fewer freezable embryos recovered compared with the use of PGF2\g=a\and pFSH given at mid-cycle. The use of frozen semen did not affect the number or quality of embryos recovered following the use of PGF2\g=a\ and pFSH at mid-cycle, but it did decrease the number of grade 1 and 2 embryos recovered following the use of PRID and pFSH; the number (two versus three) of straws used did not affect the yield or quality of embryos recovered.