Sheep - AgRIS
Sheep - AgRIS Sheep - AgRIS
7. Reproduction Reproduction, a normal physiological process, is one, of the most remarkable and imprtant phenomena for the sheep husbandry and breeding. Regular and repeated reproduction is the basis of the income from sheep. This is true not only of sheep but of other livestock; indeed, it is true of the whole realm of agriculture. Reproduction is not only the source of continuing income; it is upon this function that the whole science of genetics is based. As new animals are produced, it is possible to influence the fundamental characteristics of some of the members of the species. Influential as the reproductive function is, it is in turn influenced by other factors, particularly age, health, and nutrition. Some other features of environment may also be important in relation to reproduction. There are many erroneous notions regarding the processes of reproduction. The breeder should have a sufficiently intimate knowledge of the anatomy and functions of the male and female reproductive systems to enable them to understand the reproductive processes in the management a flock. Without such knowledge and understanding, there are no basic principles of guidance and no modern basis for the development of scientifically sound breeding programs. Sheep in tropics generally breed throughout the year. Rams of indigenous breeds donate good quality semen under proper management conditons throughout the year. However, the rams of temperate breeds if not protected from high temperature, high humidity and high solar radiation will not produce good quality semen during hot dry and hot humid seasons. Females of tropical breeds cycle throughout the year unlike the temperate breeds which are affected by hours of day light and breed with declining day length. They come in heat in autumn from August to November, although some may breed upto February. Indigenous breeds usually breed during nutritionally better time of year, i.e. July to August, or immediately after the onset of the monsoon. A small percentage of them also breed during March-April when they are stubble grazed on harvested fields and supplemented with Acacia and Prosopis pods, and thus get flushed. Sheep breed in different seasons depending upon the availability offeed. To some extent breeding season is controlled by the sheep breeder on the basis of availability of feed resources and physical environmental conditions prevailing both at breeding and lambing, though the consideration would be more in case of lambing as extremes of weather will more seriously affect lamb survival. The breeding should, therefore, be arranged in a manner that the lambs are dropped when plenty of vegetation is available and weather conditions are mild. 7.1 Male reproduction Although there are individual exceptions, most male in the exotic breeds reach the stage of development at an age of approsimately 4 to 6 or 7 months, when reproduction is possible. The sex organs of males are apparently fully developed by that time, and the so-called secondary characteristics which are dependent upon the presence and functioning of these organs are very noticeable. Because the reproductive cells of the male, the sperm or spermato-zoa, are formed in the testicles, the testicles are generally considered the primary organs of reproduction of the male. In addition to the formation of sperm cells, a testicular secretion testosterone when absorbed by the body causes the development of the secondary sexual characteristics. When these features are very marked in an individual, he is described as masculine. Masculinity is in no way associated with the transmission of any features such as color, length of fleece, or other factors of the ram to his offspring. The well-developed, masculine features merely indicate an abundance of the hormone which stimulates the developrment of the features. The removal of the testicles, and hence the removal of the source of the hormone, results in stopping the development of the masculine characters. This is the reason. why wethers differ so much in appearance from rams. The testicles are normally carried outside the body cavity in a sac called the scrotum. The normal temperature of the testicles is below that of the body. One of the main functions of the scrotum is to regulate the temperature of these organs. This explains the lack of fertility in rams if the testicles are not descended into the scrotal sac. It also explains why some rams that have 397
een heavily fed and therefore may have had an increased body temperature accompanied by some increase in scrotal temperature may be sterile. Breeding rarns that have been showrn or heavily fed may hasten the recovery of the reproductive function of the testicles. The production of the hormone by the testicles does not seem to be influenced greatly by temperature, and hence a cryptorchid or ridgling may appear thoroughly masculine. Rams with one testicle retained within the body cavity should not be used for breeding, as there is a tendency for the condition to occur in succeeding generations, although it will not appear in all cases. The testicles are covered with a fibrous capsule, the tunica albuginea. Within this capsule are blood vessels, nerves and connective, interstitial, and spermatogenic tissue. The connective tissue divides the interior of the testicles into very small sections or compartments. In each of these sections are the seminiferous tubules within which is the secreting tissue where the billions of sperm cells are developed. It is