REPRODUCTIVE efficiency has significant implications for the economic success of equine stud farms, especially large thoroughbred breeding farms. This is particularly evident if one looks at one episode of mare reproductive loss syndrome. An outbreak of pregnancy losses in mares in 2001 in Kentucky, USA, is estimated to have cost the US thoroughbred industry US $ 300 to 500 million (Thalheimer and Lawrence 2001). The impact of the overall reduced reproductive efficiency on economic returns has also been quantified using multiple logistic regression models. These studies indicate that a mare needs to produce a foal six out of seven years to be financially viable (Bosh and others 2009a). While the loss of a fetus late in pregnancy has a more obvious financial impact, failure to conceive and early pregnancy losses can also have a significant impact due to a ‘drift’ in foaling date, which results in a season without producing a foal. Although conception rates in thoroughbred breeding programmes have significantly improved over the past two decades, pregnancy loss rates remain similar and represent the most significant source of reproductive wastage (Morris and Allen 2002, Allen and others 2007, Bosh and others 2009b). An epidemiological study of reproductive efficiency in the 2002 breeding season involving 33 English stud farms and 3373 thoroughbred mares indicated 14.3 per cent of pregnancies that are confirmed at day 15 are subsequently lost prepartum (Allen and others 2007). Studies in other countries are not dissimilar, reporting pregnancy loss rates of between 12.2 …