Zinc and the equine diet 

Zinc is an essential trace element and the equine body needs zinc for more than just intact skin and healthy hooves. Many of our local soils and plants contain only small quantities of zinc, so horses need added zinc in their feed. But what does zinc do, and how much does your horse need?  

Why do horses need zinc? 

Zinc is the second most abundant trace element in the equine body after iron. It can be found in different concentrations in various parts of the body. The largest share is found in muscle (60% in the skeletal muscles), but it also plays an important role in the pancreas, hoof horn, skin, and the liver. Again, an adequate supply of zinc is essential for the regeneration of skin and mucous membranes as well as for the cell renewal process. Zinc is also used in enzymatic processes in carbohydrate, fat and protein metabolism, and is needed for nerve tissue to function smoothly. Together with other micronutrients, zinc is important for hormone synthesis (insulin, growth hormones, sex hormones), for building keratin, for good vision and for a healthy immune system.  


Zinc digestion 

The horse consumes zinc through its feed. Actual zinc absorption takes place primarily in the small intestine. The absorption rate is not always consistent and depends on various factors, for example its type of bond. Zinc can be organically bound or inorganically bound. Trace elements that are organically bound, mostly to amino acids, are said to have a higher bio-availability.
Zinc binds to the transport protein albumin in the blood plasma and is carried to the liver. The liver returns 30 to 40% of the zinc it receives back into the blood. In the blood, the highest concentration of zinc s found in the red blood cells (75 to 88%), with a much smaller share in the blood plasma and white blood cells, and a mere 3% in the blood platelets. 

The bowel isn’t just where the most zinc is absorbed, but also where most of it is eliminated, while very little elimination of zinc happens in the kidneys. Zinc can also be released through perspiration, bleeding, the regeneration of skin and mucous membranes, and hair and horn growth. 

Zinc uptake and release are subject to homeostatic regulation. This means that there are mechanisms that ensure a balance between absorption and release so that the body maintains the concentration it needs. The equine body therefore regulates absorption in the bowel and elimination according to what it needs. How does this process work? Basically, more/less zinc is absorbed/released through the intestinal wall. Along with this simple process there are other mechanisms to control zinc balance: reduced zinc elimination in the kidneys, increased zinc conversion in the blood, and conservation of zinc released from the blood cells. These mechanisms are only activated when there are large fluctuations in zinc levels or with prolonged cases of excess or insufficient zinc. In the liver, kidneys, pancreas and bowel, zinc can bind to or be released from metallothioneins, depending on whether zinc levels in the organism are too high or too low. Metallothioneins are transport proteins that can regulate zinc levels by their increased or decreased production in the mucous membrane cells and their binding or release. 

How much zinc does my horse need? 

The German Society of Nutrition Physiology recommends 4 mg of zinc per kg of metabolic body mass (BM0.75 ) for horses in maintenance metabolism. For a 600 kg horse, that corresponds to 485 mg of zinc daily. Pregnancy, lactation, growth and increased work raises the recommendation to 4.5 mg per kg BM0.75. A 600 kg horse that gets daily work in all gaits for an hour or more should therefore get 546 mg of zinc. 

The metabolic body mass of the horse BM0.75 
A horse's need for energy, nutrients, minerals, trace elements and vitamins does not increase linearly with live weight, as metabolic activity is related to the ratio of body surface to weight. This is, for example, because of heat dissipation via the skin. The metabolic body mass therefore describes the metabolically active part of the body weight that is included in the calculation of requirements, as this consumes energy. 

Example for the calculation of BM0.75 and the need for zinc in maintenance metabolism:
A horse with a weight of 600 kg has a body mass of 600 kg and therefore a metabolic body mass of 600 kg0.75, i. e. approx. 121 kg. 
The “Gesellschaft für Ernährungsphysiologie” [Society for Nutritional Physiology] recommends a need for zinc of 4 mg per kg of metabolic body mass in maintenance metabolism, i. e. 121 kg x 4 mg = 484 mg zinc for a horse weighing 600 kg.

To know how much zinc to feed your horse, naturally you must first determine what he’s getting now. One way to do that is through a ration calculation. You can use this to determine how much zinc your horse consumes through his feed. However, up-to-date hay analyses are rarely available and zinc levels in pasture grass can only be estimated. So unless all feeds have recently been analysed, a ration calculation can only give you a general overview.  

