Altrenogest treatment before weaning improves litter size in sows

Contents The study aimed to assess whether altrenogest treatment, fed before weaning (from −8 to −2 days), could improve fertility of sows showing reproductive seasonality. Ninety sows (50 in winter–spring [ WS ] and 40 in summer–autumn [ SA ]) were randomly selected and assigned to control (C; 27 in WS and 20 in SA ) or altrenogest treatment (A; 23 in WS and 20 in SA ) groups. The diameter and number of ovarian follicles were transrectally scanned at the onset of oestrus. Oestrus was evaluated twice daily from weaning to day 8 post‐weaning. Sows in oestrus were post‐cervically inseminated at 0 and 24 hr after the onset of oestrus with liquid stored semen (1.5 × 10 9 sperm/doses), and farrowing rates ( FR ) and total piglets born ( LS ) were recorded. More ( p  < .01) sows showed no signs of oestrus within 8 days after weaning in SA (30%) than in WS (2%), without differences between A and C groups. The diameter (cm) of the follicles at the onset of oestrus was larger in A than in C sows (0.76 ± 0.01 vs 0.73 ± 0.01; p  < .01), irrespective of the season. No differences in the number of follicles were found. FR did not differ between seasons and groups, being always above 85%. LS was larger ( p  < .01) in A (14.00 ± 0.46) than C (12.27 ± 0.44) sows, irrespective of the season. In conclusion, a short‐term altrenogest treatment at the end of lactation improves the total number of piglets born from weaned sows, probably by promoting a better and more homogeneous follicular development at the start of oestrus.