Foaling Tendencies and Dystocia Rates in New Zealand

Foaling Tendencies and Dystocia Rates in New Zealand

The average gestation length for pregnant mares in New Zealand is seven to eight days longer than mares in the Northern Hemisphere, which could mean larger, heavier foals and an increased risk of difficulty during birthing.

Photo: iStock

Dystocia (difficulty during birthing) in pregnant mares is most commonly attributed to an abnormal foal position in the womb. But researchers have recently learned that the foal’s size (and, indirectly, sex) also appear to contribute to dystocia, especially among mares in New Zealand.

The average gestation length for pregnant mares in New Zealand is seven to eight days longer than mares in the Northern Hemisphere. And the team believes this could mean larger, heavier foals and an increased risk of difficulty during birthing.

“Colts appear to be at a significantly higher risk of neonatal death compared to fillies,” said Cristina Rosales, BVSc (Hons), MVS, MANZCVS (Equine Medicine), MRCVS, a resident at the University of Melbourne’s Equine Centre, in Victoria, Australia. “This could be related to colts being typically heavier than fillies at birth.”

Rosales and colleagues recently performed a study to evaluate periparturient (around the time of birth) characteristics of mares and foals in New Zealand.

Management could be one factor that influences a mare’s gestation length. The mares in Rosales’ study were kept at pasture year-round and exposed to environmental conditions and daylight hour changes as the season progressed.

“We suspect that the gestation length is influenced by changes in internal melatonin concentrations (a hormone in the mare’s brain) and, thus, follows a more ‘natural’ course with regards to duration,” Rosales said. “Conversely, mares subject to artificial light can have their gestation lengths artificially shortened.”

Other findings included:

  • Dystocia was reported in 8.4% of foalings, with large foals being the most common cause;
  • 2.7% of foals died in the perinatal (time around birth) period;
  • The odds of foal mortality decreased with increasing gestation length and were higher for colts than fillies and when dystocia was reported;
  • 22% of foalings occurred during daylight, a figure higher than what’s reported from the Northern Hemisphere, Rosales said. “This may relate to when mares feel most at ease or least stressed,” she added. “There was no association between daytime foaling and neonatal death in this study.”
  • 95% of mares expelled the fetal membranes within four hours of parturition. “The large majority of mares pass their membranes within the first hour following birth, but if it’s longer than four hours, the mare needs immediate veterinary attention,” Rosales said.

Moving forward, Rosales said differences in periparturient behavior could be important in the management of mares and newborn foals. “This is an interesting area for further research,” she said.

The study, “Periparturient characteristics of mares and their foals on a New Zealand Thoroughbred stud farm,” was published in the New Zealand Veterinary Journal

About the Author

Katie Navarra

Katie Navarra has worked as a freelance writer since 2001. A lifelong horse lover, she owns and enjoys competing a dun Quarter Horse mare.

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