Connie Larson received a B.S. in animal science and an M.S. degree in animal nutrition at Montana State. While earning a Ph.D. degree in animal alarsonnutrition at New Mexico State University, she investigated the influence of trace mineral supplementation on reproduction, immune response, performance and trace mineral status. Her duties at Zinpro Corporation include conducting equine and beef cattle research trials, and providing technical service.


Introduction
Many questions often arise within the equine industry regarding nutrients that are important for maintaining sound feet and legs in the horse. When we consider the most critical interactions between nutrients and hoof maintenance or growth, we have to keep in mind that the first priority is balanced nutrient delivery to meet the over-all requirement of the horse. Nutrient requirements are based on the mature weight, age and performance or production demands of individual horses. It is necessary to meet all the requirements for energy, protein, vitamins and minerals before we start to focus on nutrients for a specific function such as hoof growth. As we evaluate the diet more closely to determine the interactions between nutrients and growth of the various hoof tissues, we turn our attention to specific amino acids, minerals and vitamins. In addition, it is helpful to start with an understanding of a few hoof function basics and the cornification process that produces the hoof capsule.

 

Hoof Function
The architectural design, combined with the integrated function of external and internal components, contribute towards much of the foot’s strength. Ultimately; however, this strength begins at the cellular level. Flexibility and strength rely on proper synthesis and organization of proteins and lipids lying within and between the cells. Therefore, providing adequate nutrient balance to the cells is critical for building strong functional hooves and lameness prevention. In order to understand the nutritional aspect of lameness prevention, a brief overview of function is presented.

Function overview
The foot has three major functions: 1) biomechanical, 2) growth and hoof wall regeneration, and 3) react and adapt to the external environment. The hardness and insensitivity of the hoof’s exterior surface are required for biomechanical and protective roles, while the internal components are required for its ability to reproduce and adapt to the environment. From a mechanical standpoint, the foot must sense the ground surface, accept the force, absorb and dissipate a portion of the load, change the direction of the loads within the foot up through the skeleton. Friction against the ground during motion continually erodes away the hoof wall and sole. Growth is required to regenerate the wall and sole due to normal wear. Growth mechanisms also apply to the foot’s ability to remodel in respond to stress, or healing following injury or disease.

Hoof wall growth
Hoof wall growth requires epidermal cell differentiation to form the visible hard outer layer. Therefore, growth begins inside the hoof capsule along the dermis tissue that connects the bone to the hoof capsule. Cells in the hoof capsule begin as living viable cells that undergo a differentiation process referred to as cornification. The end product result is fully cornified cells responsible for the physical characteristics we observe on the outer hoof surface. Changes in the hoof wall appearance are often a signal that a nutritional deficiency or imbalance is present in the diet.

The differentiation process occurs in a specific, highly ordered and tightly regulated pattern. It can be described as five major events: 1) formation of the keratin cytoskeleton, 2) keratin protein synthesis, 3) synthesis of the membrane coating material, 4) cell envelope synthesis, and 5) terminal cornificiation. The first two steps involve keratin, a family of structural proteins that are a major component of skin, hair and hooves. Synthesis of these proteins in the cells followed by assembly into cell structures provides the functional characteristic of skin, hair and hoof wall as barriers against the environment. This requires that they are tough and insoluble, yet pliable.

During the third and fourth events of the differentiation process, lipids and proteins are being synthesized and then deposited outside of the cells to form the membrane coating material and cell envelope. These processes are providing the mechanism for adhering cells together, increasing strength of the tissue without severely limiting flexibility and forming a permeable barrier that will limit water movement into the cells. The final event, terminal cornification, results in cell death. The cells have essentially transformed from metabolically active cells to rigid protein-filled nonviable cells that are tightly adhered together. Due to the organization of the proteins and lipids, the end product is practically an indestructible barrier that protects the foot in numerous environmental conditions.

Nutrient Balance Is Key Concept
To achieve optimum hoof growth and function, a supply of balanced nutrients is critical. Overfeeding a single nutrient or combination of nutrients can be as detrimental as underfeeding. Sound nutrition decisions require consideration of nutrient requirements in relation to the feeds and the horse’s work load or level of production. When dietary changes are made to affect hoof wall integrity, ample time must be allowed for improvement to occur.

The hoof wall grows at a rate of 0.25 to 0.40 inches per month for an adult horse. The time required to replace the entire hoof wall from the coronary band down to the toe is nine to twelve months. Hoof wall replacement at the quarter requires six to eight months and for the heels it takes four to five months. In addition to nutritional factors, genetics, age, season and environment can all influence hoof wall characteristics.

