Page 437 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

412 CHAPTER 2

VetBooks.ir 2.8 No single method exists to predict reliably the
precise moment of ovulation; however, the following
criteria are of use:

• Size of ovulating follicle. Follicles grow
at 3–5 mm/day during oestrus and very few
follicles ovulate within 24 hours when <33 mm
in diameter. The size of the follicle at ovulation
is NOT determined by the size of the mare.
Small ponies will ovulate with the same size
follicles as large draught breeds. Some mares,
however, will routinely ovulate off larger (50–60
mm) or smaller (25–30 mm) follicles, and good
record keeping is essential if these mares are to
be identified and managed appropriately.
• Change in consistency of ovulating follicle.
Consistency changes from a tense, tight sphere
to a soft, fluctuant structure occur in 85–90% of
pre-ovulatory follicles.
• Change in shape of ovulating follicle.
A change in shape (detected on ultrasound
examination) from a round shape to a pear or
conical shape indicates impending ovulation.
Fig. 2.8 Introduction of a sterile disposable vaginal However, this may not be seen in all follicles
speculum to allow visualisation of the vagina and and occurs in 85–90% of pre-ovulatory follicles
cervix and collection of uterine swabs.
(Figs. 2.9–2.12).
• Change in endometrial oedema. Oedema
examined at the appropriate time for pregnancy. (detected on ultrasound examination) is maximal
They can also be used to predict when a mare is at around 1–2 days prior to ovulation and starts
likely to return to oestrus following her last known to decline in prominence around 24 hours prior
oestrus if she was not mated. A mare can be expected to ovulation.
to tease at about 14–16 days after the last day of her
last oestrus (of course this will NOT apply if she is It is important to collate all information in order
pregnant). to help make an estimation of when the mare will
ovulate and therefore when to breed her. Fertility is
THE DIFFICULTY OF OVULATION highest when mares are bred within a window from
PREDICTION IN THE MARE 48 hours prior to ovulation to 6 hours post ovula-
tion (although it is possible to achieve conception up
The mare has a relatively long oestrous period to 20 hours post ovulation). The ideal time to breed
(5–7 days) and ovulation normally occurs at around a mare depends on the semen type (natural cover,
24 hours before the end of the oestrous period. fresh/chilled/frozen semen insemination).