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Body Condition Score of Mare Impacts Sex of FoalBy Kentucky Equine Research Staff · October 15, 2015

Most of us are aware of the importance of body condition on equine reproductive success. What some of us might not know is that there is some evidence that the sex of the foal can be impacted by the mare’s body condition score (BCS). In industries that do not permit artificial insemination, this could prove handy for breeders looking for big, strong colts born early in the breeding season, such as Thoroughbreds.

Previous research in wild horses in New Zealand has shown that mares in good condition at the time of conception produced more colts. When mares were light, they produced more fillies. This finding was further explored by a researcher at Massey University in New Zealand. Out of 24 mares on two separate studs with BCSs between 3.5 and 8, he found:

  • Mares with a BCS between 5.0 and 6.2 (mean 5.6) at the time of cover produced fillies; and
  • Mares with a BCS between 6.3 and 7.3 (mean 6.8) at the time of cover produced colts.

The study also reported a significant and linear relationship between BCS at the time of conception and the interval from parturition to date of first service, which was unexpected. Those data prompted the study author to recommend that “...breeders should not attempt to have their mares on a higher BCS at the moment of covering than 6.5-7, because the interval from parturition to first date of service will prolong when mares gain weight.” 

“The general recommendation is to have mares at a BCS of 5 to 6 at the start of the breeding season, which means owners and managers need to plan ahead and regularly assess their mares’ condition using established scoring systems like the Henneke 1-9 scale or the Australian 0-5 scale,” said Peter Huntington, B.V.Sc., M.A.C.V.Sc., director of nutrition at Kentucky Equine Research (Australia).  

He added, “The practice of ‘flushing’ or a gain in BCS has been shown to increase fertility in ruminants, but is not beneficial in horses unless the mare is thin, as in just retired from racing or competition. If mares are too fat, they can have delayed conception or an increased risk of foals with developmental orthopedic disease.”

Huntington’s recommendation holds true regardless if mares are bred back shortly after parturition and are lactating or open from the previous season.