As with the ram, ewes might fail to be bred, or be bred but fail to become pregnant. In addition, the ewe might not maintain the pregnancy or might have decreased prolificacy. Again, the possible reasons are numerous. A ewe might not be bred for the following reasons:
The ewe might already be pregnant.
It might be anovulatory season, which is longer in ewe lambs.
The ewe lamb might 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 can temporarily or permanently suppress estrus.
Ewes might be too thin, lactating, or recently weaned.
Ewe lambs that are overfed postpubertally might not cycle.
Ewes (especially maiden ewes) might display behavior such shyness or dominating the ram.
There might be a freemartin/pseudohermaphrodite condition.
Ewes might not conceive or maintain pregnancy because of the following:
The synchronization program is not correct; eg, rams are joined too early with the ewes before they are in estrus or ovulation does not occur.
Pathology of the reproductive tract exists.
Early embryonic death occurs, which can be due to a variety of disorders—eg, selenium deficiency, specific causes of abortion (such as border disease, toxoplasmosis, chlamydiosis), stress, heat shock in early pregnancy, or high levels of soluble protein in the feed, leading to high BUN concentration.
Abortion (mid- to late-term), if not observed, can 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, early embryonic death, and genetics.
