Understanding animal reproduction is a key element of animal science because reproductive efficiency is a trait of great economic performance in domestic animals. To achieve reproductive efficiency requires a good understanding of reproductive physiology and behavior. Reproductive efficiency is usually measured as animals per 100 breeding females or animals per litter. Domestic animals generally have limited periods of estrus or sexual receptivity. In contrast most primates are sexually receptive during most of their reproductive cycle. Some domesic animals (e.g.cats, sheep and horses) show seasonal periods of estrus.
Male reproductive system.
The functions of the male reproductive system are similar for all domestic animals namely - the production, storage and deposition of sperm cells. It also produces male sex hormones and serves as a passageway for the expulsion of urine from the bladder. The detailed tructure varies with species, breed, age and sexual maturity. Testes (singular testis) produce sperm and the hormone testosterone. This hormone is responsible for the development of secondary sexual characteristics and libido. The sperm cells are produced in the testes (spermatogenesis) which are enclosed by the scrotum and held outside the body at a temperature slightly below body temperature. The sperm produced in the testes move to the epididymis which is a long coiled tube connected to each testicle. The epididymis is responsible for the maturation, storage and transportation of sperm. The Vas Deferens connects the epididymis to the urethra which is a tube extending from the bladder to the tip of the penis. Accessory glands secrete seminal fluid that function as a transportation and protection medium. The penis consists of erectile tissue which becomes engorged with blood during sexual arousal allowing its insertion into the vagina. In some species (bull boar and ram) the penis is S shaped when relaxed because of a structure called a sigmoid flexure. This straightens out to produce erection and in these species there is a smaller amount of erectile tissue present.
Female reproductive system.
Functions are to produce ova to be fertilized, to serve as a receptacle for the penis during copulation, to house and nourish the fetus during pregnancy and to deliver the fetus at term. Also responsible for the production of female hormones estrogen and progesterone which are involved in sexual development, the reproductive cycle and lactation. There are fewer anatomical differences between females of different species compared to males.
The two ovaries function to produce ova, estrogen and progesterone. The ovum develops inside a follicle (Graffian Follicle) which then ruptures releasing the egg into a funnel like structure, the infundibulum, which collects the ova and directs them into the Fallopian tubes or oviducts. Fertilization occurs in the Fallopian tubes and after a period of 3-5 days the embryo travels down the Fallopian tube and attaches to the wall of the uterine horn or the uterus depending upon the species. The uterus is separated from the vagina by the cervix. This is a mass of concentric connective tissue. It seals the uterus from the vagina during pregnancy and serves also as a passageway for semen during copulation. The vagina is the receptacle for the penis during copulation.
Reproductive cycle.
The estrous cycle is the period from the beginning of one heatperiod to the beginning of the next. This varies for different livestock species. e.g. in cattle it averages 21 days. The estrous cycle begins with an ovum developing inside a follicle. This follicle secretes estrogen which begins the heat period during which the animal will be receptive to the male. The outward signs of heat differ between species. Length of heat also varies with species. At some point the follicle erupts leading to ovulation. After ovulation the follicle becomes a corpus luteum and secretes the hormone progesterone. This causes the absence of heat until the next follicle matures. If the ovum (ova) is fertilized the corpus luteum persists on the ovary and continues to produce progesterone. This will suppress further ovulationa nd play a role in maintaining pregnancy. If fertilization has not occurred the CL will regress, progsterone secretion declines and the next follicle will begin to develop, leading to the next cycle.
The events of the reproductive cycle are a consequence of an interplay between the ovarian hormones discussed above and hormones produced from the anterior pituitary. Follicular stimulating hormone (FSH) stimulates the follicle cells of the ovary to secrete estrogens. As estrogen in blood increases FSH production ceases and then estrogen subsides. As estrogen drops FSH production will increase again and the cycle continues. Progesterone production also occurs in the ovary and is influenced by another hormone of the anterior pituitary luteinizing hormone (LH) which also induces ovulation.
FSH and LH also influence male reproductive physiology. LH stimulates cells to produce testosterone and FSH stimulates seminiferous tubule cells to nourish spermatozoa.
