Page 922 - The Veterinary Care of the Horse
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Figure 19.8 This mare is not receptive to the stallion
Control of the oestrous cycle
The mare has a seasonal breeding period which is influenced by factors such as daylight
length, temperature and nutrition. In the cold winter months, most mares stop having oestrous
cycles and their ovaries become small and inactive. In February and March when they begin
cycling, some mares have prolonged periods of oestrous behaviour but fail to ovulate. There
is considerable individual variation between mares. Artifical lighting is sometimes used prior
to the breeding season to encourage early return to oestrus.
The oestrous cycle is under hormonal control. When problems arise, oestrus or ovulation
can sometimes be induced with hormone treatments.
Gestation length
The gestation (pregnancy) length is 11 months (340 days), but considerable variation occurs
with a range of 320–360 days and sometimes even longer.
