A surprising reason for foal rejection

Researchers may soon be able to explain why some mares reject their offspring.

Israeli researchers are a step closer to understanding the hormonal imbalances that may cause mares to reject their foals.

A mare running with a foal
Research shows that mares who rejected their foals had a drop in progesterone and prolactin levels from day one to day three post-foaling.

For a study, conducted at the Koret School of Veterinary Medicine—Veterinary Teaching Hospital of the Hebrew University of Jerusalem, researchers selected eight mares who behaved normally after foaling and 15 who demonstrated rejection behavior, such as indifference to the foal, refusing to let the foal nurse or even attacking the foal. All of the study mares were Arabian, a breed in which this behavior has been well-documented. “Primary or ‘true’ foal-rejecting phenomenon has not been documented as a consistent problem in other breeds,” says Dalia Berlin, DVM.

On the first and third day after foaling, the researchers drew blood from the mares to be tested for levels of three reproductive hormones: progesterone, prolactin and estradiol. “Progesterone is the main hormone responsible for maintaining the pregnancy,” says Berlin. “Its concentration decreases toward parturition0. Prolactin is the main hormone of milk production. The role of estradiol is not clear in the post-foaling period.”

The data showed that mares who rejected their foals had a drop in progesterone and prolactin levels from day one to day three post-foaling, while estradiol remained unchanged. In the mares displaying normal maternal behavior, progesterone concentrations decreased as well, but the estradiol and prolactin concentrations did not change. The most statistically significant difference between the two groups, however, was found in the interplay between two of the hormones: The estradiol-to-progesterone ratio in the first day after foaling was significantly lower in rejecting mares than it was in mares exhibiting normal maternal behavior.

“The role of these two hormones in initiating and maintaining maternal behavior is unclear in all animals,” says Berlin. “Progesterone has been related to aggressive behavior in female animals guarding their offspring, and estradiol concentrations were not found to be directly related to maternal behavior. We do not know how this ratio may affect the maternal behavior. Our results are novel and further studies are required to further investigate them.”

The ratio of these hormones in the rejecting mares increased in the three days after foaling, suggesting that, over time, it might normalize and the aberrant behavior could diminish. “Most of the mares in our study did indeed improve over time,” says Berlin. “In our experience, most rejecting mares improve, and it is possible that in these mares the hormonal imbalance is gradually corrected. In some mares, however, such as the three ‘permanent rejecters’ in our study, the behavior does not improve over time. Perhaps in these mares the imbalance persists. We did not have enough mares to prove such a theory.”

Click here to learn how the seasons affect “marish” behavior. 

Berlin adds it’s premature to consider hormonal treatments for rejecting mares. “It is too soon to deduce from our results that hormonal treatment could be curative. Because only the ratio was found to be significantly different between the groups, we still don’t know what the optimal concentration for each hormone is.”

Reference:Postpartum concentrations of serum progesterone, oestradiol and prolactin in Arabian mares demonstrating normal maternal behaviour and Arabian mares demonstrating foal rejection behaviour,” The Veterinary Journal, February 2018

This article was originally published in EQUUS 488

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