Split-weaning Before Altrenogest Synchronization of Multiparous Sows Alters Follicular Development and Reduces Embryo Survival

This study used split-weaning (SW) to induce differences in follicle size at weaning and study its consequences for follicle development during and after post-weaning altrenogest feeding and for reproductive performance. Multiparous sows (n = 47) were assigned to SW (n = 23; litter size reduced to the six smallest piglets 3 days before weaning) or control (C; n = 24; normal weaning). Altrenogest (20 mg/day) was fed to all 47 sows from Day -1 till Day 5 (complete weaning = Day 0). Follicle size on Day 1, 2 and 8 was smaller in C than in SW (p = 0.05). Ovulation rate was similar, but C sows had higher embryo survival rate (ESR) than SW sows (83 ± 19 and 58 ± 31%, respectively; p = 0.001). SW sows with low ESR (63%; n = 10; p = 0.04). A decrease in follicle size between Day 5 and 6 of altrenogest feeding was associated with increased ESR in both treatments (p = 0.002). Follicle pool analyses (assessment of all follicles >2 mm) revealed that on Day 3, sows with low ESR had a higher % of follicles >5 mm compared with sows with high ESR (30% vs 10%; p = 0.04). Thus, sows in which follicle growth was less suppressed during altrenogest feeding had a lower ESR. These effects on follicle development and ESR were more pronounced in split-weaned sows.