Zinc (Zn) and copper (Cu) are dietary trace minerals with different functions in the body, but they are often discussed together. That’s because they use the same “vehicle” for absorption from the digestive tract into circulation and compete for a ride on that vehicle. As such, horses must consume Zn and Cu in proper amounts so that one mineral doesn’t outcompete the other.
The National Resource Council’s Nutrient Requirements of Horses (2007) recommends that Zn and Cu be fed in a 4:1 ratio (4 parts Zn to 1 part Cu) for mature horses to ensure proper absorption of both minerals.
Zinc is part of over 100 different enzymes in the body, many of which participate in protein and carbohydrate metabolism. The highest Zn concentrations are found in the iris and choroid (between the sclera and retina) of the eye, pancreas, hoof horn, liver, and muscle.
An average 1,100-pound (500-kilogram) horse requires a minimum of 400-500 milligrams of Zn in the diet per day. Wheat bran, wheat middlings, and brewer’s grains are good Zn sources. Most forages contain low- to marginal Zn concentration, but fortified commercial concentrates typically contain sufficient amounts. Horses on forage-only diets might require supplementation.
Zn deficiency is rare, and while horses seem to tolerate dietary excess well, overconsumption of Zn results in decreased Cu absorption. This can have devastating consequences, especially for growing foals (zinc deficiency is possibly linked to some developmental orthopedic disease, or DOD).
In horses Cu has a number of vital roles, including:
- Maintaining healthy connective tissue (e.g., ligaments and tendons);
- Aiding antioxidant enzymes in cellular mitochondria;
- Melanin (pigment of skin, hair, and eyes) synthesis; and
- Iron stores mobilization.
The liver stores and metabolizes Cu. An average, 1100-lb (500- kilogram) horse requires a minimum of 100-120 milligrams of dietary Cu per day. Requirements vary for growing horses and pregnant mares.
Generally, Cu in forage hovers around adequate concentration. Molasses, brewer’s grains, and soybean meal are other good Cu sources. Again, horses on forage-only diets might require supplementation, depending on the nutritional content of pasture and hay.
Deficiency results in abnormal bone growth in growing horses. In mature animals, connective tissues weaken and elastin (main protein in connective tissue) synthesis declines. Horses seem to tolerate excess dietary Cu well, perhaps because the more Cu a horse ingests, the less is actually absorbed.
Both Zn and Cu have important roles in growth and connective tissue health, making them especially critical for growing animals. Forage-only diets could be borderline with respect to providing adequate amounts of Zn and Cu, potentially requiring a ration balancer to ensure proper intake. A qualified nutritionist can guide horse owners to provide properly fortified diets.