Specifics of Selected Estrous Cycles

Mare

Puberty begins at 10 to 24 months, with an average onset of about 18 months.

Length of Estrous Cycle. Cycle lengths varying from 7 to 124 days have been reported. However, average lengths are about 21 to 22 days.

Length of Estrus. The average length of estrus in the mare is approximately 6 or 7 days. Estrus tends to shorten from spring to midsummer. Early in the breeding season, through March and April, estrus tends to be irregular and long, frequently with no ovulation. From May to July the periods become shorter and more regular, with ovulation as a normal part of the cycle. Ovulation usually occurs 1 to 2 days before the end of estrus.

Time of Breeding. Fertility rises during estrus to a peak 2 days before the end of estrus, then falls off abruptly. Mares with longer estrous periods should be bred on the third or fourth day and again 48 to 72 hours later. If heat is longer than 8 to 10 days, it is better to wait until the next heat period. Mares with regular short heat periods throughout the year can be successfully bred at any time of the year.

Early in the breeding season, some mares show intense sexual desire during long heat periods but do not ovulate. These mares prob­ably will not conceive until their heat periods become shorter and more regular. Other mares may have only silent heat periods, in which ovulation occurs but no sexual desire is evident. Many of these mares conceive if the time for breeding can be identified by rectal palpation and appearance of the vulva, vagina, and cervix.

Foal heat (or postpartum estrus) may occur 1 to 2 weeks after foaling. Ovulation may occur during this period of estrus, and conceptions are possible if mares are bred during a foal heat.

Histologic (tissue) changes in the lining of the genitalia of the mare during the estrous cycle approximate the general pattern found in all mammals.

Cow

In cattle, puberty varies considerably with the breed and level of nutrition. Holstein heifers show first estrus at an average of 37 weeks of age on a high level of nutrition, 49 weeks on a medium level, and 72 weeks on a low level of feeding. Puberty appears to occur when the heifer is about two-thirds of her adult body size, measured by height and length rather than weight.

Length of Estrous Cycle. The estrous cycle averages 20 days for heifers and 21 to 22 days for mature cows.

Length of Estrus. The estrous period in the cow may be defined as the time she will stand when mounted by another cow or bull. This standing heat averages about 18 hours in both dairy and beef cows, somewhat less in heifers.

The normal range is 12 to 24 hours. Ovula­tion normally occurs about 10 to 14 hours after the end of estrus in the cow.

Time of Breeding. Conception has occurred in cattle bred as early as 34 hours before ovula­tion and as late as 14 hours after ovulation. It has been suggested that bovine spermatozoa must be present for at least 6 hours in the uterus or uterine tubes of the cow before they are capable of fertilizing an ovum. For artificial insemination, cows that come into standing heat in the morning are bred the same after­noon, and cows that come into standing heat in the afternoon are bred the next morning.

Bleeding from the vulva occurs in a high percentage of heifers and cows 1 to 3 days after the end of estrus. This phenomenon is metes- trus bleeding; fertility is reduced if breeding is done during bleeding. However, fertility is not impaired if breeding has taken place before the bleeding.

Ewe

Puberty usually occurs during the first breeding season (usually fall) after ewes are 4 to 12 months of age if the ewes are well fed. If ewes fail to reach puberty during their first potential breeding season, they may be over 12 months old before reaching puberty.

Breeding Season. The ewe is probably the best example of a seasonally polyestrous animal, with a long period of anestrus followed by a breeding season that may vary from 1 to 20 consecutive estrous cycles. The length of breed­ing season appears to be related to the severity of climate in which the breed developed. in severe climates, a suitable lambing period is restricted; hence, the breeding season is like­wise restricted so that lambing occurs only during the favorable time (scotch Blackface is an example). Breeds developed in milder cli­mates may lamb successfully over a longer period, so the breeding or sexual season is also extended (e.g., Merino sheep).

Length of Estrous Cycle. The average estrous cycle in the ewe is 16.5 to 17.5 days. Unusually long or short cycles tend to appear during the early and later parts of the sexual season, rather than during the middle part.

Length of Estrus. Duration of estrus averages about 30 hours, with a range for most ewes between 24 and 48 hours. The ram may be attracted during proestrus and metestrus as well as estrus, but the ewe will accept him only during the actual estrous period. ovulation occurs near the termination of estrus, and two or three ovulations may occur in the same estrous period.

Time of Breeding. The best time for breeding ewes is middle to late estrus.

Sow

sexual maturity in the gilt usually occurs about 7 months of age.

Length of Estrous Cycle. The average estrous cycle in swine is about 21 days, with a range of 18 to 24 days considered normal.

Length of Estrus. The estrous period may range from 15 to 96 hours, with an average duration between 40 and 46 hours. The first estrus after weaning is usually longer and may average 65 hours; it occurs about 7 to 9 days after weaning of the piglets.

Many sows exhibit infertile estrus 1 to 3 days following parturition. in nearly all of these animals, ovulation does not occur. it has been suggested that this heat may be caused by estro­gen from some source other than the ovary. ovulation occurs during the latter part of estrus, about the second day of the cycle. At each period, 10 to 25 ova are shed, with an average of about 16 for most breeds.