Post-weaning Altrenogest treatment in primiparous sows; the effect of duration and dosage on follicular development and consequences for early pregnancy

Our objective was to investigate follicle development in sows during and after different Altrenogest treatments post-weaning and relate this to subsequent ovulation rate and embryonic development. Primiparous UPB sows (n = 47) were randomly assigned to (weaning = Day 0): control (no Altrenogest, n = 12), RU8-15 (15 mg of Altrenogest, Day-1 till Day 7, n = 12), RU8-20 (20 mg of Altrenogest, Day-1 till Day 7, n = 12) or RU15-15 (15 mg of Altrenogest, Day-1 till Day 14, n = 11). From weaning onwards, trans-abdominal ultrasound was performed daily. Sows were slaughtered on Day 4 or 5 after ovulation. Follicle size increased during Altrenogest treatment and reached a plateau around Day 6, regardless of dose (4.6 ± 1.5, 4.6 ± 1.6 and 4.6 ± 1.6 mm for RU8-15, RU8-20 and RU15-15, respectively). This increase resulted in larger follicles (P = 0.0002) at the onset of the follicular phase (i.e. time of weaning for control sows and 24 h after last administration of Altrenogest for treated sows); 4.8 ± 1.8, 4.8 ± 1.4, 4.9 ± 0.9 mm and 2.9 ± 0.8, for RU8-15, RU8-20, RU15-15 and controls, respectively. Pre-ovulatory follicle size tended (P = 0.07) to be larger for treated animals (7.9 ± 2.4, 7.9 ± 0.7, 8.6 ± 1.3 and 6.9 ± 0.9 mm for RU8-15, RU8-20, RU15-15 and controls, respectively). The interval follicular phase-oestrus was shorter (P = 0.005) for treated animals. Treatment did not affect ovulation rate or early embryonic development. However, for treated animals, the increase in follicle size during treatment was positively related with ovulation rate (P = 0.05). In conclusion, post-weaning treatment with Altrenogest of first litter sows influenced follicle size and shortened the follicular phase, but did not affect ovulation rate or early embryonic development.