Page 433 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

408 CHAPTER 2

VetBooks.ir THE OESTROUS CYCLE naturally cycling in February. The push for foals to
be born earlier in the year (for racing and commer-
During the winter months, in the majority of mares
there is a period of anoestrus, characterised by small cial sales reasons) contrasts with the physiological
breeding season for the mare, which starts around
firm ovaries with minimal follicular activity. The March to April in the northern hemisphere (August
uterus is atonic and thin walled, with a pale, dry and to mid-September in the southern hemisphere) and
partially relaxed cervix. The mare is neither receptive runs through to the early autumn. The aim is to get
towards nor rejects the stallion. Levels of LH are low the mare through the transitional period in advance
and levels of FSH fluctuate randomly. Plasma proges- of February to enable the mare to be covered on
terone levels are <3.18 nmol/l (<1 ng/ml), due to the fertile cycles resulting in foals being born early in
absence of any corpora lutea. the year. It is necessary to appreciate the unpredict-
As day length increases, there is an increase in able nature of the transitional period as mares may
ovarian activity and the mare enters the ‘spring appear in season and stand to be mated for prolonged
transitional’ stage of her annual cycle. In the spring periods of time without resulting in a pregnancy.
the ovaries develop numerous follicles of varying Once she ovulates and enters the cyclical phase, an
sizes that grow, regress and do not ovulate. These individual pattern emerges specific to the mare her-
can be palpated per rectum as the ‘small bunches of self, and notable for its inconsistency as compared
grapes’ ovaries noted in the literature. Initially, the with other domestic species (e.g. ovulation without
uterus will be flaccid and the cervix pale in colour, oestrous behaviour, oestrus without ovulation, split
but as follicular activity increases, uterine oedema oestrus). The oestrous cycle and where the mare is
develops and the cervix relaxes and becomes pink in within the cycle can be monitored by her behaviour
colour. Eventually, a follicle progresses through to towards a stallion (or teaser) and by ultrasonography
maturity and ovulates, thereby initiating the ovu- of the reproductive tract (ovaries, uterus) and visu-
latory phase. Plasma FSH levels rise early in the alisation of the cervix. The durations of the various
transitional period, causing follicular development, components of the cycle are shown below.
but steadily fall approximately 15–20 days prior to
the first ovulation. Plasma LH concentrations are • Oestrous cycle: time period from one ovulation
initially low, meaning ovulation will not occur, but to another; 19–24 days, average 21 days.
they increase slowly until a few days immediately • Oestrus: time when the mare shows ‘heat’,
prior to the first true oestrus, with a peak around is receptive to the stallion and stands to be
ovulation. Developing follicles in the ovary pro- mated; 4–9 days, average 6 days. Dominant
duce oestrogen, but in the transitional mare their hormone – oestrogen.
levels are not high enough to trigger rises in GnRH • Dioestrus: time between oestrus periods
and LH, which will lead to the first ovulation. when the mare is not receptive to the stallion;
Eventually, a follicle does develop sufficiently to 12–16 days, average 14 days. Dominant
trigger these changes and the mare will show oes- hormone – progesterone.
trous behaviour and ovulate. Once the first ovula-
tion has occurred, the mare usually continues to Ovulation is generally considered to occur
ovulate regularly throughout the remainder of the approximately 24 hours before the end of oestrus,
ovulatory season. The sometimes prolonged transi- often during the evening or night.
tional phase before a mare starts to ovulate regularly
each spring can cause problems for breeders who are HORMONAL CHANGES IN
keen to breed early foals. This is particularly acute THE OESTROUS CYCLE
in the northern hemisphere Thoroughbred indus-
try where all foals are given a 1st January birthday Early luteal phase
regardless of the actual day of birth. The difficulty A follicle will generally reach a size of 35–40 mm and
arises because approximately only 30% of mares are produce oestrogen, causing the uterus to develop