Domestic sheep reproduction

Last updated
A cross-bred ewe suckles her lamb, which was the first of the 2008 spring lambing at a farm in Coventry, England First Lamb 2008 suckles.jpg
A cross-bred ewe suckles her lamb, which was the first of the 2008 spring lambing at a farm in Coventry, England

Domestic sheep reproduce sexually much like other mammals, and their reproductive strategy is furthermore very similar to other domestic herd animals. A flock of sheep is generally mated by a single ram, which has either been chosen by a farmer or has established dominance through physical contest with other rams (in feral populations). [1] Most sheep have a breeding season (tupping) in the autumn, though some are able to breed year-round. [1]

Contents

Largely as a result of the influence of humans in sheep breeding, ewes often produce multiple lambs. This increase in the lamb births, both in number and birth weight, may cause problems in delivery and lamb survival, requiring the intervention of shepherds.

Sexual behavior

Ewes generally reach sexual maturity at six to eight months of age, and rams generally at four to six (ram lambs have occasionally been known to impregnate their mothers at two months). [1] Sheep are seasonally polyoestrus animals. [2] Ewes enter into oestrus cycles about every 17 days, which last for approximately 30 hours. [1] In addition to emitting a scent, they indicate readiness through physical displays towards rams. The phenomenon of the freemartin, a female bovine that is behaviorally masculine and lacks functioning ovaries, is commonly associated with cattle, but does occur to some extent in sheep. [3] The instance of freemartins in sheep may be increasing in concert with the rise in twinning (freemartins are the result of male-female twin combinations). [3] The Flehmen response is exhibited by rams when they smell the urine of a ewe in oestrous. The vomeronasal organ has receptors which detect the oestrogens in the ewe's urine. [4] The ram displays this by extending his neck and curling his lip. [5]

Bighorn sheep exhibiting the Flehmen response Courting, Bighorn Sheep Style (22623154384).jpg
Bighorn sheep exhibiting the Flehmen response

Rutting

Without human intervention, rams may fight during the rut to determine which individuals may mate with ewes. Rams, especially unfamiliar ones, will also fight outside the breeding period to establish dominance; rams can kill one another if allowed to mix freely. [1] During the rut, even normally friendly rams may become aggressive towards humans due to increases in their hormone levels. [6]

A pregnant St Croix ewe Pregnant ewe on Tortola.jpg
A pregnant St Croix ewe

Historically, especially aggressive rams were sometimes blindfolded or hobbled. [7] Today, those who keep rams typically prefer softer preventative measures, such as moving within a clear line to an exit, never turning their back on a ram, and possibly dousing with water or a diluted solution of bleach or vinegar to dissuade charges. [1] [8]

Pregnancy

Without ultrasound or other special tools, determining if a sheep is pregnant is difficult. [1] [7] Ewes only begin to visibly show a pregnancy about six weeks before giving birth, so shepherds often rely on the assumption that a ram will impregnate all the ewes in a flock. [1] However, by fitting a ram with a chest harness called a marking harness that holds a special crayon (or raddle, sometimes spelled reddle), ewes that have been mounted are marked with a color. Dye may also be directly applied to the ram's brisket. This measure is not used in flocks where wool is important, since the color of a raddle contaminates it.[ citation needed ] The crayon in the marking harness can be changed during the breeding cycle to allow for lambing date predictions for each ewe. [9]

After mating, sheep have a gestation period of around five months. Within a few days of the impending birth, ewes begin to behave differently. They may lie down and stand erratically, paw the ground, or otherwise act out of sync with normal flock patterns. An ewe's udder will quickly fill out, and her vulva will swell. Vaginal, uterine or anal prolapse may also occur, in which case either stitching or a physical retainer can be used to hold the orifice in if the problem persists. [6] Usually ewes that experience serious issues while lambing such as prolapse, will be discarded from the flock to avoid further complications in upcoming years. [10]

Artificial insemination and embryo transfer

In addition to natural insemination by rams, artificial insemination and embryo transfers have been used in sheep breeding programs for many years in Australia and New Zealand. [11] These programs have become more commonplace in the United States during the 2000s as the number of veterinarians qualified to perform these types of procedures with proficiency have grown. [12] However, ovine AI is a relatively complicated procedure compared to other livestock. Unlike cattle or goats, which have straight cervices that can be vaginally inseminated, ewes have a curved cervix which is more difficult to access. Additionally, breeders were until recently unable to control their ewe's estrus cycles. [13] The ability to control the estrus cycle is much easier today because of products that safely assist in aligning heat cycles. Some examples of products are PG600, CIDRs, Estrumate and Folltropin V. These products contain progesterone which will bring on the induction of estrus in ewes (sheep) during seasonal anestrus. Seasonal anestrus is when ewes do not have regular estrous cycles outside the natural breeding season.

