Trace Minerals

This article is from the August 2019 Horse Deals magazine.

Photo: Rachel Clayfield

Dr. Mark Barnett has had a lifetime association with horses, riding and showing horses from an early age, before completing a Bachelor’s Degree and a PhD in Animal Nutrition and Physiology. Mark was appointed the Lecturer of Animal and Equine Nutrition at Charles Sturt University before leaving the academic world and taking up the role of Equine Nutritional Consultant for a nutrition company in SE Queensland. Since then, Mark has created MTB Equine Services, with his wife Tracey. Mark’s focus is on improving the health, appearance and performance of horses at all levels using balanced nutrition that is scientifically sound.

While they make up only a small portion of a horse’s entire diet by weight, minerals play a critical role in the everyday function of the horse. Without their inclusion in the diet, horses would not be able to metabolise their feed, mares wouldn’t be able to go in foal, and their bones would not be able to support their own weight. They are necessary components of virtually every enzyme the horse uses and are integral parts of vitamins, hormones, and amino acids, yet they make up only 4% of the horse’s total body weight. Amounts absorbed by the horse depends on the mineral involved, its need in the body, and the form it is in. Either way, they are vital in the fertility of the broodmare and stallion, and the growth and development of the offspring. This article briefly looks at some of the more important trace minerals needed by the broodmare, stallion and foal.

Copper (Cu)

Copper is an essential component of the equine diet and possibly one of the greatest contributors to health, appearance, performance, and fertility in horses in Australia. Known to be associated with reproduction, bone and connective tissue development, growth, coat condition, immunocompetence and red-blood cell formation (just to name a few), it is vital that an adequate concentration of copper be found in the horse’s diet.

Australian soils are notoriously deficient in copper and, to exacerbate the issue, are overloaded with iron. Copper deficiency occurs either when there are insufficient amounts in the diet, or where absorption and storage are adversely affected by a high intake of iron and/or manganese.

Your horse’s iron to copper ratio in its diet should be no more than 8:1 iron to copper, otherwise a copper deficiency may result.

Broodmares need about 200 to 250 mg of copper a day to avoid deficiencies and to assist in preventing instances of limb deformities in their foals (higher if the iron to copper ratio is greater than 8:1). Studies have shown that the inclusion of a copper supplement in the diet of mares during the last few months of her pregnancy, can reduce the incidence of these leg issues in the foals.

Requirement
Growing horse: 30 mg/kg of the diet (e.g. 150 mg for a 200 kg foal eating 2.5% of its body weight)
Broodmare: 20 mg/kg of the diet (e.g. 200 mg for a 500 kg mare eating 2.0% of its body weight)
Stallion: 15 mg/kg of the diet (e.g. 150 mg for a 500 kg stallion eating 2.0% of its body weight)

Zinc (Zn)

Zinc is a trace mineral required in many physiological roles of the horse, including enzyme development and functioning, gene expression, appetite, fertility, coat and hoof conditioning and a properly functioning immune system. Horses who are deficient in zinc often display symptoms such as reduced appetite, retarded growth, thickening of the skin and loss of hair (often in patches). Research has shown that excessive magnesium in the diet can cause reduced absorption of zinc, inadvertently creating a deficiency, as can certain anti-nutrients in particular feeds, especially grains. Phytases, which are the principle storage form of phosphorous in grains, can impair the absorption of zinc in horses by as much as 40%.

Foals appear to require a higher daily intake of zinc compared to older horses. In 2013, a study by de Simone et al. recommended that the daily dosage set by the National Research Council (2007) for zinc in the diets of weanling, yearling and 2-year-old Thoroughbreds be increased due to apparent compromised immune systems. Both broodmares and stallions greatly benefit from the inclusion of extra zinc in their diets, with many research studies indicating improved semen quality and mare cycling with elevated zinc in the diet.