Amino Acids
Amino acids are “building blocks” or individual components of protein. Dietary protein digestion yields amino acids that can be delivered via the circulation to provide for protein synthesis in the newly formed cells that will become the hoof wall, sole and frog. As discussed above, the first step in this process involves the synthesis of keratin proteins. These proteins in the hoof will contribute to both the strength and flexibility required for hoof function. Keratin proteins require sulfur-containing amino acids such as methionine and cysteine. The sulfur is a key component required for the formation of bridges that occur between the proteins to form a mesh-like structure. The structure of these proteins is a major factor contributing to the physical capabilities of the hoof.

Lysine, methionine and threonine are three amino acids that equine nutritionist take into consideration when formulating complete diets. Having the correct balance among the amino acids and then providing sufficient protein levels for the horse’s requirements is important. When too much protein is fed to the horse, the metabolic process is to convert that to energy so from a protein perspective it is not beneficial. It is common to find methionine added to commercial hoof supplements. However, there has been a limited amount of research reported to date, and none of the results have shown that amino acid supplementation affects hoof growth rate or hoof quality.

Minerals
Minerals are segregated into two categories, macro-minerals (calcium and phosphorus, for example) that are needed in larger quantities, and micro- or trace minerals that are needed in much smaller amounts. In relation to the foot, minerals are needed as structural components in bone, joints, connective tissue and hoof wall. The most easily recognized mineral role is that of calcium and phosphorus for skeletal structure. Calcium makes up 35% of the bone while phosphorus accounts for 14 to 17%. Feeding excess phosphorus in relation to calcium disrupts calcium absorption and creates skeletal alterations. During the cornification process, calcium provides the signal to initiate the formation of the rigid cell envelope. This occurs during later stages of cornification and is critical for strength in the exterior hoof wall.

The interaction between minerals and hoof growth occur throughout the entire growth process, starting with the proliferation of new cells and continuing until those cells have become fully cornified in the hoof wall, sole and frog. Among the trace minerals, those that are most critical for hoof growth are zinc, copper and selenium. Zinc is essential for hoof growth and is required for more metabolic functions during hoof growth than any other trace mineral. In regards to minerals, it could easily be considered as the most critical.

Zinc is instrumental in over 300 enzyme systems and has three key functions in the cornification process in the hoof: 1) required to drive the reactions forward, 2) serves as a structural role in some of the proteins, and 3) helps to regulate the processes. The role of zinc in hoof growth includes the following: cell proliferation, protein synthesis (keratin and cell envelope proteins) within the cells, and protecting the cell membranes from damage. Essentially, the cells can be equated to bricks used to build a structure. The new cells have to maintain a functional membrane while components are being synthesized inside the cell to provide for strength. In later stages, the cell envelope is formed to provide a hard protective layer around the cell and then cementing substance adheres the cells together. In the hoof wall, zinc concentration is higher than other trace elements. In order to meet hoof growth requirements, it is important that zinc intake and absorption are adequate.

Copper has a very specific role in hoof growth that is instrumental in building the keratin protein bridges. The sulfur in methionine and cysteine serve as a structural component of these bridges. There is a copper-dependent enzyme in the cells that is responsible for building the bridges between proteins. If copper is deficient, then the enzyme activity is decreased and the structural bridging is compromised during growth. Ultimately, the new growth is likely to exhibit cracking and splitting which impacts functionality of the hoof.

