EFFECTS OF PROGESTERONE PRETREATMENT ON FERTILITY OF GILTS MATED AT AN INDUCED PUBERTAL ESTRUS

The effects of progesterone (100 mg/d,im) on pubertal fertility were e xamined in 247 gilts over 3 experiments. In the first experiment, 128 gilts were exposed to progesterone for 0, 2, 4 or 8 d before receiving PMSG (750 IU) 1 d later. The number of large (>4mm) follicles or corp ora lutea (CL) were determined on the day of PMSG injection, Day 0 (on set of estrus), Day 1 or Day 10 (n=8). In the second experiment, embry onic survival was observed in 68 gilts after induction of estrus with FG600 (400 IU PMSG, 200 IU hCG). Vehicle or progesterone was previousl y administered for 2 d to these gilts, and they were allowed 1, 2, or 3 d between the last progesterone injection and FG600. In Experiment 3 , a field trial was conducted in which 51 gilts received vehicle or pr ogesterone for 2 d, followed by a 3-d interval before injection of PG6 00 to induce estrus. The gilts were allowed to farrow. Treatment with progesterone 1 d before PMSG increased (P<0.05) the number and size of preovulatory follicles ard increased (P<0.05) the number of corpora l utea. However, the percentage of gilts pregnant by Day 10, the number of embryos recovered per gilt and embryonic survival were reduced (P<0 .05) with progesterone pretreatment. Utilizing a smaller dose of PMSG (750 vs 400 m) with PG600 negated the effects of progesterone pretreat ment on ovulation rate. When the interval between progesterone treatme nt and FG600 was lengthened to 3 d embryonic survival to Day 30 improv ed but was similar to that of the vehicle/PG600 treated gilts. Fertili ty, as defined as conception rate and litter size, was similar between gilts exposed to vehicle or progesterone. These results indicate that pretreatment with progesterone up to the day before PMSG might improv e follicular development and ovulation rate at the pubertal estrus wit h a dose of 750 m of PMSG but not with the 400 IU (PG600). Reducing th e dose of PMSG to 400 IU and allowing for 3 d between progesterone and gonadotropin treatment reduced the incidence of uterine infections bu t resulted in a fertility rate similar to that of gilts receiving RG60 0 alone.