estimated that these tubules are so numerous that their aggregate length would be many thousand feet. It will be necessary to consider cell formation more in detail in studying the inheritance of characters, but, for the purpose of reproduction, it is sufficient to remember that the contribution of the male to the new offspring originates in the seminiferous tubules. There, spermatogonia or sperm mother cells enlarge and divide. Thus, two cells arise. Both of these divide again. During these processes the cells have moved toward the end of the tubule. Nourishment is provided for the completion of the development. When this occurs. spermatozoa or sperm cells have been produced. A normal sperm cell consists of a head, body, and tail. The formation of spermatozoa is a process which continues from the time it first starts until the end of life in the case of most sheep. The sperm as found in the testicles are not very active. They become active later when they come into contact with some secretions of other glands. Besides carrying the factors which the offspring will inherit from the sire, the spermatozoa initiate cell division on the part of the egg after its fertilization. There are other important parts of the male reproductive system. Ducts, called efferent ducts or vasa efferentia, formed by the joining of many of the tubules, lead to the epididymis which is essentially a large tube that is found on the outside of the testicles and leads from the top of the testes to the base. The epididymis provides a passageway and temporary storage for the sperm. Ducts leading from the epididymis form the vasa deferentia. These tubes pass up through the inguinal canal—the small opening into the abdominal cavity and connect with the urethra, which, leading through the penis, completes the passage from the testes to the exterior. In rams there is a small filiform appendage at the end of the penis. This is not, however, a vital matter in reproduction. At various points along this passageway are accessory glands whose chief function seems to be the secretion of fluids which serve as media for the transference and perhaps nourishment or stimulation of the spermatoza. Alongside of the ducts leading from the testes to the abdominal cavity are blood vessels, nerves, and supporting tissues. These are cut or broken when castration is performed. Rupture and separation of the main blood vessel so that blood does not reach the testicle is the basis of bloodless methods of castration. Under such conditions the testes shrink or atrophy. Not all of the sperm that are formed reach maturity. In many individual there are thousands of imperfectly formed sperm, and many of the sperm have no motility. Infections and above normal temperatures are two factors that are known to affect the sperm cells. Any males that have a large percentage of abnormal or nonmotile sperm are likely to be unreliable as breeders, and in severe cases they are sterile. Microscopic examinations of the semen are neeeded to determine whether such conditions exist. The semen of rams emitted at one time is from 0.5 to 2.0 cubic centimeters in volume. Very frequent service may reduce the volume as well as the number of sperm that the semen contains. The numberofsperm ina cubic millimeter of semen ranges from 500,000 to 6,000,000 or more. The average number per cubic millimeter is probably about 1,000,000 sperm. Since only one sperm is required to fertilize each egg cell produced by the female, the lavish production of sperm is apparent. 398
- Page 11 and 12: een possible because of its substan
- Page 13 and 14: (USA). The Rambouillet as purebreds
- Page 15 and 16: ii) Marwari Deriving its name from
- Page 17 and 18: Pradesh and Bihar, Bijapur, Gulbarg
- Page 19 and 20: elatively tall with little hair exc
- Page 21 and 22: 4.2 Distribution of type breeds in
- Page 23 and 24: Table 4.1 continued...... Region/Br
- Page 25 and 26: Table 4.2 continued....... SI. Bree
- Page 27 and 28: 5. Genetics 5.1 Chromosome Profile
- Page 29 and 30: esponsible for the formation of one
- Page 31 and 32: the exercise these animals did not
- Page 33 and 34: For understanding the inheritance o
- Page 35 and 36: with a dense growth of relatively l
- Page 37 and 38: produced on rations which contain i
- Page 39 and 40: The Coefficient of Variation The co
- Page 41 and 42: 5.6.3 Wool Production/Wool yield an
- Page 43 and 44: 6. Breeding 6.1 Components of Sheep
- Page 45 and 46: elationships. Certain coat colors a
- Page 47 and 48: Selection on this basis means that
- Page 49 and 50: ii) Independent Culling Method In t
- Page 51 and 52: eflected by increase in average fle
- Page 53 and 54: at 75 or 8-4 monthly intervals comp
- Page 55 and 56: Based on the crossbreeding results,
- Page 57 and 58: Table 6.8 Means and standard errors
- Page 59 and 60: month post-weaning individual feedl
- Page 61: ams) in November, 1975 for evaluati
- Page 65 and 66: need for one or more "teasers" to d
- Page 67 and 68: ii) Consistency The normal consiste
- Page 69 and 70: collection frequencies per day in 3
- Page 71 and 72: iv) Altitude High altitude and poor
- Page 73 and 74: containing diluents enriched with a
- Page 75 and 76: 6. Nutrients Although spermatozoa u
- Page 77 and 78: 7. 3.1 Puberty Puberty in the femal
- Page 79 and 80: Estrogen in large quantities inhibi
- Page 81 and 82: The udder becomes firm and enlarged
- Page 83 and 84: 7. 3. 8 Interlambing interval It is
- Page 85 and 86: 8. Nutrition 8.1 Components of Shee
- Page 87 and 88: allied to the various amino acids o