Historically, vaginal insemination of sheep only produced 40-60% success rates, and was thus called a "shot in the dark" (SID). In the 1980s, Australian researchers developed a laparoscopic insemination procedure which, combined with the use of progestogen and pregnant mare's serum gonadotropin (PMSG), yielded much higher success rates (50-80% or more), and has become the standard for artificial insemination of sheep in the 21st century. [13]

Semen collection is naturally an integral component of this entire process. Once semen has been collected it can be used immediately for insemination or slowly frozen for use at a later date. Fresh semen is recognized as the method of choice as it lives longer and yields higher conception rates. Frozen semen will work but it must be the highest quality of semen and the ewes must be inseminated twice in the same day. [14] The marketing of ram semen is a major part of this industry. Producers owning prize winning rams have found this to be a good avenue to leverage the accolades of their most famous animals.

During embryo transfer (ET) a minor surgical procedure with almost no risk of injury or infection when performed properly, sheep laparoscopy allows the importation of improved genetics, even of breeds which may otherwise be non-existent in certain countries due to the regulation of live animal imports. [13] Embryo transfer procedures are used to allow producers to maximize those females that produce the best lambs/kids either for retention into the flock or for sale to other producers. ET also allows producers to continue to utilize a ewe/doe that may not physically be able to carry or feed a set of lambs. ET can allow a producer to grow his flock quickly with above average individuals of similar bloodlines. The primary industry to utilize this technology in the United States is the club lamb breeders and exhibitors. It is a common practice in the commercial sheep industries of Australia, New Zealand, and South America. [15]

Average success rates in Embryo Transfer in terms of embryos recovered can vary widely. Each breed will respond differently to the ET process. Typically white faced ewes and does are more fertile than black faced ewes. A range of zero to the mid 20s in terms of viable embryos recovered from a flush procedure can be expected. Over the course of a year the average is 6.8 transferable eggs per donor with a 75% conception rate for those eggs. [14]

Infertility

Infertility can be attributed to many aspects in managerial practices as well as health factors. One of the main reasons low lambing percentages can be seen in a flock is due to mineral and vitamin deficiencies. [16] The main vitamins and minerals that play a major role in fertility are selenium, copper, vitamin A and D. [16] Other factors that affect fertility and potentially cause abortion are infectious diseases, inappropriate body condition or toxins in feed. [17]

Lambing

As the time for lambing approaches, the lamb will drop causing the ewe to have a swayback, exhibit restless behaviour and show a sunken appearance in front of the hipbone area. [18] When birth is imminent, contractions begin to take place, and the fitful behavior of the ewe may increase. A normal labor may take one to several hours, depending on how many lambs are present, the age of the ewe, and her physical and nutritional condition prior to the birth. [1] Though some breeds may regularly produce larger litters of lambs (records stand around nine lambs at once), most produce either single or twin lambs. [6] [19] The number of lambs a ewe produces per year is known as the lambing percentage. [20] The condition of the ewe during breeding season will impact the lambing percentage as well as the size of the lambs. [21] At some point, usually at the beginning of labor or soon after the births have occurred, ewes and lambs may be confined to small lambing jugs. [7] These pens, which are generally two to eight feet (0.6 to 2.4 m) in length and width, are designed to aid both careful observation of ewes and to cement the bond between them and their lambs. [1] [22]

The second of twins being born on a New Zealand pasture Emerging lamb cropped.jpg
The second of twins being born on a New Zealand pasture

