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

II. THE BREEDING SEASON

A. Principles

The principle upon which control of the time of ovulation and
oestrus has been based is the prolongation, at will, of the luteal
phase of the oestrous cycle by the administration of progesterone
or an analogue. It is based upon the belief that progesterone is
the major, if not the sole, hormone produced by the corpus luteum
and that a suitable progestagen administered artificially beyond the
life span of the normal corpus luteum will mimic exactly the mani-
fold actions of that gland. Since the initial demonstration 20
years ago by the Wisconsin group (Dutt & Casida, 1948; O'Mary, Pope
& Casida, 1950) that the time of ovulation and oestrus could be
reasonably well controlled by progesterone administered daily to
sheep, there has been a spate of work, mostly of a trial and error
nature, on sheep, cattle and pigs and, to a lesser extent, on goats
and mares. This, associated with the stimulus given to steroid
research by the need to control human fertility, has led to the
testing and characterization of new and highly active progestagens,
notably MAP, CAP, MGA, Cronolone, Nilivar and many others. In this
context the spayed ewe assay technique, as used by Shelton, Robinson
& Holst (1967), has proved to be a particularly useful method for
the evaluation of the relative potency and duration of activity of
progestagens for use in the sheep (Table 1).

Simplified methods of administration by feeding, intravaginal
application, and subcutaneous implant have been developed. In
general, suppression and release of ovulation in sheep and cattle
can be effected by a wide variety of progestagens given in any one
of these ways. Effective synchronization can be obtained with some
but not all progestagens, and this appears to be a function of
method of administration and of duration of activity as assessed in
the spayed ewe (Table 2). A very high degree of synchronization
can be obtained in sheep (Robinson, 1965; Barker, 1966; Gordon &
Keane, 1967) and cattle (Mauléon & Rey, 1966; Carrick & Shelton,
1967; Shimizu, Toyodo, Takeuchi, Kawai & Adachi, 1967) using intra-
vaginal sponges impregnated with steroids of the 9a-halogenated
series (Shelton, 1965), and one of these - SC-9880 or Cronolone
(Searle) - has been very widely tested (see Robinson, 1967). Other
progestational steroids, such as MAP, are also effective when given
by this manner, but the time relationships are different and, on
present evidence, appear less precise (Clarke, Roberts, Carter &
Kirton, 1966; Robinson, 1966; Robinson, Moore, Holst & Smith,
1967). The removable subcutaneous implant, as used by Dziuk &
Cook (1966), Dziuk, Cook, Kaltenbach & Niswender (1966) and Dziuk,
Cmarik & Greathouse (1966), appears also to result in precise
synchronization but to date it has not been as widely tested as has

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