Effects of altrenogest treatments before and after weaning on follicular development, farrowing rate, and litter size in sows

In a previous study, we showed that follicle size at weaning affects a sow’s response to a short altrenogest treatment after weaning. In this study, an attempt was made to prevent growth of follicles into larger size categories before weaning using different altrenogest treatments before weaning to improve reproductive performance after postweaning altrenogest treatments. Sows (87 primiparous and 130 multiparous) were assigned to: control (no altrenogest treatment; n = 59), RU0-20 (20 mg altrenogest, d -1 to d 6; weaning = d 0; n = 53), RU40-20 (40 mg altrenogest, d -3 to d 0 and 20 mg altrenogest d 1 to d 6; n = 53), and RU20-20 (20 mg altrenogest, d -3 to d 6; n = 52). Follicle size was assessed daily with trans-abdominal ultrasound. Follicle size on d -3 (3.6 ± 0.7 mm) and at weaning (4.0 ± 0.7 mm) was similar for all treatments. Altrenogest-treated sows had larger follicles at the start of the follicular phase than control sows (5.4 ± 0.1 and 3.8 ± 0.2 mm, LS Means, respectively; P <0.0001) and on d 4 of the follicular phase (8.0 ± 0.1 and 6.7 ± 0.2 mm, LS Means, respectively; P <0.0001). Multiparous sows had larger follicles than primiparous sows at the start of the follicular phase (5.3 ± 0.1 and 4.7 ± 0.1 mm, LS Means, respectively; P <0.01) and on d 4 of the follicular phase (8.0 ± 0.1 and 7.0 ± 0.1 mm, LS Means, respectively; P <0.0001). Farrowing rate and litter size (born alive + dead) were not affected by treatment or parity. However, in primiparous sows, when mummies were included in litter size, altrenogest sows had larger litters than control sows (13.4 ± 0.5 and 11.9 ± 0.7 piglets, respectively; P = 0.02). In primiparous control sows, backfat depth at weaning and litter size were positively related (slope of the regression line = 0.82; P <0.05), which was not the case in primiparous altrenogest sows. In conclusion, the different altrenogest treatments before weaning did not prevent growth of follicles before weaning and similarly affected subsequent follicle development and fertility. In primiparous sows, altrenogest treatment after weaning increased the number of fetuses during pregnancy but positive effects seemed limited by uterine capacity. Altrenogest treatment after weaning improved litter size in primiparous sows with low backfat depth at weaning, which suggests a specific positive effect of a recovery period after weaning in sows with low body condition scores at weaning