Ovine obstetrics can be problematic. By selectively breeding ewes that produce multiple offspring with higher birth weights for generations, sheep producers have inadvertently caused some domestic sheep to have difficulty lambing. [6] However, it is a myth that sheep cannot lamb without human assistance; many ewes give birth directly in pasture without aid. [7] Balancing ease of lambing with high productivity is one of the dilemmas of sheep breeding. [6] While the majority of births are relatively normal and do not require intervention, many complications may arise. A lamb may present in the normal fashion (with both legs and head forward), but may simply be too large to slide out of the birth canal. This often happens when large rams are crossed with diminutive ewes (this is related to breed, rams are naturally larger than ewes by comparison). [7] Lambs may also present themselves with one shoulder to the side, completely backward, or with only some of their limbs protruding. [1] Lambs may also be spontaneously aborted or stillborn. Reproductive failure is a common consequence of infections such as toxoplasmosis and foot-and-mouth disease. [23] [24] Some types of abortion in sheep are preventable by vaccinations against these infections. [25]

In the case of any such problems, those present at lambing (who may or may not include a veterinarian, most shepherds become accomplished at lambing to some degree) may assist the ewe in extracting or repositioning lambs. [1] In severe cases, a cesarean section will be required to remove the lamb. [26] After the birth, ewes ideally break the amniotic sac (if it is not broken during labor), and begin licking clean the lamb. The licking clears the nose and mouth, dries the lamb, and stimulates it. [1] Lambs that are breathing and healthy at this point begin trying to stand, and ideally do so between a half and full hour, with help from the mother. [1] Generally after lambs stand, the umbilical cord is trimmed to about an inch (2.5 centimeters). Once trimmed, a small container (such as a film canister) of iodine is held against the lamb's belly over the remainder of the cord to prevent infection. [7]

Postnatal care

A lamb in Australia which, unusually, has not had its tail docked. Lamb - no docking.jpg
A lamb in Australia which, unusually, has not had its tail docked.

In normal situations, lambs nurse after standing, receiving vital colostrum milk. Lambs that either fail to nurse or are prevented from doing so by the ewe require aid in order to live. If coaxing the pair to accept nursing does not work, one of several steps may then be taken. Ewes may be held or tied to force them to accept a nursing lamb. If a lamb is not eating, a stomach tube may also be used to force feed the lamb in order to save its life. [1] In the case of a permanently rejected lamb, a shepherd may then attempt to foster an orphaned lamb onto another ewe. Lambs are also sometimes fostered after the death of their mother, either from the birth or other event.

Scent plays a large factor in ewes recognizing their lambs, so disrupting the scent of a newborn lamb with washing or over-handling may cause a ewe to reject it. [1] [6] [22] Conversely, various methods of imparting the scent of a ewe's own lamb to an orphaned one may be useful in fostering. If an orphaned lamb cannot be fostered, then it usually becomes what is known as a bottle lamb—a lamb raised by people and fed via bottle. [1]

An Oxford Down ewe and her twins in a lambing jug, note the iodine stains on the lambs Mudchute farm sheep with lambs.jpg
An Oxford Down ewe and her twins in a lambing jug, note the iodine stains on the lambs

After lambs are stabilized, lamb marking is carried out – this includes ear tagging, docking, castration and usually vaccination. [1] Ear tags with numbers are the primary mode of identification when sheep are not named; it is also the legal manner of animal identification in the European Union: the number may identify the individual sheep or only its flock. When performed at an early age, ear tagging seems to cause little or no discomfort to lambs. [1] [6] However, using tags improperly or using tags not designed for sheep may cause discomfort, largely due to excess weight of tags for other animals. [1]

