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Factors Affecting Reproductive Performance of Sheep

By

John WA Larsen

, BVSc, PhD, University of Melbourne Faculty of Veterinary & Agricultural Sciences

Last full review/revision May 2021 | Content last modified May 2021

Reasons for reproductive failure in the ewe and ram are numerous. Performance must be considered in light of the management system and genetics of the breed. If targets are not being met, the following can be used as a guideline to investigate contributing factors.

The Effects of Breed on the Reproductive Performance of Sheep

Breed selection can greatly influence reproductive performance, particularly prolificacy and age at first lambing. The characteristics and performance of sheep breeds around the world vary enormously. For example, terminal sire breeds such as Suffolk, Hampshire, and Texel are often used to obtain rapid growth and muscling. Traditionally, these breeds are only moderately to lowly prolific and so are not preferred in the ewe flock except to produce replacement terminal sire rams. Maternal breeds should be fertile and prolific, of easy lambing with good maternal instincts (eg, bonding), have high milk production and longevity, and be suitable both for the environment (eg, grassland, confinement, hilly country) and, for some management systems, long ovulatory seasons. Within each geographic region and production system, specific breeds tend to be more commonly used. Some of the more popular maternal breeds in northeastern North America are the Finnish Landrace, Romanov, and Polled Dorset. In the western US, range breeds are more popular.

Some regions commonly use a crossbred sheep that includes traits from several breeds. In a crossbreeding system, maternal heterosis is used to improve performance, ie, the offspring (the F1 generation) possess traits that are better than the average of their parents, also known as hybrid vigor. A well-known example is the British “mule” sheep in which ewes of a hill breed (eg, Scottish Blackface) that possess the traits of hardiness and good mothering but are not prolific are crossed with a ram of a lowland prolific heavy-milking breed (eg, Bluefaced Leicester). The resultant F1 ewe is a prolific, heavy-milking mother with longevity and hardiness. No ram lambs are retained for breeding. These ewes are then bred to a terminal sire (eg, Suffolk or Texel). The offspring of this mating—a terminal cross—grow well and have good carcass characteristics.

Composite breeds are the result of a planned crossbreeding program coupled with stringent selection based on preselected production traits. The more successful are phenotypically stable and are considered new breeds. In the US, the Polypay, a composite of the Finnish Landrace, Targhee, Dorset, and Rambouillet, is very prolific and hardy and has excellent wool production. The Rideau sheep, developed in Canada, is a composite of Finnish Landrace, Dorset, Suffolk, and East Friesian. These ewes are very prolific, heavy milkers, and excellent mothers and are frequently used in accelerated lambing programs. Other examples of popular composites include the Katahdin (North American haired sheep), Dorper (South Africa and North America), Coopworth (New Zealand and Australia), Corriedale (New Zealand), and British Milk Sheep (UK and Canada).

The Effects of Reproductive Failure of the Ram on the Reproductive Performance of Sheep

Rams may fail to mate the ewes, even when the ewes are in estrus, or mating may occur but pregnancy does not ensue.

For failure to mate, the many reasons include:

Mating not followed by pregnancy may present as ewes being marked repeatedly, a spread-out lambing period, and/or poor prolificacy in the ewes. Many of the reasons for failure to mate can also influence failure to achieve pregnancy. Additional reasons for lack of pregnancy after mating include:

Reproductive Failure in Ewes

As with the ram, ewes may fail to be bred, or be bred but fail to become pregnant. In addition, the ewe may not maintain the pregnancy or may have decreased prolificacy. Again, the possible reasons are numerous. Ewes may not be bred because of the following:

  • The ewe may already be pregnant.

  • It may be anovulatory season, which is longer in ewe lambs.

  • The ewe lamb may be prepubertal, influenced by growth (nutrition) and breed.

  • If a synchronization/estrus induction program is used, there could be a fatal error in the program (eg, loss of a controlled intravaginal drug-releasing device, too low a dose of eCG, or improper melengestrol acetate feeding program).

  • Phytoestrogens or specific mycotoxins may temporarily or permanently suppress estrus.

  • Ewes may be too thin, lactating, or recently weaned.

  • Ewe lambs that are overfed postpubertally may not cycle.

  • Ewes (especially maiden ewes) may display behavior such shyness or dominating the ram.

  • There may be a freemartin/pseudohermaphrodite condition.

Ewes may not conceive or maintain pregnancy because of the following:

Reasons for poor prolificacy in ewes that become pregnant include insufficient energy at breeding when in poor body condition (low body condition score), very young or very old age, anovulatory or transition season, insufficient dose of eCG, early embryonic death (see above), and genetics.

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