Requirement
Growing horse: 90 mg/kg of the diet (e.g. 450 mg for a 200 kg foal eating 2.5% of its body weight)
Broodmare: 60 mg/kg of the diet (e.g. 600 mg for a 500 kg mare eating 2.0% of its body weight)
Stallion: 45 mg/kg of the diet (e.g. 450 mg for a 500 kg stallion eating 2.0% of its body weight)

Manganese (Mn)

Manganese is essential for carbohydrate and fat metabolism, cartilage formation and a functional immune system. Studies have shown that one of the symptoms of a manganese deficiency in horses is enlarged hocks and, by affecting the bone growth plates, the shortening of legs with characteristic knuckling-over of joints. When young horses are fed high calcium diets (e.g. high lucerne/alfalfa diets) they can develop flexural deformities of the legs (the inability to extend a joint completely). Research has shown that supplementation with manganese can rectify the problem. There is some suggestion that a deficiency in manganese may result in twisted limbs and shortened forelimb bones in foals, but the results have not been conclusive. Young horses on manganese deficient diets have an increased incidence of lameness and incoordination of movement, while dams severely deficient in manganese have a much higher incidence of resorption of the foetus in utero, or death of the foal at birth. Mares that are experiencing only a slight deficiency of manganese in the diet have a greater tendency of irregular oestrus cycles, making them much more difficult to get
in foal.

Selenium (Se)

Selenium plays an important role in the maintenance of membrane integrity, growth, reproduction and the immune system. Selenium deficiency in horses can cause a condition called white muscle disease, a skeletal and cardiac muscle degenerative disease that affects foals from birth to 11 months of age. The disease results in weakness, impaired movement, difficulty in suckling and swallowing, respiratory distress, and impaired cardiac function. In extreme cases, the disease can even result in death.

Selenium is also a powerful antioxidant, capable of eliminating free radicals produced from oxidative stress, which can cause cellular and tissue damage in the body. Studies have suggested that foals and their dams respond positively to high daily doses of selenium. A study in the USA demonstrated that supplementing mares in late pregnancy with 3 mg/day of selenium resulted in foals with higher concentrations of the immune system immunoprotein, IgG, compared to foals from dams receiving the currently recommended 1 mg/day. As IgG is essential in the early stage immune defences of a foal, increased supplementation of the dam may be very beneficial.

Requirement
Growing horse: 0.3 mg/kg of the diet (e.g. 1.5 mg for a 200 kg foal eating 2.5% of its body weight)
Broodmare: 0.3 mg/kg of the diet (e.g. 3 mg for a 500 kg mare eating 2.0% of its body weight)
Stallion: 0.2 mg/kg of the diet (e.g. 2 mg for a 500 kg stallion eating 2.0% of its body weight)

Iodine (I)

Iodine is an essential nutrient for reproduction, metabolism and normal physiological function in the horse. Iodine plays an important role in the horse’s overall health, heat regulation, feed utilisation, bone growth and maturation. Approximately 75% of the iodine in a horse’s body is stored in the thyroid gland. Iodine deficiency can result in the development of a goitre, an enlargement of the thyroid gland, something common in horses receiving a ration deficient in iodine, especially in foals whose mothers receive a low iodine diet during pregnancy. Symptoms of an iodine deficient foal are stillborn foals or very weak foals who cannot stand and suckle, a rough haircoat, contracted tendons and angular limbs or other abnormal bone developments. A study in Russia even showed that mares with anovulatory cycles (develop follicles each breeding cycle but they don’t ovulate and release an egg for potential fertilisation) responded positively to iodine supplementation.

Requirement
Growing horse: 0.25 mg/kg of the diet (e.g. 1.25 mg for a 200 kg foal eating 2.5% of its body weight)
Broodmare: 0.3 mg/kg of the diet (e.g. 3 mg for a 500 kg mare eating 2.0% of its body weight)
Stallion: 0.2 mg/kg of the diet (e.g. 2 mg for a 500 kg stallion eating 2.0% of its body weight)

SUMMARY
A balanced diet can often mean the difference between a mare going in foal, the stallion successfully serving that same mare and a quality foal produced 11 months later. Before commencing a feeding program for your horse, it is important to properly analyse the conditions that your horse lives in, its access to and type/quality of pastures, type and quality of fibre available and activities expected of them. A qualified equine nutritionist can evaluate your horse’s diet, detecting any imbalances, shortfalls and excesses, and provide a suitable diet designed to enhance the health, appearance, performance and fertility of your horse.

Written by Dr. Mark Barnett PhD MTB Equine Services

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