Ram lambs not intended for breeding are castrated, though some shepherds choose to avoid the procedure for ethical, economic or practical reasons. [1] Ram lambs that will be slaughtered or separated from ewes before sexual maturity are not usually castrated. [22] In most breeds, lambs' tails are docked for health reasons. [7] The tail may be removed just below the lamb's caudal tail flaps (docking shorter than this may cause health problems such as rectal prolapse), [7] but in some breeds the tail is left longer, or is not docked at all. Docking is not necessary in short-tailed breeds, and it is not usually done in breeds in which a long tail is valued, such as Zwartbles. Though docking is often considered cruel and unnatural by animal rights activists, it is considered by sheep producers large and small alike to be a critical step in maintaining the health of sheep. [1] [7] [8] [22] Long, wooly tails make shearing more difficult, interfere with mating, and make sheep extremely susceptible to parasites, especially those that cause flystrike. [1] Both castration and docking can be performed with several instruments. An elastrator places a tight band of rubber around an area, causing it to atrophy and fall off in a number of weeks. This process is bloodless and does not seem to cause extended suffering to lambs, who tend to ignore it after several hours. [1] [6] [8] [22] In addition to the elastrator, a Burdizzo, emasculator, heated chisel or knife are sometimes used. [1] After one to three days in the lambing jugs, ewes and lambs are usually sufficiently stabilized to allow reintroduction to the rest of the flock. [6]

Commercial sheep breeding

Merino ewes and lambs in Walcha, New South Wales Merino ewes & lambs-crop.JPG
Merino ewes and lambs in Walcha, New South Wales

In the large sheep producing nations of South America, Australia and New Zealand sheep are usually bred on large tracts of land with much less intervention from the graziers or breeders. Merinos [ sentence fragment ], and much of the land in these countries does not lend itself to the mob intervention that is found in smaller flock breeding countries. [27] In these countries there is little need, and no option but for ewes to lamb outdoors as there are insufficient structures to handle the large flocks of ewes there. [28] New Zealand ewes produce 36 million lambs each spring time, which is an average of 2,250 lambs per farm. [29] Australian graziers, too, do not receive the financial support that governments in other countries provide to sheep breeders. Low-cost sheep breeding is based on large numbers of sheep per labour unit and having ewes that are capable of unsupervised lambing to produce hardy, active lambs. [30]

Managerial aspects

For breeders intent on strict improvements to their flocks, ewes are classed and inferior sheep are removed prior to mating in order to maintain or improve the quality of the flock. [31] Muffled (wooly) faces have long been associated with lower fertility rates. [32] Stud or specially selected rams are chosen with aid of objective measurements, genetic information and evaluation services that are now available in Australia and New Zealand. [33] [34] The choice of mating time is governed by many factors including climate, market requirements and feed availability. Rams are typically mated at about 2.5% depending on the age of the sheep, plus consideration as to the size and type of mating paddocks. [32] The mating period ranges from about 6 to 8 weeks in commercial flocks. Longer mating times result in management problems with lamb marking and shearing etc. [31]

A Middle age Paridera in a Natural Cave near to Monasterio de Piedra. Paridera Cueva del Rio Piedra.jpg
A Middle age Paridera in a Natural Cave near to Monasterio de Piedra.

Good nutrition is vital to ewes during the last 6 weeks of pregnancy in order to prevent pregnancy toxaemia, especially in twin bearing ewes. Overfeeding, however, may result in over large single lambs and dystocia. Shearing ewes before lambing reduces the number of ewes that are cast (i.e. unable to rise unassisted), and the number of lambs and ewes that are lost. Lambs, too, are aided in finding the udder and suckling a shorn ewe. [31] In addition, shearing the ewe before lambing can increase the quality of the fleece as wool breaking can occur since giving birth is such a major stress on the ewe's body. [35] It is important to keep in mind weather conditions prior to shearing ewes, especially in colder climates. [36]

After shearing ewes are typically placed in well sheltered paddocks that have good feed and water. Attention to ewes that are lambing varies according to the breed, size and locations of properties. Unless they are stud ewes it unlikely that they will receive intensive care. On stations with large paddocks there is a policy of non-interference. On other properties the mobs are inspected by stockmen at varying intervals to stand cast ewes and deal with dystocia. Producers also sometimes quietly drift pregnant ewes away from ewes that have already lambed, in order to prevent mis-mothering. [32] [31]

Lambs are usually marked at three to six weeks of age, but a protracted lambing season may necessitate two markings. [31]

Inbreeding depression

Inbreeding tends to occur in flocks of limited size and where only a single or a few rams are used. Associated with inbreeding is a decline in progeny performance usually referred to as inbreeding depression. Inbreeding depression has been found for lamb birthweight, average daily weight gain from birth until two months, and litter size. [37] Inbreeding depression can be the cause of diseases and deformities to arise in a flock. [38]

Other countries

In the major sheep countries of Argentina, Uruguay, Brazil, Peru and Chile, breeders are also utilizing fleece testing and performance recording schemes as a means of improving their flocks. [39]

New research

In 2008, for the first time in history, researchers at Chiswick CSIRO research station, between Uralla and Armidale, New South Wales have used stem cells to develop surrogate rams and bulls. These males then produce the viable semen of another male.

