150
46TH
ANNUAL
CONGRESS
OF
THE
SAEVA
SKUKUZA
16-‐20
FEBRUARY
2014
Predicting reproductive
performance outcomes in
Thoroughbred breeding systems
Schulman, ML1* and May, CE1
1
Section of Reproduction, Faculty of Veterinary Science, University of Pretoria
Introduction
Reproductive performance of the Thoroughbred continues to be influenced by
several additional factors; primarily including a traditionally imposed official breeding
season partially at odds with the normal physiological breeding season. This
obviously influences the opportunities for mating and establishment of pregnancy.
There have additionally been no selection criteria in the breed other than for
pedigree and athletic performance. There is furthermore a complete restriction on
any form of assisted or artificial reproduction technology. A number of factors
thought to contribute to reproductive performance have been reported, these
include: book-size, mare availability, mare age, mare status, economic factors, Stud
Health programmes, stallion factors and month of mating. Very few of these factors
have been assessed through controlled studies.
The conception rates in Thoroughbred breeding systems world-wide have shown
significant improvement over recent decades. There has, however, been little
improvement in the reported overall pregnancy loss rates over the same period and
although year-to-year variations are recorded, they essentially remain similar and
represent the most significant source of reproductive wastage. While the loss of a
fetus late in pregnancy has a more obvious financial impact, the failure of a
broodmare to conceive and early pregnancy losses will also have a significant impact
due to a ‘drift’ in the foaling date, which ultimately results in a season without
producing a foal. The impact of the overall reduced reproductive efficiency on
economic returns in Kentucky Thoroughbreds was recently quantified using multiple
logistic regression models. This indicated that for financial viability a broodmare
needed to produce a foal at least in six of any seven years at stud.
Epidemiological studies on reproductive efficiency from Thoroughbred mares
resident in several countries reported that 12.2 to 20.6 % of pregnancies that had
been confirmed at Day 15 were subsequently lost prior to parturition. The most
common losses occurred in the embryonic and pre-implantation period, often
reported as “early embryonic death” (or EED), and this accounted for > 50 % of all
pregnancy attrition. Relatively little is currently known about the causes of EED in
mares and this very high rate of unexplained EED is in contrast to equine abortion,
where a causative diagnosis is relatively common and is often infectious in nature.
The majority of epidemiological studies have focused on identifying individual risk
factors for pregnancy loss, and are primarily concerned with mare factors that have
been traditionally associated with pregnancy failure. Most reports have identified
increasing mare age as a major risk associated with early pregnancy losses. This has
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