Domesticated sheep are herd animals that are bred for agricultural trade. A flock of sheep is mated by a single ram, which has either been chosen by a farmer or, in feral populations, has established dominance through physical contests with other rams. Sheep have a breeding season (tupping) in the autumn, though some can breed year-round.
As a result of the influence of humans on sheep breeding, ewes often produce multiple lambs. This increase in lamb births, both in number and birth weight, may cause problems with delivery and lamb survival, requiring the intervention of shepherds.
Sexual behaviour Ewes generally reach sexual maturity at six to eight months of age, and rams generally between four and six (though ram lambs have occasionally been known to impregnate their mothers at two months). Sheep are seasonally polyoestrus animals. Ewes enter into estrus cycles about every 17 days, which last for approximately 30 hours. They indicate sexual readiness through physical displays towards rams and emitting a scent. The Flehmen response is exhibited by rams when they smell the urine of a ewe in estrus. The vomeronasal organ has receptors which detect the estrogen in the ewe's urine. The ram displays this by extending his neck and curling his lip.
Freemartins, female bovines that are behaviorally masculine and lacking functioning ovaries, are commonly associated with cattle, but do occur to some extent in sheep. The instance of freemartins in sheep may be increasing alongside the rise in twinning (freemartins are the result of male-female twin combinations).
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; they can kill one another if allowed to mix freely. During the rut, even normally friendly rams may become aggressive towards humans due to increases in their hormone levels.
Historically, especially aggressive rams were sometimes blindfolded or hobbled. 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.
Pregnancy Without an ultrasound or other special tools, determining if a sheep is pregnant is difficult. 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. 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 colour. Dye may also be directly applied to the ram's brisket (lower chest region). This measure is not used in flocks where wool is important, since the colour of a raddle can stain the wool. The crayon in the marking harness can be changed during the breeding cycle to allow for lambing date predictions for each ewe.
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. A 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. Usually ewes that experience serious issues while lambing, such as prolapse, will be discarded from the flock to avoid further complications in upcoming years.
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. These programs became 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. 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 that is more difficult to access. Additionally, until recently, breeders were unable to control their ewe's estrus cycles. 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 a 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.
Semen collection is an integral component of this 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. 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.
Embryo transfer (ET) is 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 that may otherwise be non-existent in certain countries due to the regulation of live animal imports. ET 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. With ET, a flock can be grown quickly with above average individuals of similar bloodlines. The primary industry to utilize this technology in the United States is club lamb breeders and exhibitors. It is a common practice in the commercial sheep industries of Australia, New Zealand, and South America.
Average success rates of ET in terms of embryos recovered can vary widely. Each breed will respond differently to the ET process. Typically, white-faced ewes and dogs 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.
Infertility Infertility can be attributed to many aspects of managerial practices as well as health factors. One of the main reasons for the conditions of low lambing percentages seen in a flock is due to mineral and vitamin deficiencies. The main vitamins and minerals that play a major role in fertility are selenium, copper, vitamins A and D. Other factors that affect fertility and potentially cause abortion are infectious diseases, inappropriate body conditions or toxins in feed.
Lambing As the time for lambing approaches, the lamb will drop, causing the ewe to have a sway back, exhibit restless behaviour and show a sunken appearance in front of the hipbone area. When birth is imminent, contractions begin to take place, and the fitful behaviour of the ewe may increase.
Normal labour 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. Though some breeds may regularly produce larger litters of lambs (records stand around nine lambs at once), most produce either single or twin lambs. The number of lambs a ewe produces per year is known as the lambing percentage. The condition of the ewe during breeding season will impact the lambing percentage as well as the size of the lambs. At some point, usually at the beginning of labour or soon after the births have occurred, ewes and lambs may be confined to small lambing jugs. 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.
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. However, it is a myth that sheep cannot lamb without human assistance; many ewes give birth directly in pasture without aid. Balancing ease of lambing with high productivity is one of the dilemmas of sheep breeding. 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 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 breeding. Rams are naturally larger than ewes by comparison). Lambs may also present themselves with one shoulder to the side, completely backward, or with only some of their limbs protruding. Lambs may also be spontaneously aborted or stillborn.
Reproductive failure is a common consequence of infections such as toxoplasmosis and foot-and-mouth disease. Some types of abortion in sheep are preventable by vaccinations against these infections.
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. In severe cases, a caesarean section will be required to remove the lamb. After the birth, ewes ideally break the amniotic sac (if it is not broken during labour), and begin licking to clean the lamb.
The licking clears the nose and mouth, dries the lamb, and stimulates it. 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. Generally after lambs stand, the umbilical cord is trimmed to about an inch (2.5 centimetres). 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.
Postnatal care
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. 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.