The approach in these sheep experiments involves irradiating a ram's testes while placing stem cells from a second ram into the testes of the first, ram A. In the following weeks ram A produces semen the usual way, but is using the stem cells of ram B and therefore producing semen carrying the genetics of ram B rather than those of his own. Ram A therefore has effectively become a surrogate ram.

The viable semen is then implanted in the ewe and the many lambs born through this process are proving to be normal and healthy. DNA tests have proved that up to 10% of the lambs are sired by the surrogate ram and carry the genetics of the donor ram. [40] [41]

Another area of research that is growing in importance is the reduction of greenhouse gas emissions, mainly methane, from livestock. Ruminants are responsible for contributing the highest emissions out of all types of animals. Many researchers are conducting studies to determine how manipulating sheep diets may help reduce these dangerous emissions. [42]

Related Research Articles

<span class="mw-page-title-main">Herdwick</span> Breed of sheep

The Herdwick is a breed of domestic sheep native to the Lake District in North West England. The name "Herdwick" is derived from the Old Norse herdvyck, meaning sheep pasture. Though low in lambing capacity and perceived wool quality when compared to more common commercial breeds, Herdwicks are prized for their robust health, their ability to live solely on forage, and their tendency to be territorial and not to stray over the difficult upland terrain of the Lake District. It is considered that up to 99% of all Herdwick sheep are commercially farmed in the central and western Lake District.

<span class="mw-page-title-main">Sheep farming</span> Animal husbandry

Sheep farming or sheep husbandry is the raising and breeding of domestic sheep. It is a branch of animal husbandry. Sheep are raised principally for their meat, milk, and fiber (wool). They also yield sheepskin and parchment.

<span class="mw-page-title-main">Border Leicester</span> Breed of sheep

The Border Leicester is a British breed of sheep. It is a polled, long-wool sheep and is considered a dual-purpose breed as it is reared both for meat and for wool. The sheep are large but docile. They have been exported to other sheep-producing regions, including Australia and the United States.

<span class="mw-page-title-main">Jacob sheep</span> British breed of domestic sheep

The Jacob is a British breed of domestic sheep. It combines two characteristics unusual in sheep: it is piebald—dark-coloured with areas of white wool—and it is often polycerate or multi-horned. It most commonly has four horns. The origin of the breed is not known; broken-coloured polycerate sheep were present in England by the middle of the seventeenth century, and were widespread a century later. A breed society was formed in 1969, and a flock book was published from 1972.

<span class="mw-page-title-main">Romney sheep</span> Sheep breed

The Romney, formerly called the Romney Marsh sheep but generally referred to by the local farmers as the Kent, is a breed of sheep originating in England. The Romney is a "long-wool" breed recognized in England by 1800. Exported to other continents, the Romney is an economically important sheep breed, especially to the sheep-meat and wool export trades of New Zealand.

<span class="mw-page-title-main">Katahdin sheep</span> Breed of sheep

The Katahdin is a breed of domestic sheep developed by breeder Michael Piel in Maine, United States. He named this new breed after Mount Katahdin - the state’s highest peak. The breed was developed during the second half of the 20th century by crossing selected St. Croix sheep from the Virgin Islands with various other breeds, including the Suffolk. Lambs were selected based on hair coat, meat-type conformation, high fertility, and flocking instinct.

The Finnish Landrace, Finn or Finnsheep is a breed of domestic sheep native to Finland. It is one of several Northern European short-tailed sheep breeds, but is notable for its high incidence of multiple births – it is common for a ewe to have three, four, or even five lambs at once.

