by
Susan Schoenian
Date of last revision:
28-Dec-2005 0:36
Reproductive rate is defined as the number of live lambs born per ewe exposed
for breeding. Optimal reproductive rates are essential to profitable sheep production.
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Optimal
reproductive rates are
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Puberty
Puberty is when a ewe first exhibits estrus (heat). Puberty is influenced by
age, breed, genetic selection, body size, nutrition, and season of birth. Most
ewe lambs reach puberty between 5 and 12 months of age. Ewe lambs will tend
to reach puberty their first fall. For this reason, spring-born ewe lambs tend
to exhibit puberty earlier than fall-born ewe lambs. Lambs born early in the
season reach puberty earlier than those born late in the season, due to their
increased age and body weight. High levels of feed pre- and post-weaning reduce
the age at puberty. Single lambs cycle at a younger age than twin and triplet-born
ewe lambs, due to their size advantage. Ewe lambs from fine-wool, coarse wool,
and late-maturing medium-wool breeds reach puberty later than many of the meat
(Suffolk, Dorset, etc.) and hair sheep (Katahdin, St. Croix, and Barbados Blackbelly)
breeds. Finnsheep and Romanov ewe lambs and their crosses reach puberty at an
early age. Crossbred ewe lambs cycle at a younger age than purebred ewe lambs.
The
Estrus Cycle
Reproduction in non-human mammals is regulated by an estrus cycle. In sheep,
the length of the estrus cycle ranges from 13 to 19 days and averages 17 days.
The phases of the estrous cycle are proestrus, estrus, metestrus, and diestrus.
Estrus is the period of time when the ewe is receptive to the ram and will stand
for mating. It lasts approximately 24 to 36 hours. Ovulation (release of eggs
by the ovary) occurs in mid to late-estrus. Metaestrus begins with the cessation
of estrus and lasts for about 3 days. Primarily it is the period of the formation
of corpus luteum (CL). The corpus luteum produces progesterone and maintains
pregnancy in the ewe. Diestrus is the period of the estrus cycle when the CL
is fully functional. Proestrus begins with the regression of the CL and drop
in progesterone and extends to the start of estrus. Rapid follicular growth
is occuring during this period. It usually extends from day 4 to day 13-15 of
the cycle. Anestrus refers to a state where the normal cycle stops.
Estrous cycles are usually affected by the seasons. The number of hours daily that light enters the eye of the animal affects the brain, which governs the release of certain precursors and hormones. Most sheep are seasonally polyestrus and short-day breeders. They will begin to exhibit estrus when length of day begins decreasing. They will come into heat every 16 to 17 days until they are bred or return to anestrus. Thus, the most natural time for sheep to breed in the U.S. and Canada is the fall (Oct-Nov). Some sheep breeds are less seasonal. They breed almost year-round or have an extended breeding season. The less seasonal breeds include Dorset, Rambouillet, Merino, Finnsheep, Romanov, and hair sheep. The most seasonal breeds are the British long wool and meat breeds. The closer the flock is located to the equator, the longer the breeding season and the less complete and shorter will be the seasonal anestrus.
The signs of estrus in the ewe are much less pronounced than in the cow or doe and can usually not be detected unless a ram is present. When mature ewes are in heat, they will seek out the ram and stand still for him to mount them. Sometimes they wag their tails vigorously. They may nuzzle the ram around the belly or scrotum and even try to mount the ram. Young ewes rarely exhibit these behaviors. There is evidence to suggest that rams and ewes prefer to mate with their own breed, but when there is no alternative ewes will mate with almost any breed of ram.
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Dorsets
are less seasonal than other breeds.
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Pre-Breeding
Prior to breeding, ewes should be treated for internal parasites and have their
hooves trimmed. If there is a history or risk of abortions in the flock, ewes
should be vaccinated prior to breeding. It goes without saying that only healthy,
reproductively sound ewes should be exposed to rams for breeding. The udder
of every ewe should be examined. Those with hard lumps, abscesses, or unresolved
should be culled. Ewes that prolapsed should not be kept for breeding, because
there is a high probability for reoccurance. Ewes that did not raise a lamb
should be culled. Ewes that are in poor body condition due to age and/or missing
teeth should be culled. Ewes with chronic hoof problems (e.g. foot rot) should
be culled.
