Pregnant ewe lambs
Gestation averages 147-148 days.
Polypays: less seasonal
Cull ewes that prolapse
Trim hooves prior to breeding
Ewe lamb in heat
Hampshire ewes being fed
Barbados Blackbelly ewes
Ewe with quads
Replacement ewe lamb
Reproduction in the ewe
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. Optimal reproductive rate varies
by farm, production system, and geographic area.
Puberty (sexual maturity)
Puberty is when a ewe reaches sexual maturity and exhibits estrus
(heat) for the first time. The age of puberty is influenced
by breed, genetics, size (weight), nutrition, and season
of birth. Most ewe lambs reach puberty between 5 and 12 months
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 later in the season, due to their
increased age and body weight. One way to select for early puberty is to expose ewe lambs for breeding and scan them for pregancy, culling any ewe lambs which fail to become pregnant.
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 earlier age than most breeds. Crossbred ewe lambs cycle
at a younger age than purebred ewe lambs.
The estrus (or heat) 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. There are four phases to the estrous
cycle: 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 season. 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, Karakul,
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.
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 option ewes will mate with almost any breed of ram.
Reproductive characteristics of ewes
|Age at puberty,
5 to 12 months
|Length of estrus cycle, days
|Duration of estrus, hours
|Timing of ovulation
20-30 hours after start of estrus
Prior to breeding, ewes should be evaluated for their need for
deworming. They should have their hooves trimmed, if necessary.
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 unsound udders should be culled. Ewes that prolapsed
their vaginas the previous lambing 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.
The average gestation length in sheep varies from 142 to 152
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. Male lambs and heavy birth weight lambs are usually carried longer than female lambs.
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. Mid-gestation is important for development of the placenta.
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
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.
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. Ewe lambs should be fed higher forage diets. Frame growth is more imporant than fat deposition.
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 proper, 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.
Minimum weight to breed ewe lambs
weight of ewes in flock
to breed ewe lamb
Controlling reproduction in the ewe
When satisfactory results are not obtained under natural breeding
conditions, it is possible to artificially manipulate the reproductive
cycle of sheep.
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
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. CIDR devices are approved for use in sheep in the U.S.
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 most of the pharmacological treatments
described above have been approved by the U.S. Food and Drug
Administration for use in sheep, though they may be available to
producers in other countries.
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.
The "ram effect"
The "ram effect" is when non-cycling ewes are stimulated
to ovulate by the sudden introduction of an intact male or "teaser" (vasectomized) ram.
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 genrally 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
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. There are also differences among ewes, in their response to the ram effect.
The greatest 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 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.
Artificial insemination (AI)
Artificial insemination is possible in sheep, but not common in
the United States. This is because 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:
vaginal, cervical, trans-cervical, and intrauternine.
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
The technique utilizes an endoscope, a special telescope with
a fiberoptic light, which permits the technician to view the
ewes 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 with laparoscopic AI.
Embryo Transfer (ET)
Embryo transfer is a technique whereby embryos are harvested
from the ewe's reproductive tract before they have 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 all the offspring. 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.
<== SHEEP 201 INDEX