<span class="mw-page-title-main">Texel sheep</span> Dutch breed of sheep

The Texel is a Dutch breed of domestic sheep originally from the island of Texel. It is a heavy and muscular sheep, and produces a lean meat carcass. It is polled, clean-faced and clean-legged, with white face and wool. The fibre diameter of the wool averages about 32 μ, with a staple length of 8–15 cm; it is used mainly for knitting and hosiery wools.

<span class="mw-page-title-main">Shetland sheep</span> Breed of sheep

The Shetland is a small, wool-producing breed of sheep originating in the Shetland Isles, Scotland, but is now also kept in many other parts of the world. It is part of the Northern European short-tailed sheep group, and it is closely related to the extinct Scottish Dunface. Shetlands are classified as a landrace or "unimproved" breed. This breed is kept for its very fine wool, for meat, and for conservation grazing.

<span class="mw-page-title-main">Roslag</span> Breed of sheep

The Roslag sheep is a breed of endangered domesticated sheep that originated in and is almost exclusively found in Sweden.

<span class="mw-page-title-main">St. Croix sheep</span> Breed of sheep

The St Croix is a breed of domestic sheep native to the U.S. Virgin Islands and named for the island of Saint Croix. They are often also called Virgin Island White because those that were imported into North America were selected for white coloration. On the Island of St. Croix, they come in shades of brown, white, and black.

<span class="mw-page-title-main">Glossary of sheep husbandry</span>

The raising of domestic sheep has occurred in nearly every inhabited part of the globe, and the variations in cultures and languages which have kept sheep has produced a vast lexicon of unique terminology used to describe sheep husbandry.

<span class="mw-page-title-main">Clun Forest sheep</span> Breed of sheep

The Clun Forest is a breed of domestic sheep originating from the area surrounding the Clun Forest in Shropshire, England. Similar to many of the British breeds of upland sheep, Clun Forest are hardy, adaptable, good foragers, and are long–lived. With sleek heads and wide pelvic structures, Clun Forest ewes lamb easily. The breed has a short to medium–length wool and dark brown faces. They are a multi–purpose animal, kept for meat, wool, and milk. Like other dark faced sheep, Clun produce quality lamb and mutton. However, in contrast to more common meat breeds such as Suffolks, their wool is free of undesirable black fibres and kemp, and is suitable for handspinning. The breed's alert and stylish appearance, together with its reputation for hardiness and fecundity have made it popular with hobby farmers and large commercial flock owners alike. The Official Clun Sheep Breeders Society Show & Sale for males and females is held annually in early September at Ludlow livestock market by McCartneys.

<span class="mw-page-title-main">Charollais sheep</span> Breed of sheep

The Charollais is a breed of domestic sheep originating in east central France, in the same region in which Charolais cattle originated, Charolles and Saône-et-Loire. It is known for ease of lambing and is used as a terminal sire to increase muscling and growth rate of the lambs. It has been exported internationally, and is commonly used in the United Kingdom as a sire to produce market lambs from pure-bred ewes and mules.

<span class="mw-page-title-main">Sheep</span> Domesticated ruminant bred for meat, wool, and milk

Sheep or domestic sheep are a domesticated, ruminant mammal typically kept as livestock. Although the term sheep can apply to other species in the genus Ovis, in everyday usage it almost always refers to domesticated sheep. Like all ruminants, sheep are members of the order Artiodactyla, the even-toed ungulates. Numbering a little over one billion, domestic sheep are also the most numerous species of sheep. An adult female is referred to as a ewe, an intact male as a ram, occasionally a tup, a castrated male as a wether, and a young sheep as a lamb.

The Devon Closewool is a British breed of domestic sheep. It is distributed almost exclusively on Exmoor in North Devon, in south-west England. It is raised primarily for meat.

The British Milksheep is a robust, dual-purpose sheep commonly known for its milking characteristics.

The Rideau Arcott is a breed of domestic sheep native to Canada, one of only a few livestock breeds native to the country.

The Eliottdale is a breed of domestic sheep originating from Tasmania. It is a carpet wool breed raised primarily for its wool. Due to the amount of wool grown by the Eliottdale, it needs to be shorn twice per year.

<span class="mw-page-title-main">Polled Dorset</span> American breed of sheep

The Polled Dorset is an American breed of domestic sheep. It is a polled (hornless) variant of the British Dorset Horn. It was developed at the North Carolina State University Small Ruminant Unit in the 1950s after a genetic mutation led to the birth of a polled ram. After some years of breeding work, a true-breeding polled strain was established.

