Page 432 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

CHAPTER 2

REPRODUCTIVE SYSTEM
VetBooks.ir 407

2.1 The female reproductive tract 407

Charles Cooke and James Crabtree

2.2 The male reproductive tract 512

Tracey Chenier
2.3 Equine castration 577

Luis Lamas

2.1 The female reproductive tract

NORMAL REPRODUCTIVE SYSTEM

REPRODUCTIVE SEASONALITY by the pineal gland. Short winter days lead to high
melatonin secretion and suppressed gonadotropin-
Mares, like other domestic animals, have an oes- releasing hormone (GnRH) levels from the hypo-
trous cycle that is a coordinated series of anatomical, thalamus. With increased day length the periods
endocrine and behavioural changes that eventually of high melatonin production decrease, leading to
lead to ovulation. Mares are seasonally polyoestrous increased pulses of GnRH, which stimulate follicle-
breeders, meaning that they have multiple oestrous stimulating hormone (FSH) and luteinising hor-
cycles during a ‘breeding season’. Most mares are mone (LH) production.
‘long day’ or spring/summer/early autumn breed- In terms of increased ovarian activity, mares
ers, although about 20% of mares continue to ovu- seem to respond best to a photoperiod of around
late throughout the winter months. The majority of 15–16 hours (i.e. summer). The period from the first
mares enter a period when they stop cycling, known spring ovulation to the last in the autumn is the ovu-
as anoestrus, during the winter months. Photoperiod latory phase of the cycle. The periods between the
or day length is the most important factor influ- anoestrus and the ovulatory phases, which occur in
encing this seasonality and ovarian activity, but the early spring or late autumn, are called the tran-
the transition between cyclicity, non-cyclicity, and sition phases. These are associated with periods of
back again is gradual. The effect of the day length no, irregular, unpredictable or prolonged oestrous
is mediated by the hormone melatonin, produced behaviour.