The importance of iodine in pregnant mares’ diets

A multi-year study led by Dr. Claire Card at the Western College of Veterinary Medicine (WCVM) is aimed at ensuring that mares receive adequate levels of iodine in their diets as a means of preventing the devastating effects of an iodine deficiency on their foals.

Iodine is an essential nutrient required to produce the thyroid hormones that play a key role in metabolism and growth. During the last trimester of pregnancy when the foal’s thyroid begins to develop, an iodine deficiency can prevent the gland from functioning at an adequate level.

The result is a foal that’s considered “hypothyroid,” meaning that its thyroid gland isn’t making thyroid hormones to meet the foal’s needs. 

“I was seeing many mares with long gestations, delivering foals that were hypothyroid and very contracted-legged (with contracted leg muscles),” says Card, a board-certified theriogenologist and professor at the WCVM.

“Many had to be euthanized, and we started on a journey to figure out why these problems were developing during gestation. Over time the evidence pointed to a nutritional basis.”

While previous research has established that iodine is critical for healthy foals, Card’s most recent study is aimed at determining the exact amount of iodine that’s needed for a mare to produce a healthy foal with a healthy, functioning thyroid gland.

Her investigation includes a careful analysis of iodine excretion – a measure of the amount of iodine taken in compared to the amount that’s excreted. The study also examines other factors that can affect iodine absorption.

For example, iodine absorption can be affected by nitrates such as those found in fertilizer. The researchers are particularly concerned about the effects of glucosinolate — a compound that’s found in oil seeds such as mustard and canola. 

Some mares have been known to selectively seek out and eat these plant species containing glucosinolates that are commonly found in Saskatchewan pastures.

“If you combine a low iodine diet that leads to a marginal or deficient iodine state in the pregnant mare and add another environmental stressor such as consumption of mustard family plants … or nitrate from various sources, that mare is at risk of producing a hypothyroid contracted legged foal with disturbed musculoskeletal and bone development,” says Card.

The WCVM study included two groups of pregnant mares. While one group received iodine supplements throughout their pregnancy, the other group received none. Since a previous study indicated that a diet containing glucosinolate compounds results in lower iodine levels, this investigation also explored that finding by feeding some of the mares a diet containing mustard seed. 

To track changes in the mares over the course of their pregnancies, researchers administered thyroid function tests to each animal before and during the study period. They also used ultrasonography to record the size of each animal’s thyroid gland and to reveal any abnormalities. In addition, the team periodically analyzed blood and urine samples to determine the iodine levels of each mare.

The research team members carefully examined each foal after birth to detect any signs of hypothyroidism such as poor muscle development, contracted tendons and abnormal jaw alignment.

Although many horse owners are already aware of the importance of adequate iodine levels for their pregnant mares, they may find it difficult to ensure that their horses are ingesting a sufficient amount. 

Since it’s difficult to monitor a horses’ uptake from a salt block, relying on them for iodine supplementation can result in insufficient iodine levels, especially when the animals are housed together. In addition, weather causes wear on the salt blocks and makes it even more difficult to accurately measure how much each horse is getting. 

In some cases, the amount of iodine indicated in the nutritional analysis on a feed bag doesn’t match the actual iodine content of feed samples once they’re tested in the lab.

If horse owners want to ensure that their mares receive adequate amounts of iodine in their diet, Card recommends that they mix a high iodine content salt (200 milligrams per kilogram) one-to-one with a loose mineral supplement.

A relatively simple blood test can also inform horse owners if iodine levels are low. Since most veterinary clinics can conduct this test, owners should have their veterinarian check iodine levels in their animals about twice a year.

Although iodine levels can vary among individual horses, their requirements increase after periods of exercise or during physiological states such as lactation and pregnancy.

As Card and her research team continue to gather information from this study, they’re optimistic that their investigation will provide important information that can help horse owners ensure their mares are receiving a healthy diet and can produce healthy foals.

Card points out that researchers need to further investigate the role of dietary nitrate and iodine.

“With very thin data, it has been stated that horses tolerate higher levels of nitrate than cattle. While it’s true that horses have fewer clinical signs related to consumption of nitrate compared to cattle, the safe level of nitrate that does not interfere with iodine uptake has never been established,” says Card.

“Due to our changing climate, determining what a safe level of nitrate is has never been more important.” 

This latest congenital hypothyroidism initiative has financial support from the Mark and Pat DuMont Equine Research Fund — funding that has made her team’s work possible and for which she’s extremely grateful, says Card.   

“After 31 years we finally have answers for many people whose lives were affected by the birth of these foals. We are confident that all horse owners will benefit from this information. The information on dietary iodine is unique and comprehensive, and it will be cited in the future.”

Laura Callaghan of Medicine Hat, Alta., is a third-year veterinary student at the WCVM who worked as a summer research student in 2021. Her story is part of a series of articles written by WCVM summer research students.