Gestation
The average gestation length in sheep varies from 144 to 151 days. The average
is 147 days. Individual pregnancies may vary from 138 to 159 days. There are
breed differences in gestation length. The earlier maturing breeds (e.g. Finnsheep)
tend to have shorter pregnancies than the late maturing breeds (e.g. Rambouillet).
Ewes carrying multiple births tend to have shorter gestations.
The period of early gestation most critical to success during the lambing season is the first 30 days after fertilization. The first 21 to 30 days after breeding is when embryonic implantation occurs. This first 30 days is when most embryonic mortality occurs. Thus, anything that can be done to reduce embryonic mortality and should result in more lambs born. Shearing, vaccinating, working ewes, pronounced changes in feeding practices should be avoided during the first 30 days of gestation. Ultrasonic pregnancy scanning can be done on ewes from 35 to 60 days after breeding, depending on equipment used and operator skill. Nutrition during early gestation is quite simple. Ewes need only slightly above maintenance levels of nutrition for the first 15 weeks of pregnancy.
Late gestation (last 4 to 6 weeks) is a critical period for ewe reproduction. This is when the majority of fetal growth is occurring, placing increasing nutritional demands on the ewe. Ewes consuming inadequate diets are prone to pregnancy toxemia and milk fever. Nutrition in late-pregnancy affects the size and vigor of lambs and the milk producing ability of the ewe.
Parturition
There are three stages to parturition (lambing): 1) dilation of the cervix;
2) expulsion of the fetus(es); and 3) expulsion of the placenta. Stage one usually
takes 3 to 4 hours. The birth of a lamb usually occurs within an hour of less
from the rupture of the first water bag. A ewe lambing for the first time or
with multiple births may take longer. If labor takes over an hour for mature
ewes and over 2 hours for ewe lambs, assistance may be required. The placenta
is passed 2 to 3 hours after delivery is finished. In multiple births, there
are separate afterbirths for each lamb. After the lamb is born, the ewe will
lick and nuzzle it to begin the bonding process.
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Ewe lambs should achieve 70% of their mature weight before being bred. They should be fed and managed separately from mature ewes.
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Breeding
Ewe Lambs
Ewe
lambs should not be bred until they achieve approximately 70 percent of their
mature size (weight). At the same time, care should be taken not to overfeed
replacement ewe lambs. Research has shown that overfeeding pre-pubertal females
(2 to 4 months of age) has a detrimental effect on mammary development (they
deposit excess fat in their udders) and affects subsequent milk producing ability.
Replacement ewe lambs should be fed separately than market lambs or ram lambs
being fed for market.
Pregnant and lactating ewe lambs should be kept separate from mature ewes. Ewe lambs require extra nutrition because they are still growing. They will not compete well at the feed bunk with mature ewes and will not gain weight properly. Ewe lambs that are forced to run with the ewe flock will lose weight in late gestation and lactation. Single-births are common with ewes lambs, but twins and triplets are not uncommon with the more prolific breeds. Even when management and nutrition are practices, more lambing problems will be encountered with ewes lambing for the first time at 12 to 14 months of age.
While breeding ewe lambs increases their lifetime productivity, it may not be economically advantageous for all producers. Intensively managed sheep operations usually benefit from breeding ewe lambs.
Click HERE to read a Word document about Breeding Ewe Lambs.
When satisfactory results are not obtained under natural breeding conditions, it is possible to artificially manipulate the reproductive cycle of sheep.
Click HERE 
to view the publication Reproductive Management in the Ewe Flock by Induction
or Synchronization of Estrus by West Virginia University.
Hormonal
Control
A
common method of inducing estrus in non-cycling ewes is progesterone-based therapies.
Progesterone prevents the ewe from returning to estrus and ovulating. It is
produced by the corpus luteum (CL) of the ovary following ovulation and sustains
pregnancy. When progesterones are introduced artificially, they fool the body
into thinking it is pregnant and the animal will not ovulate or come into estrous
(heat). When the progesterone source is totally removed, the body realizes it
is no longer pregnant and will ovulate within a very predictable period. Progestins
refer to the synthetic compounds with the properties of progesterone. These
substances mimic the function of the CL. Progestogens
(synthetic analogs of progesterone) can be provided by feeding (MGA), implants
under the skin (Synchro-Mate B®), sponges (or pessaries) inserted into the
vagina, or plastic delivery devices inserted into the vagina (CIDR).
