VI® CONG. INTERN. REPROD. ANIM. INSEM. ARTIF., PARIS, 1968, VOL. II

THE INDUCTION OF OESTRUS AND OVULATION
DURING THE LACTATIONAL ANOESTRUS OF THE SOW

D. B. CRIGHTON
University of Nottingham School of Agriculture,
Sutton Bonington, Loughborough, Leics., U.K,

The regular oestrous cycles of the pig are interrupted
during its normal reproductive life by pregnancy and lact-
ation. There is agreement that post-partum oestrus occurs in
the sow but the percentage of sows exhibiting oestrus has
varied widely in different studies (1),(2),(3),(4). Post-
partum ovulation does not occur, regardless of whether or not
there is a post-partum oestrus. This has been established by
examining the ovaries of sows slaughtered shortly after the
post-partum oestrus (1),(2) and about 10 days after parturition
where post-partum oestrus had not occurred (1),

Throughout the remainder of lactation, the sow does not
normally exhibit oestrus or ovulation (2),(3),(4),(5),(6),(7).
When lactation is terminated by removal of the young some eight
weeks after parturition, oestrus and ovulation occur generally
within about four to seven days (2),(4),(5),(8). This is the
basis of the traditional husbandry pattern whereby conception
occurs at the post-weaning ovulation after an eight-week
lactation. Weaning is now commonly carried out at six weeks,
Thus lactation and the post-weaning period prior to oestrus
represent together about 25-32% of the interval between suc-
cessive farrowings depending upon when weaning is performed
between six and eight weeks.

To reduce the interval between farrowings to increase the
productivity per breeding unit per annum, attention must be
concentrated on lactation. The problem is not only a reprod-
uctive one but also a nutritional one since provision has to
be made for the adequate nutrition of the existing litter.
Several approaches have been attempted, the main one being
that of "early weaning" at stages of lactation ranging from 5
weeks to 2 days. Such techniques, although widely investig-
ated, have proved generally unsuccessful in practice, partic-
ularly when employed during the first 3 weeks of lactation,
mainly because of nutritional problems but also because sows
have frequently taken longer to show post-weaning oestrus
after "early weaning". A secopd approach has been to attempt
to induce oestrus and ovulation with exogenous gonadotrophin
during lactation (3),(9). It was found that prior to about 40
days of lactation a single injection of pregnant mare's serum
gonadotrophin (FMSG), failed to induce oestrus consistently,

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