Sheep 201: Reproduction in the ewe

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Triplet birth

Single lamb

Twins lambs

Katahdin ewe

short-day breeder
Seaonally polyestrus

Katahdin ewes

Young ewe lamb

Pre-pubertal ewe lamb

Pregnant ewe lambs

Gestation averages 147-148 days.

Parturition

Just born

Polypay ewes
Polypays: less seasonal

Cull ewes that prolapse

Trim hooves prior to breeding

Breeding

In heat

Hampshire ewes being fed

Barbados Blackbelly ewes

Laparoscopic AI

Trans-cervical AI

Ewe with quads

Whiteface ewes

Reproduction in the ewe

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, 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 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. 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, 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

Characteristic

Average

Range

Age at puberty,
5 to 12 months

Length of estrus cycle, days
17

13-19

Duration of estrus, hours
30

18-48

Timing of ovulation
20-30 hours after start of estrus

Gestation, days
146-147

138-149

Pre-Breeding

Prior to breeding, ewes should be evaluated for their need for anthelmintic treatment. They should 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 (pregnancy)

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.

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 (lambing)

Breeding Ewe Lambs

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.

Minimum weight to breed ewe lambs

Avg. mature weight of ewes in flock

Minimum weight to breed ewe lamb

Lb.

Kg

Lb.

Kg

90

41

63

29

100

45

70

32

120

55

84

38

140

64

98

45

160

73

112

51

180

82

126

57

200

91

140

64

220

100

154

70

240

109

168

76

Controlling reproduction in the ewe

Hormonal control

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 were recenty 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
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
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.

Light Control

The "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

Artificial insemination (AI)

There are four methods of artificially inseminating a ewe: vaginal, cervical, trans-cervical, and intrauternine.

Vaginal AI
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 AI
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.

Trans-cervical AI
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
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 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.

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