Intravaginal sponges (or pessaries) have been the traditional method of inducing and/or synchronizing estrus in ewes. They contain progestagens that are effective at lower doses than natural progesterone. Two types of sponges are Chronogest (FGA) and Veramix® (MAP). Intravaginal sponges are usually inserted over periods of 9 to 19 days and are used in conjunction with PMSG, injected at the time of sponge removal or 48 hours prior to sponge removal. Intravaginal sponges have high retention rates (>90%) and females usually exhibit estrus 24 to 48 hours after removal. Responses to intravaginal sponges have varied according to breed, protocol, co-treatment, management, and mating system.
CIDR™ (controlled internal drug release) devices are made of progesterone-impregnated medical silicone elastomers and were developed in New Zealand. Protocols for the use of CIDR™ devices is usually identical to protocols for intravaginal sponges. Research has shown that CIDR™ devices and intravaginal sponges yield similar results.
Synchro-mate-B® is a cattle implant that contains 6 mg of the synthetic progestagen norgestomet. One-third or one-half of the Synchro-mate-B® implant is typically used in ewes. Implantation periods range from 9 to 14 days. Two days before the end of the implantation period, injections of PMSG and /or PGF2a are usually given.
Melengesterol acetate (MGA®) is an orally active, synthetic progestagen developed and used to suppress estrus in feed lot heifers. The use of this product requires the feeding of a supplement containing MGA® once or twice daily for a duration of 8 to 14 days. Protocols usually include co-treatments with PMSG, P.G. 600® or Ralgro® (zeranol). Ralgro® is a commercially available growth promotant for cattle and sheep with estrogen-like effects on LH and FSH concentrations. P.G. 600® is the only veterinary grade source of PMSG readily available in the U.S. Estrus responses to MGA feeding vary, but are usually higher with co-treatment.
Prostaglandin-based protocols are only applicable to cycling ewes and are restricted to use during the breeding season. The two commonly used products are Lutalyse™(PGF2a) and Estrumate® (cloprostenol). Prostaglandins cause regression of the CL, telling the body than no pregnancy exists. The ewe will ovulate within a very predictable time. When a single treatment of prostaglandin is given to a flock of cycling ewes, 60 to 70 percent of the flock will exhibit a synchronized estrus beginning 30 to 48 hours later. A double injection system (11 days apart) is most common in sheep.
Melatonin
treatments have been shown to be an effective method of inducing estrus in non-cycling
ewes. Melatonin is called the "hormone of darkness," because it is
released by the pineal gland during the night. Treatment with melatonin therefore
mimics the short days of fall and induces estrus after a minimum of approximately
35 days of treatment.
It is important to note that none of the pharmacological treatments described
above have been approved by the U.S. Food and Drug Administration for use in
sheep, though it is possible that the MUMS
Act may result in the approval of some of these products for sheep in the
future. These products may be available to producers in other countries.
Light
Control
Controlled lighting can be used to initiate estrus. Short-day breeders like
sheep may be programmed to cycle if they are maintained in a light-tight building
where the photoperiod is reduced gradually over an 8 to 12 week period. Rams
should be exposed to the same light regiment to obtain high fertility. Light
control is usually impractical for most producers.
Ram
Effect
The "ram effect" is when non-cycling ewes are stimulated to ovulate
by the sudden introduction of a ram or "teaser." Rams produce a chemical
substance called a pheromone, the smell of which stimulates the onset of estrus.
When ewes and rams are in constant contact (sight or smell), the pheromones
are much less effective at inducing estrus. Ewes that are not cycling when a
ram (or teaser) is introduced will ovulate in 3 to 4 days. This first ovulation
will be a "silent" heat, which cannot be detected by the ram. Following
this silent heat, there will be two normal estrus peaks, with some ewes cycling
around day 18 and the remainder around day 25. Ewes that do not conceive at
either of these times may return to heat in another 17 days. It is expected
that 60 to 70 percent of the ewes will conceive at the first normal estrus.
Of the remaining ewes, 60 to 70 percent should conceive at the second estrus.