- Page 89 and 90: animals. A deficiency of salt is sh
- Page 91 and 92: seem to have most significance in s
- Page 93 and 94: Table 8.2 Chemical composition of g
- Page 95 and 96: Table 8.3 Nutritive value of fodder
- Page 97 and 98: Mineral and vitamin Requirement If
- Page 99 and 100: Table 8.5 Total digestible nutrient
- Page 101 and 102: Table 8.7 continued....... c) Lacta
- Page 103 and 104: shirinking due to reclaimation of l
- Page 105 and 106: Sorghum-Berseem-Maize Improved vari
- Page 107 and 108: 9. Housing and Management 9.1 Syste
- Page 109 and 110: ) Shearing, skirtng and primary cla
- Page 111 and 112: The availability of pastures which
een heavily fed and therefore may have had an increased body temperature accompanied by<br />
some increase in scrotal temperature may be sterile. Breeding rarns that have been showrn or<br />
heavily fed may hasten the recovery of the reproductive function of the testicles. The<br />
production of the hormone by the testicles does not seem to be influenced greatly by<br />
temperature, and hence a cryptorchid or ridgling may appear thoroughly masculine. Rams with<br />
one testicle retained within the body cavity should not be used for breeding, as there is a<br />
tendency for the condition to occur in succeeding generations, although it will not appear in all<br />
cases.<br />
The testicles are covered with a fibrous capsule, the tunica albuginea. Within this capsule<br />
are blood vessels, nerves and connective, interstitial, and spermatogenic tissue. The connective<br />
tissue divides the interior of the testicles into very small sections or compartments. In each of<br />
these sections are the seminiferous tubules within which is the secreting tissue where the<br />
billions of sperm cells are developed. It is estimated that these tubules are so numerous that<br />
their aggregate length would be many thousand feet. It will be necessary to consider cell<br />
formation more in detail in studying the inheritance of characters, but, for the purpose of<br />
reproduction, it is sufficient to remember that the contribution of the male to the new offspring<br />
originates in the seminiferous tubules. There, spermatogonia or sperm mother cells enlarge and<br />
divide. Thus, two cells arise. Both of these divide again. During these processes the cells have<br />
moved toward the end of the tubule. Nourishment is provided for the completion of the<br />
development. When this occurs. spermatozoa or sperm cells have been produced.<br />
A normal sperm cell consists of a head, body, and tail. The formation of spermatozoa is a<br />
process which continues from the time it first starts until the end of life in the case of most<br />
sheep. The sperm as found in the testicles are not very active. They become active later when<br />
they come into contact with some secretions of other glands. Besides carrying the factors which<br />
the offspring will inherit from the sire, the spermatozoa initiate cell division on the part of the<br />
egg after its fertilization.<br />
There are other important parts of the male reproductive system. Ducts, called efferent<br />
ducts or vasa efferentia, formed by the joining of many of the tubules, lead to the epididymis<br />
which is essentially a large tube that is found on the outside of the testicles and leads from the<br />
top of the testes to the base. The epididymis provides a passageway and temporary storage for<br />
the sperm. Ducts leading from the epididymis form the vasa deferentia. These tubes pass up<br />
through the inguinal canal—the small opening into the abdominal cavity and connect with the<br />
urethra, which, leading through the penis, completes the passage from the testes to the exterior.<br />
In rams there is a small filiform appendage at the end of the penis. This is not, however, a vital<br />
matter in reproduction. At various points along this passageway are accessory glands whose<br />
chief function seems to be the secretion of fluids which serve as media for the transference and<br />
perhaps nourishment or stimulation of the spermatoza. Alongside of the ducts leading from the<br />
testes to the abdominal cavity are blood vessels, nerves, and supporting tissues. These are cut or<br />
broken when castration is performed. Rupture and separation of the main blood vessel so that<br />
blood does not reach the testicle is the basis of bloodless methods of castration. Under such<br />
conditions the testes shrink or atrophy.<br />
Not all of the sperm that are formed reach maturity. In many individual there are thousands<br />
of imperfectly formed sperm, and many of the sperm have no motility. Infections and above<br />
normal temperatures are two factors that are known to affect the sperm cells. Any males that<br />
have a large percentage of abnormal or nonmotile sperm are likely to be unreliable as breeders,<br />
and in severe cases they are sterile. Microscopic examinations of the semen are neeeded to<br />
determine whether such conditions exist.<br />
The semen of rams emitted at one time is from 0.5 to 2.0 cubic centimeters in volume.<br />
Very frequent service may reduce the volume as well as the number of sperm that the semen<br />
contains. The numberofsperm ina cubic millimeter of semen ranges from 500,000 to 6,000,000<br />
or more. The average number per cubic millimeter is probably about 1,000,000 sperm. Since<br />
only one sperm is required to fertilize each egg cell produced by the female, the lavish<br />
production of sperm is apparent.<br />
398