by Barbara S. Veritas
There has been much discussion among equine nutritionists regarding the need for increased methionine in the diet of horses suffering from a variety of hoof growth problems. A study of the protein structure of the hoof wall sheds light on why this amino acid has received so much attention.
The hoof wall is made primarily of keratin, a protein also found in hair, nails and feathers. Keratin is an unusually stable protein; it is insoluble in either hot or cold water, and it is not broken down by proteolytic enzymes, as are most proteins. The durability of keratin results from the numerous disulfide bonds of the amino acid cystine. In fact, cystine may account for as much as 24% of the total amino acids in keratin. Cystine (the oxidized form of cysteine) is able to contribute the strengthening disulfide bonds to the structure of keratin because it is a sulfur-bearing amino acid. There are two other sulfur-bearing amino acids, taurine and methionine. While taurine is not a factor in keratin formation, methionine is a very key player.
Unlike cystine, methionine is an essential amino acid. Eight of the twenty-two "common" amino acids are essential; they must be in the horse's diet. No matter what else is present, the body cannot make them on its own and they must be fed. Methionine itself is present in keratin only in small amounts, but it is absolutely necessary for the production of cystine. Through a process called transamination, the horse's body converts dietary methionine into the cystine needed to produce many important proteins, including: muscle tissue, hemoglobin, glutathione peroxidase (the crucial selenium-containing enzyme that protects against cell damage), and keratin. Vitamin B6, the "protein potentiator", is a key factor in this process.
While plant foods contain much more methionine than cystine, they are still considered relatively poor sources of this essential amino acid. Methionine is often cited as the second limiting amino acid in horse diets, after lysine. So when farriers and veterinarians see cracking, crumbly, poor growing hoof walls, they now routinely recommend substantial increases in dietary methionine. They know that, in conjunction with other cofactors, it will work to get cystine production into high gear.
Of all the building blocks for protein, lysine is most often the key to improving protein availability, especially in grass hay based diets. Lysine is another of the eight essential amino acids. Whenever protein synthesis is needed without the problems of excess crude protein, additional lysine is an excellent solution.
A review of the recommendations of the National Research Council, as found in Nutrient Requirements of Horses, Fifth Edition, shows how often lysine deficiency can be a problem. Based on several studies, the NRC states that a yearling at moderate growth levels would have a daily lysine requirement of 36 grams. But, based on their average crude protein delivery of lysine (about 3.5%, according to NRC) in the "typical" diet at the recommended level of digestible energy, the horse would be receiving only 27.7 grams of lysine - a shortfall of 8.3 grams daily.
For two-year olds the picture is no better. The NRC lysine requirement for 2 year-olds in training is 45 grams daily. Based again on the recommended level of digestible energy, the horse would get only 36.8 grams of lysine from the "typical" diet, a shortfall of 8.2 grams daily.
Now, we all know there's no such thing as the average diet. But the implication is clear that it's hard to deliver enough lysine, especially for growing horses and those in high-demand activities (e.g., where muscle, blood, hoof building should be occurring at accelerated rates). Dosing horses with steroids to build muscle mass may be missing the point when i'ts likely the horses aren't able to use as much as 50% of the crude protein they're being fed. Lysine is the FIRST key to unlocking protein efficiency.
Zinc, in general, is involved with growth rates and healing. Zinc deficiencies retard the synthesis of DNA, RNA and protein, which are necessary for proper hoof condition and repair. Zinc deficiencies also impair cellular division and growth and repair of connective tissue.
In healthy animals, zinc deficiency is related to depressed utilization of amino acids and sulfur. It is critical to maintain a proper ratio (from 3:1 to 5:1) of zinc to copper in order to maintain optimum mineral absorption of both minerals and in order to prevent developmental orthopedic disease (per the Ohio State study with Thoroughbred foals, and subsequent research). Zinc deficiencies may be caused by excess calcium since calcium displaces zinc, e.g. straight alfalfa hay diets without proper grain and/or mineral supplementation. Zinc deficiencies can also cause restriction of blood flow, poor wound healing and susceptibility to infection.
Sufficient dietary zinc will speed wound healing as much as threefold, increase blood volume in areas where blood vessels are constricted and act as an anti-oxidant to help protect against heavy metal poisoning. Supplemental zinc and copper in the proper ratio can substantially improve the body's ability to grow and heal hoof tissues.