Approximate assessments can also be obtained through blood plasma/blood serum and/or hair analysis. A blood analysis will reveal a recently occurring zinc deficiency, whereby a hair analysis will reveal a long-term zinc deficiency. In the blood, zinc levels in the range of 40-170 µg/dl (6-26 µmol/l) are considered adequate. However, there are studies showing that even blood samples are not reliable in showing zinc levels. It has been determined that stress can reduce the concentration of zinc in the blood plasma, although this would not necessarily mean a general zinc deficiency. Zinc levels in the blood also seem to fluctuate with the seasons, with the highest levels found in winter and the lowest in summer during the grazing season. Pasture grazing might be a factor as some studies have shown that horses who live outdoors generally have lower zinc concentrations in their plasma. However, not all studies came to this conclusion. Illness can also affect zinc levels in the blood; low zinc levels were found both in horses with severe cases of sweet itch as well as in horses with febrile infections. 

Different zinc compounds consumed through feed can also have different effects on zinc levels in the blood. In a study involving ponies, a dose-dependent effect on blood levels was found with zinc sulphate, zinc chelate and zinc lactate, but not with zinc oxide. This is probably because the body does not absorb zinc oxide as easily. 

Another approach for examining zinc absorption is through faecal samples. In one experiment, zinc excretion increased when zinc levels in the feed increased. Faecal samples may also be a good alternative to feed analysis for assessing minerals in the feed. Further research is needed here to examine and assess the exact connection between absorbed and eliminated quantities. 

How much zinc does horse feed contain? 

The amount of zinc in the base feed’s native elements (hay, haylage, straw, pasture grass) is often insufficient, as many regions have soils with low zinc concentrations. Horses who eat pasture grass exclusively may not be getting enough zinc. Even when horses in studies consistently consumed too little zinc when turned out 24/7, the exact zinc levels differed significantly. This may be due to the fluctuating zinc content in the grasses, herbs and legumes depending on location, to the vegetation stage, and/or lower concentrations of trace elements due to heavy rains. Zinc levels vary not only in pasture grass, but also in other forage feeds. A soil’s average zinc concentration can fluctuate from 10 mg/kg to 80 mg/kg. These differences are caused by regional factors. A horse that lives outside should get at least 35 mg of zinc per kg of dry mass. In the 1970s the calculation used was 28 mg of zinc per kg of hay; today that number has dropped to 22 mg. A horse will need at least 40 mg of zinc per kg of dry matter. 

A mineral feed containing zinc is generally recommended. No perfect composition exists, however, due to the high fluctuations each month; these come close to meeting a horse’s requirement. 

Making note of what your horse eats every day will give you a relatively reliable assessment of how much zinc he consumes. There are also feeds and individual ingredients that can throttle zinc’s bio-availability and absorption if consumed in very large quantities. These include calcium, copper and phytic acid (found in grains and brewer's yeast). 

Feed regulations give maximum zinc amounts to be used in feeds so that horses are not chronically oversupplied: 150 mg per kg of complete feed or per kg of dry mass of the daily ration. Zinc oxide is widely used as a zinc compound in feed, but is less effective than zinc chloride or sulphate. 


Zinc deficiency 

The effects of a zinc deficiency are not often seen from the outside. An inadequate supply can cause a condition known as parakeratosis, in which a thickening of the skin occurs with bark-like deposits along with hair loss and increased susceptibility to infection. The wounds can be slow to heal. It is assumed that inadequate zinc intake may also cause poor hoof horn. 

Low zinc intake can cause skeletal development disorders in unborn foals. It can also heighten susceptibility to infections, stunt growth and impair limb development. The third and fourth months of a foal’s life are particularly critical, before an intense growth spurt occurs in the fifth month. Growing foals need an adequate supply of all micronutrients for proper bone mineralisation. 

Can too much zinc be toxic? 

Horses can tolerate an oversupply of zinc (up to 500 mg/kg dry matter won’t pose problems). This basically means that added zinc will not do your horse any harm if he doesn’t have a zinc deficiency. The excess is eliminated and not stored in the tissue. Nevertheless, avoid too much zinc over an extremely long-period, as chronic oversupply may cause damage to the liver.  
Interactions with other trace elements must also be considered in connection with excess zinc. High zinc levels can have an effect on copper utilisation. Special care should be taken with pregnant mares, as here an oversupply can cause increased storage of zinc in the foal’s liver. There are also indications of a possible connection between developmental orthopaedic diseases/osteochondrosis and a chronic oversupply of zinc in the development of foals and young horses. 

Interested in learning more about other micronutrients? Then have a look at our article The world of micro-nutrients | agrobs.de

Janina Beule, M.Sc. Pferdewissenschaften
September 2023, © AGROBS GmbH

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