Equine Science Update
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Moderate exercise in the last trimester of pregnancy is unlikely to cause problems for mares or their unborn foals, research suggests.

A research team from the University of Maine and the Equine Science Center, Rutgers University, New Jersey compared the response of pregnant and non-pregnant mares to a graded exercise test. A full report of the study has been published in Comparative Exercise Physiology.
Written by Mark Andrews. Published online 25.02.10
© Copyright Equine Science Update  2010
Effect of exercise in pregnant mares
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They evaluated the effect of exercise on various parameters including maternal heart rate, plasma cortisol and plasma lactate and foetal heart rate.

Six unfit standardbred mares were assessed when nine months pregnant and again eight months later after weaning. The exercise test was carried out on a treadmill inclined at 6%. The speed increased in steps of one minute each at 4m/s, 6m/s and 7m/s.

An ECG monitor attached to the mares recorded both mare and foetal heart rate. It was not possible to record the foetal heart rate during exercise because of noise on the ECG signal, but it was recorded immediately after the end of the exercise test.

The mares’ heart rates during exercise were recorded with a micromanometer catheter, which detected the  changes in blood pressure with each beat.

The results revealed some interesting differences between the mares’ responses to exercise depending on whether they were pregnant or not.

At each stage of the graduated exercise test, mares had lower heart rates when pregnant than after weaning. Plasma lactate concentrations rose less in response to exercise when mares were pregnant than when they were not. The researchers speculate that this might be due to greater cardiovascular efficiency during pregnancy.

Resting plasma cortisol levels were lower during pregnancy than when the mares were not pregnant.

The researchers were surprised to find that pregnant mares showed no increase in cortisol levels during exercise In contrast, when they were not in-foal, the mares  showed a normal increase in plasma cortisol during and after the exercise test.

Exercise had no effect on foetal heart rate. This suggests that the unborn foal is not stressed by the mare undertaking moderate exercise.

The researchers point out that the foals from the mares involved in the study were born without problems and were healthy, with normal body weights.

They conclude: “these data suggest that pregnant mares benefit from greater cardiovascular efficiency during pregnancy. They should be able to perform limited moderate exercise without any major deleterious effects on their unborn foals or themselves during late gestation.”

For more details:
Maternal and foetal heart rates during exercise in horses.
Lehnhard RA, Manso Filho HC, Causey RC, Watford M, McKeever KH
Comparative Exercise Physiology (2009) 6, 43-48.
Photo courtesy Dr Mitsuo Ishii