The wean-to-service interval (WSI) — the time between weaning piglets and the sow returning to oestrus and being mated — is a critical welfare and reproductive performance indicator in pig farming. Factors affecting WSI reflect the sow's physiological recovery from lactation and her nutritional and welfare status.
Physiology
During lactation, suckling-induced prolactin suppresses GnRH pulsatility, preventing follicular development and ovulation. Weaning removes this suckling stimulus, allowing the HPG axis to recover and follicular growth to resume. The first post-weaning ovulation typically occurs 4–7 days after weaning in healthy, well-nourished sows. Delayed ovulation (extended WSI >7 days) indicates physiological stress or poor condition.
Factors Prolonging WSI
- Negative energy balance during lactation: The most important factor — sows losing excessive body condition during lactation have delayed first ovulation. Insufficient feed intake during lactation (due to feed restriction, heat stress, competition, or poor palatability) is the primary cause
- Low body condition at weaning: Sows weaned with BCS <2.5 have significantly delayed WSI and reduced subsequent litter size
- Extended lactation length: Longer lactation (21+ days) allows greater body condition recovery but delays weaning — balance depends on management system
- Parity: First-parity sows (gilts) have longer WSI than multiparous sows — they are still growing while supporting lactation
- Disease: Mastitis, metritis, and systemic illness delay ovulation
Welfare Significance of WSI
Extended WSI is a welfare indicator — reflecting physiological stress from excessive body condition loss. Sows that fail to cycle post-weaning may be classified as "anoestrous" and removed from the herd prematurely, before nutritional recovery would have allowed normal cycling to resume. Welfare-centred management aims to maintain sow body condition through lactation by:
- Maximising voluntary feed intake during lactation (ad lib feeding, high-quality diets, appropriate feed delivery systems)
- Managing environmental temperature to maintain appetite (optimal lactating sow temperature: 16–18°C)
- Monitoring individual sow intake through electronic feeding records
- Providing adequate drinking water (minimum 3L/hour flow rate)
Flushing and Post-Weaning Management
Brief post-weaning "flushing" (increasing feed intake immediately post-weaning) has mixed evidence for WSI reduction but is commonly practiced. Boar contact after weaning promotes faster oestrus expression. Monitoring return-to-service rates and WSI distribution in herd records enables identification of nutritional management problems requiring intervention.