Domestic sheep reproduction
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]
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).
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, especially older females.[6]
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
After
Artificial insemination and embryo transfer
In addition to natural insemination by rams,
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
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, exhibiting restless behaviour and show a sunken appearance in front of the hipbone area. Ovine
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
In normal situations, lambs nurse after standing, receiving vital
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. 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. In the large sheep producing nations of Commercial sheep breeding
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]
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
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
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]
References
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- ^ a b c Dally, Martin Animal Science Department, University of California. "Laparscopic Artificial Insemination: A Means to Improve Genetics". toprams.com. Retrieved 2008-11-02.
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- ^ "Wake up call for Irish sheep farming". Archived from the original on June 30, 2007. Retrieved October 19, 2008.
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- ^ "8 Reasons to Shear Before Lambing | Shroeder Shearing". shroedershearing.com. Retrieved 2019-04-01.
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External links