References

  1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Wooster, Chuck (2005). Living with Sheep: Everything You Need to Know to Raise Your Own Flock. Geoff Hansen (Photography). Guilford, Connecticut: The Lyons Press. ISBN   978-1-59228-531-0.
  2. ↑ Marai, Ibrahim Fayez Mahmoud (1987). New Techniques in Sheep Production. Butterworths. p. 222. ISBN   978-0-408-10134-9.
  3. 1 2 Padula, A.M. (2005). "The freemartin syndrome: an update". Animal Reproduction Science. 87 (1/2): 93–109. doi:10.1016/j.anireprosci.2004.09.008. PMID   15885443.[ dead link ]
  4. ↑ Lynch, J.J. (1992). The Behaviour of Sheep. Victoria: CSIRO Publications. p. 105. ISBN   978-0643053298.
  5. ↑ "SHEEP | Animal Behaviour" . Retrieved 2019-03-28.
  6. 1 2 3 4 5 6 7 8 9 Simmons, Paula; Carol Ekarius (2001). Storey's Guide to Raising Sheep. North Adams, MA: Storey Publishing LLC. ISBN   978-1-58017-262-2.
  7. 1 2 3 4 5 6 7 8 9 Smith M.S., Barbara; Mark Aseltine PhD; Gerald Kennedy DVM (1997). Beginning Shepherd's Manual, Second Edition. Ames, Iowa: Iowa State University Press. ISBN   978-0-8138-2799-5.
  8. 1 2 3 Weaver, Sue (2005). Sheep: small-scale sheep keeping for pleasure and profit. 3 Burroughs Irvine, CA 92618: Hobby Farm Press, an imprint of BowTie Press, a division of BowTie Inc. ISBN   978-1-931993-49-4.{{cite book}}: CS1 maint: location (link)
  9. ↑ "Breeding tools for better management –". www.premier1supplies.com. 10 October 2012. Retrieved 2019-04-01.
  10. ↑ Fraser, Allan (1987). Sheep husbandry and diseases. Collins. ISBN   978-0632026623. OCLC   20416719.
  11. ↑ Cottle, D.J. (1991). Australian Sheep and Wool Handbook. Inkata Press, Melbourne. ISBN   978-0-909605-60-5.
  12. ↑ "International Embryo Transfer Society". Iets.org/. Retrieved 19 October 2014.
  13. 1 2 3 Dally, Martin Animal Science Department, University of California. "Laparscopic Artificial Insemination: A Means to Improve Genetics". toprams.com. Retrieved 2008-11-02.{{cite web}}: CS1 maint: multiple names: authors list (link)
  14. 1 2 "Reproduction Specialty Group" . Retrieved 19 October 2014.
  15. ↑ "American Embryo Transfer Association" . Retrieved 19 October 2014.
  16. 1 2 Coleby, Pat (2000). Healthy sheep : naturally. Victoria: Landlinks Press. p. 133. ISBN   978-0643065246. OCLC   975196851.
  17. ↑ "Infertility and abortion in ewes". www.agric.wa.gov.au. Retrieved 2019-04-01.
  18. ↑ SIMMONS, PAULA (2019). STOREY'S GUIDE TO RAISING SHEEP : breeding, care, facilities. STOREY Books. ISBN   978-1612129846. OCLC   1041893745.
  19. ↑ "Quintuplet birth takes sheep breeder by surprise". Prague Daily Monitor. Czech News Agency. 2008-01-24. Archived from the original on January 29, 2008. Retrieved 2008-01-25.
  20. ↑ Ross, C.V. (1989). Sheep Production and Management. New Jersey: Prentice-Hall Incorporated. p. 123. ISBN   978-0-13-808510-0.
  21. ↑ Prescott, J.H.D. (1971). Lamb Production. England: U.S. Feed Grains Council. p. 12.
  22. 1 2 3 4 5 Brown, Dave; Sam Meadowcroft (1996). The Modern Shepherd. Wharfedale Road, Ipswich 1P1 4LG, United Kingdom: Farming Press. ISBN   978-0-85236-188-7.{{cite book}}: CS1 maint: location (link)
