Page 501 - Anatomy and Physiology of Farm Animals, 8th Edition

486 / Anatomy and Physiology of Farm Animals

(A)
VetBooks.ir

(B)

Figure 27-8. A clear mucous discharge from the vulva of a cow in estrus (A). If the mucus is dried on a
glass slide and examined with a light microscope, the high sodium chloride (salt) concentration results
in a “ferning” pattern (B). Source: images courtesy of Library of Reproduction Images (LORI: http://
lorimainsection.blogspot.ca).

of fertilizing an ovum. For artificial insemi- followed by a breeding season that may
nation, cows that come into standing heat vary from 1 to 20 consecutive estrous
in the morning are bred the same afternoon, cycles. The length of breeding season
and cows that come into standing heat in appears to be related to the severity of
the afternoon are bred the next morning. climate in which the breed developed. In
Bleeding from the vulva occurs in a high severe climates, a suitable lambing period
percentage of heifers and cows 1 to 3 days is restricted; hence, the breeding season is
after the end of estrus. This phenomenon likewise restricted so that lambing occurs
is metestrus bleeding; fertility is reduced if only during the favorable time (Scotch
breeding is done during bleeding. However, Blackface is an example). Breeds developed
fertility is not impaired if breeding has in milder climates may lamb successfully
taken place before the bleeding. over a longer period, so the breeding or
sexual season is also extended (e.g., Merino

Ewe sheep).
Length of Estrous Cycle. The average
Puberty usually occurs during the first estrous cycle in the ewe is 16.5 to 17.5 days.
breeding season (usually fall) after ewes are Unusually long or short cycles tend to
4 to 12 months of age if the ewes are well appear during the early and later parts of
fed. If ewes fail to reach puberty during the sexual season, rather than during the
their first potential breeding season, they middle part.
may be over 12 months old before reaching Length of Estrus. Duration of estrus
puberty.
averages about 30 hours, with a range for
Breeding Season. The ewe is probably most ewes between 24 and 48 hours. The
the best example of a seasonally polyestrous ram may be attracted during proestrus and
animal, with a long period of anestrus metestrus as well as estrus, but the ewe