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

LIMITING FACTORS IN THE INDUCTION OF
EARLY BREEDING IN SHEEP

M.G, Keane & J.P. Crowley

Animal Sciences Divisiom, An Foras Taluntais, Dunsinea,
Castleknock, Co, Dublin, Ireland

The restricted breeding season of the national ewe flock in
Ireland results in a marked seasonality of lamb supplies with its con-
sequent effects on prices, which fall on average by over 40% from April
to July - hence there is considerable interest in hormonal methods of

indueing early breeding activity in sheep.

The progesterone-pregnant mares serum (PMS) treatment devised by
Gordon (1958) for out-of=-season lambing has been further evaluated for
the induction of early breeding activity by Crowley (1964). Subsequently
oral progesterone derivatives combined with PMS have been used for this
purpese, More recently the development by Robinson (1965) of sponge—
type pessaries for prolonged administration of progesterone affords
further improvement of the hormone treatments available for early

breeding,

In the period 1965-67 an 8 to 16 days course of treatment with in-
travaginal sponges impregnated either with progesterone or some pro-
gesterone derivative in conjunction with PMS was evaluated in a series
of extensive field trials., The lambing performance was recorded and a
most interesting feature of the results was the large variation in re-
sponse to comparable treatments which occurred in different flocks., Ve
have examined some of the factors operating at farm level which may be

responsible for this inconsistency of response.

(a) Effect of PMS in Early Lambing Treatment:

The data in Table I demonstrate clearly the necessity of serum
gonadotrophin in eliciting a high oestrus response under farm conditions.
The inclusion of PMS in the treatment produced a significantly higher
mating response and as a result the proportion of ewes which lambed to

the induced oestrus was significantly higher, However, when the lambing

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