Factors affecting postweaning oestrus in the sow.

An investigation of the factors affecting postweaning oestrus in the sow, comprising 3019 weanings in 75 herds (Norwegian Landrace) located in the southeastern area of Norway, was carried out in 1977 and 1978. The percentage of primiparous and pluriparous sows which came into oestrus during the first 7 days of the postweaning period was 52.3% and 77.4% respectively. The ovaries of sows slaughtered because of failure to come into heat after weaning were examined, results showing that the ovaries in 46.2% of the primiparous and 78.3% of the pluriparous sows had corpora lutea ("silent heat"). Ovaries lacking luteal tissue (anoestrus) were found in 38.5% of the primiparous and 13.0% of the pluriparous sows. The time lapse between weaning and first oestrus in primiparous sows was significantly longer in the 3rd and 4th quarters of the year as compared with the 1st and 2nd quarters (P less than 0.01). Postweaning oestrus occurred later in sows on a low feeding level than in sows on a high feeding level (P less than 0.01). The presence of a boar in the same room in which the sows were kept after weaning did not influence the length of the weaning to oestrus period. If herd size was increased by one sow, the time which elapsed between weaning and first oestrus decreased on average by 0.17 days (P less than 0.01).