Vie CONG. INTERN. REPROD. ANIM. INSEM. ARTIF., PARIS, 1968, VOL. I

with progestagen-PMSG was low in early lactation. In the third
experiment, using pure bred Merinos, the incidence of oestrus 4 and
8 weeks after parturition was uniformly high. The data suggest

also that mating within 4 weeks results in an almost total failure
of fertilization (7-20 percent). Mating at 6 weeks results in poor
fertilization (60-65 percent) followed by embryonic mortality, and
at 8 weeks in comparable fertilization but no appreciable mortality.

These are particularly interesting data. They suggest a
graded degree of infertility associated with the state of the tract
after parturition and highlight the enormous difficulties confront-
ing us in attempting to breed ewes for twice yearly lambing. For
this we require a postpartum interval of 32 days in all ewes in a
flock!

V. CONCLUSION

I must confess that in preparing this review I became rather
depressed. I could not help but think of the many years of work
that had gone into this field and of what little practical result
has been achieved. However, in retrospect, I am rather more
optimistic. Progress has been made despite the relatively small
band which is working in this very difficult field. The biological
problems to be solved are much more difficult than those which con-
front the endocrinologist working on human birth control. He is
concerned primarily with preventing conception. We are concerned
with controlling conception and with augmenting fertility, and we
have gone a long way towards doing this in the normal breeding
season, We now have a reasonably precise, albeit expensive, means
of controlling ovulation for artificial insemination of the sow
using Methallibure plus PMSG and HCG. We have a convenient method
of synchronizing oestrus in the sheep and goat and for slightly
advancing the breeding season using progestagen impregnated intra-
vaginal sponges. We can exercise a degree of control over the time
of ovulation and oestrus in fed cattle. None of these methods is
perfect, but in their development we have learned something about
the normal endocrine pattern in the oestrous cycle and the factors
which control the normal cycle. In order that we may perfect
artificial control it is essential that we learn a great deal more
about the factors involved in normal control. We must have a much
clearer image of the overall picture of reproductive physiology in
the farm animals and a more concise understanding of the size, shape
and number of the pieces which go to form those parts of the picture
in which we are particularly interested. Without such an under-
standing we have little chance of effecting artificial control of
the cycle.

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