Page 434 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Reproductive system: 2.1 The female reproductive tr act 409

VetBooks.ir oedema, the cervix to relax and the mare to display feedback on GnRH release. The increased pulse
frequency of GnRH increases LH release. Rising
behavioural signs consistent with oestrus. The fol-
licle then ovulates to form a similar sized structure
ovulation. The dominant follicle produces oestro-
mainly containing blood, called a corpus haemorrhagi- LH levels cause follicular growth, maturation and
cum (CH), which is seen as a hyperecholic structure gen and inhibin. The latter inhibits the growth of
on ultrasound. The CH then undergoes luteinisation non-dominant follicles, hence mares generally have
to form the corpus luteum/corpus lutea (CL) (Fig. 2.1) single or double ovulations. The uterine tone begins
and a rapid rise in plasma progesterone causing the to decrease.
uterus to lose the oedema and develop ‘tone’, the cer-
vix to close and the mare to become non-receptive to Follicular phase
the stallion. The CL can be identified by ultrasound The combination of falling progesterone and ris-
as a hyperechoic structure, which is usually a little ing oestrogen levels results in oestrous behaviour
smaller than the CH from which is came. and increasing receptivity. There are correspond-
ing changes to the genital tract (the uterus losses
Mid-luteal phase tone and softens with oedema, the cervix relaxes and
High progesterone concentration (25.4–31.8 nmol/l softens, and there are increased secretions). There
[8–10 ng/ml]) leads to a negative feedback on GnRH, is gradual growth of the primary follicle/s evident
causing a decrease in its pulsatile secretion from the on rectal palpation and ultrasound. Oestrogen level
hypothalamus. This decrease in GnRH results in reaches a threshold, which results in a positive feed-
inhibition of LH release, but still stimulates release back on LH release, causing a surge in LH, which
of FSH from the pituitary. FSH release causes waves is responsible for ovulation. Ovulation occurs at the
of follicles to develop. The mare is still non- receptive hilar or ovulation fossa of the ovary
to the stallion
IDENTIFYING THE STAGE OF
Late-luteal phase THE OESTROUS CYCLE
Prostaglandin F alpha (PGF α) is released from
2
2
the endometrium and causes regression of the CL Accurate and early recognition of the stage of the
(luteolysis) and so a decline in serum progesterone. oestrous cycle in the mare is a central part of modern
Falling progesterone levels remove the negative stud farm management. A number of techniques are
used, often in combination.

2.1 Visual inspection and observation
of behavioural changes
Often practiced by individual mare owners or where
teaser animals are not available.

Teasing
The mare is exposed to a teaser, which is usually a
pony or non-valuable stallion, and her behaviour
assessed for evidence of receptivity. There are many
different teasing methods (Fig. 2.2).
The signs of oestrus when teased by a stallion are:
quiet and calm; ears forward; nuzzling; posturing;
moving towards the teaser; squatting and frequent
Fig. 2.1 Examination of an ovary at post-mortem squirts of urine; raising tail; eversion of vulval lips
clearly demonstrating the corpus luteum within the and clitoris (winking).
ovary.