Page 500 - Anatomy and Physiology of Farm Animals, 8th Edition

The Ovary and Estrous Cycles / 485

Specifics of Selected Estrous occur during this period of estrus, and
Cycles
conception is possible if mares are bred
VetBooks.ir during a foal heat.
Histologic (tissue) changes in the lining
Mare
of the genitalia of the mare during the
Puberty begins at 10 to 24 months, with estrous cycle approximate the general
an average onset of about 18 months. pattern found in all mammals. However,
these changes are not distinctive enough to
Length of Estrous Cycle. Cycle lengths make a vaginal smear useful in determin-
varying from 7 to 124 days have been ing the stage of the estrous cycle.
reported. However, average lengths are
about 21 to 22 days. The abnormally long
cycles undoubtedly include a number of Cow
skipped periods or cycles.
In cattle, puberty varies considerably with
Length of Estrus. The average length of the breed and level of nutrition. Holstein
estrus in the mare is approximately 6 or 7 heifers show first estrus at an average of
days. Estrus tends to shorten from spring 37 weeks of age on a high level of nutrition,
to midsummer. Early in the breeding season, 49 weeks on a medium level, and 72 weeks
through March and April, estrus tends to on a low level of feeding. Puberty appears
be irregular and long, frequently with no to occur when the heifer is about two‐
ovulation. From May to July the periods thirds of her adult body size, measured by
become shorter and more regular, with height and length rather than weight.
ovulation as a normal part of the cycle.
Ovulation usually occurs 1 to 2 days Length of Estrous Cycle. The estrous
before the end of estrus. cycle averages 20 days for heifers and 21 to
22 days for mature cows.
Time of Breeding. Fertility rises during
estrus to a peak 2 days before the end of Length of Estrus. Estrus in the cow may
estrus, then falls off abruptly. Mares with be defined as the time she will stand when
longer estrous periods should be bred mounted by another cow or bull. This
on the third or fourth day and again 48 to standing heat averages about 18 hours in
72 hours later. If heat is longer than 8 to both dairy and beef cows, somewhat less
10 days, it is better to wait until the next in heifers.
heat period. Mares with regular short The normal range is 12 to 24 hours.
heat periods throughout the year can be Ovulation normally occurs about 10 to 14
successfully bred at any time of the year. hours after the end of estrus in the cow, or
Early in the breeding season, some 24 to 32 hours after the onset of standing
mares show intense sexual desire during heat. Estrus detection is a primary factor in
long heat periods but do not ovulate as dairy and beef cattle herd economic perfor-
they are still in transition. These mares mance. Numerous devices have been devel-
probably will not conceive until their heat oped to aid in heat detection, but physical
periods become shorter and more regular. signs such as standing to be mounted and a
Other mares may have only silent heat clear vaginal discharge (Fig. 27‐8) can also
periods, in which ovulation occurs but no be used to confirm estrous cycle stage.
sexual desire is evident. Many of these Time of Breeding. Conception has
mares conceive if the time for breeding occurred in cattle bred as early as 34 hours
can be identified by palpation or ultra- before ovulation and as late as 14 hours
sound per rectum and the appearance after ovulation. It has been suggested that
of the vulva, vagina, and cervix. Foal heat bovine spermatozoa must be present for
(or postpartum estrus) may occur 1 to at least 6 hours in the uterus or uterine
2 weeks after foaling. Ovulation may tubes of the cow before they are capable