carefully controlled conditions and supervision, and with adequate
randomisation and distribution of treatments.

Because of these difficulties, the majority of work in the field
of control of ovulation in sheep and cattle, at least, is inconclu-
sive. Little of it will stand up to rigid analysis and as Loy
pointed out only two years ago ''research on practical control of
reproductive phenomena remains today in the realm of trial and error
experimentation'" (Loy, 1966).

The main purpose of this paper is not to review work done on
synchronization but, as the title indicates, to consider synchroni-
zation and fertility. What are the causes of the failures which
commonly follow all forms of treatment? What parameters can be
used as measures of normality of the oestrous cycle and how are these
related to fertility?

First, let us define' our objectives. They are to:

I. Control the time of ovulation and oestrus of cattle, sheep,
goats and swine in their normal breeding season so accurately
that hand service or artificial insemination can be carried
out at a predetermined time,

II. Induce breeding activity in the non-breeding season of sheep
and goats.

III. Breed during early lactation of sheep and goats.

In each case, full "normal" fertility must be obtained. This
can be defined as at least 90 percent of all ova fertilized, of which
75 percent survive the first 3 weeks. This will result, approxi-

mately, in a 90 percent conception rate in swine, in 67 percent in
cattle and in uni-ovular sheep, and in 80 percent in bi-ovular and
90 percent in tri-ovular sheep and goats. It is interesting that
Hulet & Foote (1967) observed just such figures following
laparotomies on 342 normal sheep which shed 1, 2 or 3 eggs.

There are other aspects of control of fertility which are of
regional importance, These include multiple ovulation for super-
foetation or for egg recovery and transplant, and suppression of
ovulation in lieu of spaying of heifers. However, because of their
regional nature and because, also, relatively little of significance
has been achieved in the past decade, I do not propose to deal with
these topics.