Seasonal Breeding in Sheep

Photographs

Photography by Dana Mohn

All about Ewwwwe...

Created By: Chelsea Holschbach, Dana Mohn, Kayla Eick, and Darci O'Brien.

General Information on Sheep

Domestic sheep are scientifically known as Ovis aries. They are quadruped, four legs, ruminant mammals typically kept as livestock. Sheep are members of the order Artiodactyla, the even-toed ungulates. Numbering a little over 1 billion, domestic sheep are the most numerous species in their genus (Schoenian). Sheep are most likely descendents from the wild mouflon of Europe and Asia and where one of the earliest animals to be domesticated for agricultural purposes. Sheep are raised for fleece, meat and milk. Today sheep are being utilized for human research (Schoenian).

A group of sheep is called a flock. Adult female sheep are referred to as ewes, intact males are rams, castrated males are wethers, and younger sheep are lambs. The domestic sheep is a multi-purpose animal, and the more than 200 breeds to serve these diverse purposes (Schoenian).

Seasonal Breeders

Wild species have a breeding season that is initiated at a time when the environment will allow for the best survival of the young. Spring is usually the optimal season for parturition. Time of breeding is then dependent on Gestation length. Domestication has stopped seasonal breeding in some species: Cattle & Swine. Of the farm species, Sheep, Goats and Horses are still seasonal breeders (Winters).

Sheep are short-day breeders otherwise known as fall breeders (Pathways to Pregnancy and Parturition). Breeds such as the Horned Dorset, Merino, and Rambouillet can have extended breeding (Pathways to Pregnancy and Parturition). Other short-day breeders include: goats, fox, deer, and elk.

Rams and bucks are also affected by photoperiod but less than their female partners are. As photoperiod increases in the spring and early summer, sperm production decreases, abnormal spermatozoa increases, and fertility decreases. Horses are an example of long-day breeders. Their cyclicity is dependent on photoperiod which is opposite of sheep and goats. It starts when daylight to darkness ratio increases, and ends when daylight decreases (Pathways to Pregnancy and Parturition). Other long-day breeders include: ring-tailed lemur, hamster groundhog, and mink.

(Pathways to Pregnancy and Parturition)

Puberty and Anestrus

When a ewe reaches puberty, she has reached her sexual maturity and will exhibit estrus for the first time. The time when a ewe will reach puberty depends on many different factors including, the breed, body size, nutrition, season of birth etc. While a common age that ewe lambs reach puberty is between 5 and 12 months of age. Due to their genetics and biological clock ewes will usually show puberty their first fall (Schoenian). This can have a large effect on the age variation of when they have their first estrus, since ewe lambs born in January instead of April would be three months older. While ewes have reached maturity by six months of age, they are usually not allowed to breed until they are at least eight-teen months of age ("Animal Corner").

Estrous cycles are commonly affected by the seasons in different species, ovine is one of those which are very receptive. It is due to the number of hours daily that light enters the eye of the animal. It then affects the brain, to determine the release of certain precursors and hormones. Most sheep are seasonally polyestrus and short-day breeders. They start their estrus when the length of the day begins to decrease. The cycles of estrous will continue every 16 to 17 days until the ewe is bred or returns to anestrus (Schoenian). Therefore most breeding in ovine occurs in the months between October and November. Estrus lasts approximately 24 to 36 hours, and is where the ewe will stand to be mounted by the male. Detecting estrus in sheep is much harder than that is cattle especially dairy cattle, it is almost required to have a ram present (Schoenian).

(Pathways to Pregnancy and Parturition)

Animals that are seasonally anestrus is believed to have come from preventing the ewes from conceiving during a time when the survival rate of the embryo would not be high. Anestrus is a state where a female is not exhibiting normal estrous cycles. The ovaries are relatively inactive. There are no follicles of corpus lutea present on the ovaries either. The embryo survival rate is reduced a lot by the temperature and the humidity in the environment (Pathways to Pregnancy and Parturition). Which is why fall becomes the ideal time for conception, because the embryo is at an ideal temperature and then the lamb goes through parturition during a moderate temperature also.

There are certain breeds of sheep that are less affected by the season and can have longer breeding seasons or practically breed year-round. The location of the sheep relative to the equator will also have some changing effects on the ewe.