Pregnancy and parturition
Following fertilization embryonic cells divide about every 20h moving through
the 2,4,8, 16 cell stage. Migration through oviduct to uterus or uterine
horn is typically at 16 or 32 cell stage. Chorionic and amniotic membrane
develop and the chorion attaches to the uterus wall. Provides nutrients and
discharges waste After parturition the fetus is expelled by contractions
of the uterus and passes through the cervix and vagina. The placenta is also
expelled.
Reproductive cycle data on domestic species.
| Heat duration (hours) | Cycle length (days) | |
| cattle | 12 | 21 |
| sheep | 30 | 17 |
| horses | 6 | 21 |
| dog | 9 | - |
| cat | 5 | 10 |
| swine | 44 | 21 |
| Gestation length (days) | Usual number offspring | |
| cattle | 285 | 1 |
| sheep | 147 | 1-3 |
| horses | 336 | 1 |
| dog | 52 | 4 |
| cat | 60 | 7 |
| swine | 114 | 6-14 |
Length of gestation and usual number of offspring in domestic species
Some Factors Modifying Reproductive Cycles.
Photoperiod - some animals are seasonal breeders and the female cycle is influenced by day length or light availability. This is called photoperiod. Examples are the cat, the horse and the sheep. In cats and horses ovarian activity are positively affected by increasing light. Sheep and goats are positively affected by decreasing light. Consequently cats and horses will come into their estrus in the spring while sheep will do so in the fall (in the northern hemisphere). The effects of photoperiod are mediated via melatonin which is produced by the pineal gland.
Nutrition - inadequate nutrition will lead to depression of ovarian activity and can also affect male fertility. Sometimes forages containing high levels of estrogen can also affect reproductive cycles.
Lactation - In some species lactation nand suckling can suppress ovulation. This suppression is complete in pigs and usually in cats. It does not occur in cattle beyond the first cycle or two after calving.
Reproductive problems in livestock.
Sterility - can be permanent or temporary and caused in various ways. e.g. Cryptorchidism is the failure of both testicles to descend into the scrotum. This causes complete sterility. Sometimes only one is retained (monorchid) and this leads to lowered sperm production. Both these conditions are inherited.
Freemartinism - a condition in cattle in which the female of twin calves of mixed sex is usually sterile (90%) because of an incompletely formed genital tract.
Disease - Bacterial, viral or protozoal infections of the reproductive tract often lead to abortion. e.g. Brucellosis, leptospirosis, BVD.
Nutrition - inadequate nutrition, deficiencies of vitamins and minerals, often lead to lowered reproductive performance or sterility.
Technology in livestock reproduction
Artificial insemination (AI) had a huge effect on certain areas of the livestock industry, notably the dairy industry. It is also widely used in beef cattle and in a more limited way in sheep, pigs and horses. Through AI it is possible for semen, collected from genetically superior males, to be diluted, frozen in liquid nitrogen and stored for long periods. It can be readily transported.
Multiple ovulation is a technique in which a genetically superior female is treated with hormones to induce her to produce many eggs simultaneously. These eggs can be collected, fertilized with sperm in vitro, and then implanted into surrogate mothers. In this way one female can produce many more offspring than otherwise possible. It is also possible to take the fertilized egg at an early stage of development (usually at the two or four cell stage) when it is totipotent. The cells can be separated and each cell can become a new, genetically identical individual. This is called cloning. A related technique called nuclear transfer has even greater potential. Using this technique the nucleus of a adult animal cell, say a mammary gland cell, can be placed into an unfertilized egg from which the nucleus has been removed and reimplanted in the uterus of a recipient female where it will develop into a complete animal, genetically identical to the donor animal. The recent excitement concerning the cloning of the Scottish sheep, Dolly, was that she was produced by nuclear transfer, not from an embryonic cell, but from a mature, differentiated cell from the mammary gland of an adult sheep. This raises exciting, and controversial, future possibilties.
Sex determination. In some types of livestock enterprise being able to determine the sex of offspring would be economically valuable. The dairy industry is a good example. Female calves are much more valuable than male calves. Since the X and Y chromosomes are usually of different sizes it is possible to distinguish sperm cells carrying an X versus a Y chromosome by using a fluoresecent dye and then passing them by a device which detects teh differences in fluorescence and separates them. In this way it would be possible to manipulate the sex ratio using "male" or "female" sperm.
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