Paula I. Menzies, DVM, MPVM, DECS-RHM, Ruminant Health Management Group, Department of Population Medicine, Ontario Veterinary College, University of Guelph
(Previously published online with the Merck Manual – Veterinary Manual)
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.
Breed:
Breed selection can greatly influence reproductive performance, particularly prolificacy and age at first lambing. Sheep breeds around the world are very diverse in performance, and it is advisable to be familiar with the traits of some of the more popular ones. 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 possess the following characteristics: fertile, prolific, easy lambing with good maternal instincts (eg, bonding), high milk production, longevity, suitable 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 western USA, 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–gain 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 USA, 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).
Reproductive Failure of the Ram:
Rams may fail to mate the ewes, even when the ewes are in estrus, or mating may occur but pregnancy does not ensue. Possible reasons are numerous. For failure to mate, these include: 1) The ewes are being mated but the marking harness or crayon is not functioning properly. 2) The ram lacks libido because it is ill with another disease, is too thin, is too old, it is during the anovulatory period, or the weather is too hot. 3) The ram is reluctant to breed, perhaps because of the pain of infectious balanoposthitis (pizzle rot), contagious ecthyma of the penis or prepuce, or because it is lame and cannot mount comfortably. 4) The ram may be inexperienced and has not been “taught” how to breed by observing a more experienced ram. 5) The ram may not be able to cope properly with the environment, eg, a barn-raised ram turned out to mountainous pasture. 6) Too few rams are available to breed the ewes (ram to ewe ratio) for the type of ram (age, experience), conditions (paddock vs range), time of year (ovulatory vs anovulatory), or synchronization programs. 7) Rams may have behavioral issues such as inter-ram aggression, shy rams, or rams “falling in love” with or disliking specific ewes.
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 impaired fertility due to disease such as Brucella ovis infection, chorioptic mange of the scrotum, infertility after a fever, or other causes of orchitis and/or epididymitis; impaired fertility due to mechanical reasons such as excessive heat or cold, inguinal hernia, or injury from fighting; impaired fertility due to environmental hormone disrupters (eg, phytoestrogenic plants); inadequate testicular circumference because of age, season, genetics, or disease; abnormalities of the penis due to a congenital defect or trauma; and sperm abnormalities.
Reproductive Failure of the Ewe:
As with the ram, ewes may fail to be mated, or be mated but do not become pregnant. In addition, the ewe may not maintain the pregnancy or have decreased prolificacy. Again, the possible reasons are numerous. Ewes may not be mated because of the following: 1) The ewe may already be pregnant. 2) It may be anovulatory season, which is longer in ewe lambs. 3) The ewe lamb may be prepubertal, influenced by growth (nutrition) and breed. 4) If a synchronization/estrus induction program is used, there could be a fatal error in the program, eg, loss of CIDR, too low dose of eCG, or improper MGA feeding program. 5) Phytoestrogens or specific mycotoxins may temporarily or permanently suppress estrus. 6) Ewes may be too thin, lactating, or recently weaned. 7) Ewe lambs that are overfed postpubertally may not cycle. 8) Ewes may display behavior such as dominating the ram or being shy, particularly maiden ewes. 9) There may be a freemartin/pseudohermaphrodite condition.
Ewes may not conceive or maintain pregnancy because of the following: 1) The synchronization program is not correct, eg, rams are joined too early with the ewes before they are in estrus or too low a dose of eCG is given and ovulation does not occur. 2) There is pathology of the reproductive tract. 3) Early embryonic death occurs, which can be due to a number of issues, eg, selenium deficiency, specific abortion disease (eg, border disease, toxoplasmosis, chlamydiosis), stress, heat shock in early pregnancy, or high levels of soluble protein in the feed leading to high urea nitrogen levels in the blood. 4) Abortion (mid to late term), if not observed, may also present as failure to conceive or maintain pregnancy.
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 (above), and genetics.