Although recommendations vary, rams should be isolated from ewes for at least 6 weeks in order for the ram effect to work. Ewes must have no contact with rams by either sight or smell, which means that they must be separated by distance. The ram effect is not as effective with ewe lambs. The ram effect is most effective during the transitional period when ewes have not begun to cycle, but are almost ready to. Breed of ram can affect ewes' response to the ram effect, with the less-seasonal breeds being more effective at bringing non-cycling ewes into estrus.
The great value of the ram effect is the synchronization of estrus activity which will result in large numbers of ewes ovulating, conceiving, and lambing in a relatively short period of time. To be effective, it is important to have adequate numbers of young, healthy rams. Teaser (vasectomized) rams or testosterone-treated wethers can also stimulate the ram effect.
Selection
Selection is another method to obtain early lambing flocks, but it requires
a long term commitment. Heritability is generally thought to be less than 10
percent. Researchers at Virginia Tech were able to decrease the seasonal anestrus
to 11 days in a crossbred flock of 1/2 Dorset x 1/4 Rambouillet x 1/4 Finnsheep
by selecting for spring fertility.
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Pregnancy
rates with laparoscopic AI are similar to those achieved with natural
service.
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Artificial
Insemination
Artificial insemination is possible in sheep, but not common. The ewe has a
very complicated cervix which makes trans-cervical A.I. as is done with cattle,
swine, and goats more difficult. As compared to other livestock, the ewe shows
few visible signs of heat (estrus). Breed improvement in the sheep industry
has been much slower to develop and the industry lacks a means to identify superior
genetics.
There are four methods of artificially inseminating a ewe:
1) vaginal
2)
cervical
3)
trans-cervical
4) intrauterine
Vaginal is the simplest form of insemination and involves depositing fresh semen into the anterior vagina without any attempt to locate the cervix. Reported success rates are highly variable and this method is unsuitable for use with frozen semen.
Cervical is another cheap and relatively easy method of insemination. The cervix is located, via a speculum fitted with a light source, and the semen is deposited into the first fold of the cervix. Conception rates with fresh or chilled semen are good, but generally unacceptably low with frozen, thawed semen.
The trans-cervical
method of insemination involves grasping the cervix and retracting it into the
vagina with a pair of forceps to allow an inseminating instrument to be introduced
into the cervical canal. The University of Guelph (Canada) has developed an
instrument with a special bent tip that allows passage through the cervix.
Laparoscopic AI, also known as intrauterine, by-passes the cervix and deposits
semen directly into the uterine horns. In 1982, Australian researchers developed
the laparoscopic insemination (LAI) procedure that revolutionized the sheep
AI technique. LAI is a minimally invasive, minor surgical procedure that requires
veterinary expertise. The technique utilizes an endoscope, a special telescope
with a fiberoptic light, which permits the technician to view the ewe’s
reproductive tract. The semen is injected directly into the lumen of the uterus,
and the same procedure is repeated on the other uterine horn. The procedure
takes 2 to 5 minutes per ewe. After the procedure is over, the ewe normally
starts eating within minutes. Conception rates range from 50 to 80 percent.
Embryo
Transfer
Embryo
transfer is a technique whereby embryos are harvested before they are attached
to the uterus. This technique allows the production of a greater number of offspring
from a given female, the "donor", without requiring her to carry these
offspring to term. For sheep, the embryo transfer technique most often used
is a surgical procedure, both for collecting the embryos and for transferring
them into the recipients.
Click HERE for an explanation of Embryo Transfer in small ruminants from SR Genetics, Ontario, Canada.
Ovaries - the ovaries have two principle functions: 1) production of eggs (ova); and 2) secretion of female hormones (estrogen and progesterone).
Oviduct (fallopian tubes) - the oviduct "catches" the egg when it is released. The oviduct is the site of fertilization and early embryonic development.
Uterus - the uterus is small muscular organ that provides protection and nourishment to the growing embryo. The uterus consists of two parts: body and two uterine horns. The inner lining of the uterus is made up of many button-like projections known as caruncles, which is where the placenta attaches.
Cervix - the cervix separates the uterus from the vagina. During pregnancy, it seals and protects the embryo and fetus from the external environment.
Vagina
- the vagina connects the cervix to the vulva.
Vulva - the vulva is the external opening of the female genital tract.
