Although the trace mineral zinc (Zn) was first noted as a new metal around 1374 in India, it had been used for many years prior, extracted in an impure form from zinc-ores to produce brass. Yet, it was not named until the 16th century, when a European alchemist documented it as “zinken.”
The development of a method to purify these zinc extracts led to its use in the medical field, especially for treating inflammatory eye conditions by means of zinc solutions and tablets. Interestingly, researchers know today that the highest concentrations of zinc in the body can be found in the eye’s choroid and iris.
It was only in 1934 that zinc was demonstrated to be an essential nutrient in rats’ diets by Todd et al. Rats fed a low-zinc diet had poor growth rates and abnormal fur coats. The following year, research from the same laboratory showed that supplementing zinc-deficient rats with zinc corrected the “faded, thin, and wooly” fur condition and allowed normal growth rates to resume. These findings were followed by similar research in livestock, exploring zinc deficiencies and establishing requirements. Across species, including in the horse, reduced growth rates, inappetence, and abnormalities of the skin were common observations in zinc-deficient animals.
Today, research across species, including humans, has linked zinc to many different bodily processes. These include fetal development, growth, tissue repair, reproduction, and the immune system, to name but a few. Zinc serves as a catalyst for more than 200 metalloenzymes in the body, explaining in part how it can affect such a range of different processes.
Zinc also fulfills many structural functions. For example, it can be found at the functional site of the enzyme copper/Zn superoxide dismutase, and in a diverse group of proteins called zinc fingers, where a zinc ion stabilizes protein structures involved in functions such as signal transduction and cell proliferation. Zinc has also been shown to affect gene expression regulation.
In areas where soil zinc concentrations are low or marginal, local forages and feeds might subsequently be low in zinc. Feedstuffs commonly fed to horses contain approximately 15 to 40 mg zinc per kg dry matter. The recommended total dietary zinc intake for a 500-kilogram (1,100-pound) mature horse, idle or used for light exercise, is 400 mg per day (National Research Council’s Nutrient Requirements of Horses, 2007). This recommendation increases to 500 mg per day for a lactating mare or heavily exercising horse of similar weight. Therefore, some horses might require additional zinc supplementation to meet their requirements.
Commercial horse feeds are formulated to provide additional zinc to a horse’s diet and could contain either an organic or inorganic form of zinc, as would be indicated on the feed tag. The maximum tolerable concentration for zinc in equine diets has been set at 500 mg/kg dry matter, well-above the recommended amount of zinc required by the horse. However, excessive amounts of dietary zinc could interfere with a horse’s copper status. It is not clear if zinc affects copper metabolism by interfering with absorption through shared transport mechanisms, or post-absorption, but maintaining a reasonable total dietary copper to zinc ratio is important. Commercial horse feeds are formulated keeping this in mind. Therefore, it is always a good idea to work with a nutritionist should you consider adding additional mineral supplements to a balanced commercial horse feed.
Mieke Holder, PhD, is an assistant research professor within UK’s Department of Animal and Food Sciences.
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