follicle. We conclude that, in the absence of luteal progesterone, synchronisation with a single progestagen sponge for 14 days resulted in higher LH pulse frequency and ovulation of a
persistent follicle with a larger maximum diameter, compared with controls. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Synchronisation; Ovulation; Follicles; LH; Sheep-ovary
1. Introduction
In cattle, the model of waves of follicular growth and the existence of a dominant Ž
. follicle is widely accepted Fortune, 1994; Ginther et al., 1996; Roche et al., 1998 . The
existence of a similar pattern of follicular growth in sheep has been controversial; Ž
however, it now appears that waves of follicular growth occur in sheep Noel et al., .
1993; Ginther et al., 1995; Evans et al., 2000 . Progesterone and progestagens are used widely to synchronise oestrous in cattle and
sheep. The effect of these treatments on follicular growth in cattle is well documented, but this is not the case with sheep. In cattle, if plasma progesterone concentrations are
Ž
y1
. maintained at subluteal concentrations 1 to 2 ng ml
prolonged growth of the Ž
. ovulatory follicle occurs Sirois and Fortune, 1990; Savio et al., 1993 . These concentra-
tions of progesterone are created when cattle without an active corpus luteum are treated with an intravaginal device for long periods as the progesterone-releasing ability of the
Ž .
intravaginal device decreases Sirois and Fortune, 1990 . This intermediate plasma Ž
progesterone concentration results in a higher LH pulse frequency Roberson et al., .
1989; Stock and Fortune, 1993; Kinder et al., 1996 and the development of larger ovulatory follicles than those observed in cattle with luteal concentrations of proges-
Ž .
terone Sirois and Fortune, 1990; Savio et al., 1993; Bigelow and Fortune, 1998 . The efficacy of prostagen treatments to synchronise oestrus in sheep has been
Ž .
documented Gordon, 1997 but the effects on the growth of the ovulatory follicle are unclear. Synchronisation of oestrus with progesterone or prostagens results in a concep-
tion rate of 70 to 80. It is possible that the reason why some of the 20 to 30 of ewes that fail to conceive may be because these animals ovulate a poor-quality oocyte
from a prolonged follicle. This may happen in ewes when natural luteolysis occurs early in the synchronisation treatment leaving the intravaginal device as the only source of
negative feedback on gonadotrophin secretion. If the efficacy of the sponge decreases with time it may lead to a situation where subluteal plasma concentrations of proges-
terone exist leading to an increase in the LH pulse frequency and hence prolonged
Ž .
growth of ovulatory follicles as described in cattle Mihm et al., 1999 . When progesterone concentrations in ewes were maintained at less than 1 ng ml
y1
, using customised implants or injections of oil, the diameter and the age of the ovulatory
follicle was increased compared to ewes with progesterone concentrations of greater
y1
Ž .
than 1 ng ml Johnson et al., 1996 . While progesterone is used in some synchronisa-
Ž tion protocols in sheep, many products use more potent synthetic progestagens Robin-
. son et al., 1967 . The initial release of progestagen from intravaginal sponges is high but
Ž .
decreases with time Greyling et al., 1994 . The effects of starting synchronisation
treatments at different stages of the oestrous cycle on follicle growth have been Ž
. examined in ewes Leyva et al., 1998 . The results were equivocal due to the lack of
continuous data on the growth of ovulatory follicles and the asynchronous decrease in endogenous progesterone concentrations. Hence, the efficacy of progestagen synchroni-
sation on the negative feedback regulation of LH secretion and the resultant effect on follicle growth towards the end of the treatment remains to be established.
The aim of the present experiment was to synchronise oestrus using different progestagen treatments in ewe lambs in which luteolysis was induced early in the
synchronisation period and to examine the effect on LH concentrations and the growth of the ovulatory follicle. We tested the hypothesis that in the absence of luteal
progesterone, ewes synchronised for 14 days with a single long-duration progestagen sponge will ovulate an older follicle and have higher LH pulse frequency than ewes
Ž synchronised for 14 days with three short-duration sponges removed and replaced every
. 5 days .
2. Materials and methods
