Domestic sheep reproduction
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As with other mammals, domestic sheep reproduction occurs sexually. Their reproductive strategy is very similar to other domestic herd animals. A flock of ewes is generally mated by a single ram, who has either been chosen by a farmer or has established dominance through physical contest with other rams (in feral populations).[1] Most sheep are seasonal breeders, though some are able to breed (also called tupping) year-round.[1]
Largely as a result of the influence of man 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.
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[edit] 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] Ewes enter in to estrus 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. Sheep may display homosexuality, and this occurs in about eight percent of rams.[2] Its occurrence does not seem to be related to flock hierarchy (as some homosexual behavior is in mammals), rather the ram's typical motor pattern for intercourse is directed at rams instead of ewes.[2] 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]
[edit] Rutting
Without human intervention, rams may fight during the rut to determine which individuals may mate with ewes, if the mating percentage of ewes is below the recommended level. 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.[4]
Historically, especially aggressive rams were sometimes blindfolded or hobbled.[5] 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][6]
[edit] Pregnancy
Without ultrasound or other special tools, determining if a sheep is pregnant is difficult.[1][5] Ewes only begin to visibly show a pregnancy about six weeks before the birth, so shepherds often rely on the assumption that a ram will impregnate all the ewes.[1] However, by fitting a ram with a chest harness called a marking harness that holds a special crayon (or raddle), ewes that have been mounted are marked with a color. Dye may also be directly applied to the ram's brisket. 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 lay 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 is used to hold the orifice in.
[edit] Lambing
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.[4][7] At some point, usually either at the beginning of labor or soon after the births have occurred, ewes and lambs may be confined to small lambing jugs.[5] These pens, which are generally two to eight feet in length and width, are designed to aid both careful observation of ewes and to cement the bond between them and their lambs.[1][8]
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.[4] However, it is a myth that sheep cannot lamb without human assistance: many ewes give birth directly in pasture without aid.[5]. Balancing ease of lambing with high productivity is one of the dilemmas of sheep breeding.[4] While the majority of births are relatively normal and do not require intervention, there are still a large number of possible complications that 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).[5] 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 aborted or stillborn. Reproductive failure is a common consequence of infections such as toxoplasmosis and foot-and-mouth disease.[9][10] Some types of abortion in sheep are preventable by vaccinations against these infections.[11]
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] 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 (2.54 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.[5]
[edit] 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.[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 theirs.[1][8][4] 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]
After lambs are stabilized, lamb marking—the process of ear tagging, docking, and castrating—is carried out.[1] Vaccinations are usually carried out at this point as well. 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. When performed at an early age, ear tagging seems to cause little or no discomfort to lambs.[1][4] 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]
Castration is performed on ram lambs not intended for breeding, though some shepherds choose to avoid the procedure for ethical, economic or practical reasons.[1] Ram lambs that will either be slaughtered or separated from ewes before sexual maturity are not usually castrated.[8] Docking, which is a shortening—not a complete removal—of sheep's tails is practiced for health reasons.[5] The tail is removed just below the lamb's caudal tail flaps, docking shorter than this may cause health problems such as rectal prolapse.[5] 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][6][5][8] Long, wooly tails make shearing more difficult, interfere with mating, and make sheep extremely susceptible to parasites, especially those that cause flystrike.[1] Both castrating and docking can be performed with several different 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][4] 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.[4]
[edit] References
- ^ a b c d e f g h i j k l m n o p q r s t u v w x y Wooster, Chuck; Geoff Hansen (Photography) (2005). Living with Sheep: Everything You Need to Know to Raise Your Own Flock. Guilford, Connecticut: The Lyons Press. ISBN 1-59228-531-7.
- ^ a b Schwartz, John. "Of Gay Sheep, Modern Science and Bad Publicity", The New York Times, 2007-01-25. Retrieved on 2007-12-07.
- ^ a b Padula, A.M. (2005). "The freemartin syndrome: an update.". Animal Reproduction Science 87 (1/2): 93–109. doi:.
- ^ a b c d e f g h Simmons, Paula; Carol Ekarius (2001). Storey's Guide to Raising Sheep. North Adams, MA: Storey Publishing LLC. ISBN 978-1-58017-262-2.
- ^ a b c d e f g h i Smith M.S., Barbara; Mark Aseltine PhD, Gerald Kennedy DVM (1997). Beginning Shepherd's Manual, Second Edition. Ames, Iowa: Iowa State University Press. ISBN 0-8138-2799-X.
- ^ a b c 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 1-931993-49-1.
- ^ "Quintuplet birth takes sheep breeder by surprise", Prague Daily Monitor, Czech News Agency, January 24, 2008. Retrieved on 2008-01-25.
- ^ a b c d e Brown, Dave; Sam Meadowcroft (1996). The Modern Shepherd. Wharfedale Road, Ipswich 1P1 4LG, United Kingdom: Farming Press. ISBN 0-85236-188-2.
- ^ 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. PMID 7728629.
- ^ Musser JM (2004). "A practitioner's primer on foot-and-mouth disease". J. Am. Vet. Med. Assoc. 224 (8): 1261–8. doi:. PMID 15112774.
- ^ García de la Fuente JN, Gutiérrez-Martín CB, Ortega N, et al (2004). "Efficacy of different commercial and new inactivated vaccines against ovine enzootic abortion". Vet. Microbiol. 100 (1-2): 65–76. doi:. PMID 15135514.
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