  23. ↑ Anderson ML, Barr BC, Conrad PA (1994). "Protozoal causes of reproductive failure in domestic ruminants". Vet. Clin. North Am. Food Anim. Pract. 10 (3): 439–61. doi:10.1016/S0749-0720(15)30531-4. PMID   7728629.
  24. ↑ Musser JM (2004). "A practitioner's primer on foot-and-mouth disease". J. Am. Vet. Med. Assoc. 224 (8): 1261–1268. doi:10.2460/javma.2004.224.1261. PMID   15112774.
  25. ↑ García de la Fuente JN (2004). "Efficacy of different commercial and new inactivated vaccines against ovine enzootic abortion". Vet. Microbiol. 100 (1–2): 65–76. doi:10.1016/j.vetmic.2004.01.015. PMID   15135514.
  26. ↑ Jackson, Peter; Thorne, Mike (2000-10-01). "Caesarean section in the ewe". In Practice. 22 (9): 546–553. doi:10.1136/inpract.22.9.546. S2CID   72494500.
  27. ↑ "Cunha, Ton J". World Book Encyclopaedia. Vol. 17. USA: Field Enterprises. 1977. p. 309. Sheep.
  28. ↑ "Wake up call for Irish sheep farming". Archived from the original on June 30, 2007. Retrieved October 19, 2008.
  29. ↑ "Performance-Based Selection". Archived from the original on October 14, 2008. Retrieved October 19, 2008.
  30. ↑ Beyond the Bale. AWI, Issue 36 October- November 2008
  31. 1 2 3 4 5 D’Arcy, J.B., Sheep Management & Wool Technology, NSW University Press, 1986, ISBN   0-86840-106-4
  32. 1 2 3 Livestock & Grain Producers Association of NSW, Sheep Production Guide, Macarthur Press, Parramatta, 1978, ISBN   0-9599973-2-6
  33. ↑ Blair, H. T.; Garrick, D. J. (June 2007). "Application of new technologies in sheep breeding". New Zealand Journal of Agricultural Research. 50 (2): 89–102. Bibcode:2007NZJAR..50...89B. doi: 10.1080/00288230709510285 . S2CID   85414333. INIST   18908365.
  34. ↑ "Australian Sheep CRC". Sheepcrc.org.au. Retrieved 2016-02-24.
  35. ↑ "8 Reasons to Shear Before Lambing | Shroeder Shearing". shroedershearing.com. Retrieved 2019-04-01.
  36. ↑ Read, Lianne (2007). "Shearing Your Sheep" (PDF). Alberta Lamb Producers. Retrieved April 1, 2019.
  37. ↑ Norberg E, Sørensen AC (2007). "Inbreeding trend and inbreeding depression in the Danish populations of Texel, Shropshire, and Oxford Down". J. Anim. Sci. 85 (2): 299–304. doi:10.2527/jas.2006-257. PMID   17235016.
  38. ↑ Selvaggi, M.; Dario, C.; Peretti, V.; Ciotola, F.; Carnicella, D.; Dario, M. (March 2010). "Inbreeding depression in Leccese sheep". Small Ruminant Research. 89 (1): 42–46. doi:10.1016/j.smallrumres.2009.12.005.
  39. ↑ Cardellino, Roberto C.; Mueller, Joaquín Pablo (2009). "Fiber production and sheep breeding in South America" (PDF). Proceedings of the Association for the Advancement of Animal Breeding and Genetics. 18: 366–73. CiteSeerX   10.1.1.521.7891 .
  40. ↑ "Surrogate rams through stem cell research". Nqr.farmonline.com.au. Archived from the original on 2016-03-03. Retrieved 2016-02-24.
  41. ↑ The Land, “Stem cell revolution”, Marius Cuming, p.3, Rural Press, Richmond, 13 November 2008
  42. ↑ Haque, Md Najmul (2018-06-18). "Dietary manipulation: a sustainable way to mitigate methane emissions from ruminants". Journal of Animal Science and Technology. 60: 15. doi: 10.1186/s40781-018-0175-7 . PMC   6004689 . PMID   29946475.

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.