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HOW LONG IS YOUR MARE IN FOAL?

March 2011

The common perception of the average gestation period of a horse is 340 days. However, many standardbred breeders and studmasters have suspected that this average was in fact longer for the standardbred breed.

In order to investigate this issue, a database of standardbred mare pregnancies was created. The mares featured in this database needed to be chosen at random and represent a wide range of mare ages, stallions and states around Australia. To fulfill such requirements it was decided to take the 139 mares that were bred to Art Major in the 05/06 season and that produced a live foal. For each of these mares, data was collected from the Harness Web website (http://harnessweb.harness.org.au/harnessweb/index.do) for their last date of service (LDOS) and their foaling date for all pregnancies producing a live foal in their lifetime.

From the 139 mares, this procured a total number of 758 pregnancies resulting in a live foal. The resulting pregnancies had a range of LDOS from 1988 to 2009 – a spread of 21 years. Using Excel, the gestation period was calculated in days using the LDOS and associated foaling date, and the average gestation period for each mare was calculated from these values.

The overall average gestation period was calculated using the average gestation period for each mare and this figure was 347 days (Table 1). This is exactly a week longer than the commonly accepted average gestation period for the general horse.

To further the study, each mare’s pregnancies were broken down into three groups; LDOS in September or October; LDOS in November or December; and LDOS in January or February. The average gestation period for all pregnancies in each category was calculated and overall averages obtained. The average gestation period for LDOS in September or October was 352 days; LDOS in November or December was 346 days; and LDOS in January or February was 339 days (Table 1).

From these figures, it can be concluded that the average gestation period for a standardbred mare is longer earlier in the season, about average in the middle and significantly shorter at the end of the season (Figure 1). In conclusion, this study indicates that the average gestation period of a standardbred mare is infact 7 days longer than the general horse, and that mares bred earlier in the season will carry the pregnancy for an average of 5 further days (12 days on top of the previously perceived average).

Table 1
Graph 1