| Beef cows rarely
lose a well-established pregnancy. Producers occasionally will notice
that a cow was found to be pregnant at pregnancy checking time, but fails
to have a calf during the subsequent calving season. This is not
necessarily cause for alarm. However, it is important to know what
levels of pregnancy loss are "normal" and at what level veterinary help
should be sought and a diagnosis initiated. Most researchers agree
that minimal losses (<3%) are occurring during the fetal stages
(e.g., greater than 40 days of development) of gestation. Reviews
of research on this topic were published in 1979 by Bellows and coworkers
and another by Hawk and coworkers. Furthermore, failure of fertilization
appears to occur at a rate of approximately 10%. Thus, most embryonic
losses occur during the period from fertilization to day 40, and a study
of physiological events leading to establishment and maintenance of pregnancy
is most likely to lead to developing methods to increase fertility.
It would appear that in herds in which the percentage of potentially
infertile cows is higher (e.g., heifers vs. mature cows), higher rates
of embryonic loss occur by 7 to 8 days of pregnancy. Some of the
early embryonic losses are attributed to chromosomal abnormalities that
are inherited or arise during meiosis, fertilization or early cleavage
stages. According to Wilmut and coworkers in 1986, chromosomal abnormalities
in cattle occur at a frequency of approximately 7.5%. Many of these
occur early enough that the cattle owner does not detect that fertilization
failure occurred, but rather assumes that the cow was not bred at all.
Death of these chromosomally abnormal embryos early in the preimplantation
period may account for a lower frequency of gross chromosomal abnormalities
detected later.
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