Productivity of livestock depends on reproductive efficiency and is usually measured by the number of offspring produced by an animal or herd per unit of time. Therefore, management of reproductive cycles is critical for obtaining maximum efficiency of reproducing animals.
Dairy and beef cattle are derived from common ancestors and thus have common reproductive characteristics. The period of time from one ovulation to the next ovulation is called the estrous cycle. The estrous cycle in cattle is on average 21 days in length and is characterized as being one in which the cow does not permit mating to occur except at the time of ovulation. This period of time in which mating is permitted is relatively short, being in the order of 16-18 h. The estrous cycle in cattle is continuous (poly-estrous) throughout the year and is not seasonal as is the case in some other ruminant-type animals such as sheep, deer, antelope, and elk. Cattle ovulate 10-14 h after the end of estrus, and normally one follicle ovulates per estrous cycle. The gestation period is 283 days, and twins occur only 1% of the time.
Sheep are generally seasonally polyestrous, with recurring estrous cycles during the fall of the year followed by a prolonged quiescent period. Some breeds of sheep that originated in equatorial regions and are subject to less variations in temperature and photoperiod have longer breeding seasons. The period of sexual receptivity in sheep lasts for 24-36 h, but may vary widely. The length of the estrous cycle is 14-19 days. Seasonal breeding in animals is associated with an increased frequency and size of episodic releases of luteinizing hormone (11). Ovulation normally occurs near the end of estrus. Ovulation rate is influenced by breed and nutrition, but twinning is extremely common in sheep. Gestation is 150 days in length.
The pig, like the cow, is polyestrous. The period of sexual receptivity in pigs lasts about 40-60 h and may be influenced by season, breed, and endocrine dysfunction (12). The length of the estrous cycle is 21 days. Ovulation rates are strongly influenced by weight of the gilt at breeding and the amount of inbreeding. Gestation in the pig is 114 days and average litter size is in the order of 9-12 pigs per litter.
The estrous cycle in all livestock is characterized by profound changes in behavioral patterns and blood hormone profiles. Cyclic changes during the estrous period reflect the secretory functions and interdependence of the ovary, uterus, hypothalamus, and pituitary gland. The estrous cycle is controlled by ovarian hormone secretions (estrogens and progesterone) and may be subdivided into a follicular phase, an ovulatory phase, and a luteal phase (Fig. 2). The ovary functions to provide fertilizable ova and a balanced ratio of steroid hormones to facilitate development of the reproductive tract for migration of the early embryo and successful implantation in the uterus.
The male reproductive system in livestock is comprised of two testes (producing both sperm and the male sex hormone testosterone), excretory ducts (epididymis, vas deferens, and ejaculatory duct), and accessory structures (prostate, seminal vesicles, bulbourethral glands, and penis). The scrotum, containing two testes, provides for efficient regulation of testicular temperature. Descent of both testes from the abdomen to their proper scrotal location is necessary for maximal fertility. Failure of one or both testes to descend is a common reproductive organ defect in livestock, especially in swine, where the condition is a hereditary defect transmitted by the male and is referred to as cryptorchidism (13).
Reproductive management of the male primarily involves maintenance of health and nutrition to optimize sperm production and libido. Methods for assessing individual male fertility, such as breeding soundness evaluations, are subjective and poorly correlated with pregnancy rates.
Decisions about breeding are some of the most important decisions a livestock producer must make. The livestock breeder must consider the heritability of a characteristic, such as prolificacy, feed conversion, milk yield, or carcass merit, for example. The livestock breeder also must consider the merits of inbreeding, outbreeding, crossbreeding, and the relative merits of different breeds of animals.
The development of modern breeds of livestock began in the 1700s. The origin of cattle and sheep breeds can be traced to Europe and the British Isles. For classification purposes there are four basic categories of modern beef and dairy cattle. The classification system for cattle is not consistent because two of the categories reflect geographical origin of the cattle and the other two reflect the purpose of the cattle.
1. British and continental breeds (beef): Includes Angus, Hereford, Shorthorn, Charolais, Limousin, Chianina, Gelbvieh
2. North American breeds (beef): Brahman, Brangus, Beefmaster, and Santa Gertrudis
3. Dual-purpose breeds (beef and dairy): Ayrshire, Milking Shorthorn, Red Poll, and Brown Swiss
4. Dairy breeds: Guernsey, Holstein, and Jersey
The foundation of modern-day pigs can be traced to European and Asiatic strains of pigs. Modern swine are raised to produce lean high-quality pork. However, not long ago, breeds of swine were classified as lard-type and bacon-type. Although the individual breeds of swine continue today, all swine producers strive for the same high-quality meat-type pig.
As in the case of cattle, the classification system for breeds of sheep is inconsistent. The Rambouillet and Merino breeds were developed for the production of the fine wool characteristic, but much lamb and mutton comes from these breeds. Likewise, income derived from wool represents a significant portion of the value of the meat-type breeds.
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