Photoperiod

Photoperiod: the period of time during the day when there is daylight. Photoperiod regulates reproductive cyclicity in seasonal breeders. (Pathways to Pregnancy and Parturition). Sheep are seasonal breeders that enter into the breeding season during shorter photoperiods; thus, they are short-day breeders. In the mid and high latitudes of temperate regions, breeding begins in autumn, when the daylight becomes shorter. (Go´mez-Brunet et al., 2008) The effects of photoperiod are caused by the stimulation of the retina by the amount of light present.

The Mechanism behind the effects of photoperiod involves the secretion of melatonin, which stimulates GnRH release. The retina of the eye is stimulated by the presence of light. When light is abundant, an excitatory pathway is active inhibiting melatonin release. Because of genetic differences, different breeds of sheep maintain different levels of photoresponsiveness. (Go´mez-Brunet et al., 2008) This is one reason why differences of breeding time occur between different breeds of sheep.

(Pathways to Pregnancy and Parturition)

Melatonin

Melatonin is a modified amino acid secreted by the pineal gland that communicates information about environmental lighting to various parts of the body. Melatonin has important effects on maintaining biological rhythms, and has important effects on reproductive function.

Light that is exposed to the retina is first relayed to the suprachiasmatic nucleus of the hypothalamus, an area of the brain well known for coordinating biological clock signals Fibers from the hypothalamus descend to the spinal cord and project back to the pineal gland. Thus it converts signals from the sympathetic nervous system into hormonal signals (Bowen).

Seasonal changes in day length have profound effects on reproduction in sheep, and melatonin is a key player in controlling such events. During the non-breeding season, the gonads become inactive, but as breed season approaches, the gonads must be rejuvenated. Photoperiod is the most important cue to allow animals to determine what season it is. The pineal gland is able to measure day length and adjust secretion of melatonin accordingly. Melatonin inhibits the secretion of gonadotropic hormones, LH, and FSH from the anterior pituitary by inhibiting the release of GnRH from the hypothalamus(Essential Reproduction,2007).

In species with longer gestation times, such as sheep, the anticipation of spring must occur much earlier, and seasonal changes in the fall act as a stimulus. Short photoperiods stimulate gonadol activity. Melatonin is released during the night, so as days shorten and nights lengthen in the autumn, the duration of the nocturnal melatonin peak is increased. Conversely, after the winter solstice, the photoperiod increases and the duration of the melatonin signal falls(Essential Reproduction, 2007).

One practical application of melatonin’s role in controlling seasonal reproduction is found in its use to artificially manipulate cycles in sheep that normally breed only once per year, but can be induced to have two breeding seasons by treatment with melatonin(Bowen).

Threats and Challenges

Seasonal breeding in sheep is a serious challenge for the industry. It reduces the effectiveness of accelerated lambing programs, restricts integrating lambing into other farm activities, and limits access to favorable seasonal markets (Notter). Potential genetic improvements are possible by selection to reduce seasonal breeding. Hybrid vigor must be achieved using a terminal-sire-crossbreeding program. There are breeds that are less seasonally bred such as the Dorset that make for good candidates for a breeding program to reduce seasonal breeding. These breeds will increase fertility but will reduce lamb growth rates and carcass value (Notter).

Citations

"Animal Corner".November 16, 2009 <http://www.animalcorner.co.uk/farm/sheep/sheep_about.html>.

Bowen, R.. "The Pineal Gland and Melatonin". 10/20/2009 <http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/otherendo/pineal.html>.

Schoenian, "Sheep 201". November 8, 2009 <http://www.sheep101.info/201/ewerepro.html>.

Senger, P.L.. Pathways to Pregnancy and Parturition. Pullman: Current Conceptions, Inc., 2003.

Notter, D.R, "Opportunities to Reduce Seasonality of Breeding in Sheep by Selection". <http://www.ansci.wisc.edu/Extenstion>.

Go´mez-Brunet, A., Santiago-Moreno, J., del Campo, A., Malpaux, B., Chemineau, P., Tortonese, D. J., Gonzalez-Bulnes, A., Lo´pez-Sebastia´n, A. (2008). Endogenous Circannual Cycles of Ovarian Activity and Changes in Prolactin and Melatonin Secretion in Wild and Domestic Female Sheep Maintained under a Long-Day Photoperiod. Biology of Reproduction 78, 552-562.

Johnson, M.H., and Barry Everitt. Essential Reproduction. Blackwell Publications, 1999.

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