A combination of zinc, copper and manganese has been evaluated for their impact on hoof growth in lactating mares. A more available form of these minerals as amino acid complexes were compared to standard inorganic forms in a 25-week study. Changes in sole depth and hoof wall growth were measured in the mares every 5 weeks. The mares fed the complexed minerals had a greater change in sole depth. Increasing sole depth is important for horses with thin soles, laminitis or a need to recover from a sole bruise or ulcer. However, the rate of hoof wall growth was similar among all the mares. In yearling horses, a different study has been reported that the form of the trace minerals was also important for increasing hoof growth the first 120 days of the study. Again, feeding more is not always better, but utilizing a source with improved uptake can impact changes in the hoof.
Selenium is required in very small amounts compared to zinc and copper.  The primary role of selenium in hoof growth is to protect to cell membranes. Consider the brick analogy mentioned earlier, cell membranes are much like the outside of a brick. In order to build a stable structure with the bricks, it would be important to avoid any crumbling edges or gaps in the brick. Selenium actually works in concert with vitamin E as a defense mechanism to protect body tissues from oxidation reactions. Under normal metabolic processes that utilize fat, carbohydrates and protein to provide energy for the horse, reactions occur that produce powerful oxidizing agents. This occurs at the cellular level, and the oxidizing agents referred to as free radicals can damage cell membranes causing a disruption in cell function. Free radicals can interact with the cell membrane lipid layer to form lipid peroxidases. If the peroxidases are formed and enter the cell then cell function is negatively altered. Vitamin E is positioned out on the cell membrane where it blocks free radical attacks on the membrane lipid layer. Selenium, an essential component of the selenium-dependent glutathione peroxidase enzyme system, is situated inside the cell to prevent free radical formation and also destroy lipid peroxidases that may have entered the cell. This anti-oxidant defense mechanism is needed for maintaining tissue integrity in the foot.

The cornification process provides for hoof capsule growth and regeneration. During this process, the differentiating cells have a high metabolic activity due to synthesis of several protein and lipids. These cells rely on adequate supplies of both vitamin E and selenium to provide protection from free radicals and optimize functional characteristics of the fully cornified cells in the hoof wall, sole and frog.
 
While selenium contributes toward hoof integrity, it can also create disruption in normal hoof wall growth when horses consume higher levels over a period of time. Evidence of chronic selenium toxicity is first observed in the hooves as well as mane and tail hair. When horses absorb excess selenium, the selenium will disrupt normal keratin protein synthesis during the cornification process. Many of these proteins have high levels of sulfur due to sulfur containing amino acids. Cross-linking, or the construction of disulfide bridges occur in order to build hoof capsule tissues that are strong yet retain some elasticity. Both of these characteristics are critical for biomechanics of the foot. In the presence of excess selenium levels, the selenium begins to replace the sulfur and the cross-linking does not occur. This compromises the structures and weakens the tissue to the point that horizontal cracks and a separation of the hoof wall become evident. Keratin protein structure in the hair is also compromised with excess selenium and so the weakened mane and tail hair will essentially break off

Vitamins.
Vitamins are classified as either fat-soluble or water-soluble due to how the vitamins are absorbed. The three fat-soluble vitamins are A, D, and E. The horse is capable of synthesizing several different vitamins; however, vitamins A and E have to be supplied entirely by the diet. Green growing forages typically provide sufficient levels of vitamins A and E to meet dietary requirements. However, biological activity of these vitamins often decreases due to plant maturity, harvesting or processing, and storage. It is common to provide supplemental vitamin A, D and E.

Vitamin A absorption, activation and mobilization from the liver are dependent on adequate zinc status in the animal. This primary contribution of vitamin A to foot function is maintenance of epithelial tissue. Vitamin D is a component of calcium metabolism and a deficiency can lead to abnormal bone growth, and toxicity causes calcium removal from the bone leaving them weak and brittle. The role of vitamin E as an anti-oxidant working synergisticly with selenium has already been presented above.

Water-soluble vitamins include vitamin C and the B vitamins. The most highly researched nutrient for equine hoof growth is biotin, a B vitamin. The microbial population in the cecum that is located in the hind-gut of the horse, has the capability to synthesize this vitamin. The vitamin is then absorbed and utilized by the horse. For this reason, it is very difficult to create a biotin deficiency.  However, supplemental biotin has been shown to be effective for some horses. Due to the water-soluble characteristic of this vitamin, potential for toxicity is relatively low because excesses can be excreted in the urine.

Specific to hoof wall strength and function, biotin is needed for the extracellular matrix (a lipoprotein intercellular cementing substance) formed in the epidermal cells during a later cornification step. Following release of lipids from the cell, this substance provides cell-to-cell adhesion. In the brick wall analogy, the cementing substance can be equated to the motor that holds the wall firmly in place and prevents the structure from falling apart. If biotin is lacking and the cells are not well adhered to each other, then the hoof wall integrity is diminished. The hoof wall becomes flaky, shelly and sometimes very brittle. This may explain why horses with poor hoof wall quality are more likely to respond to biotin supplementation than those horses with normal hoof wall quality.

In two long-term studies, biotin was found to have a positive influence on hoof wall quality in horses that had brittle hoof walls and chipped hooves. Over the course of the studies, those horses receiving biotin supplementation showed improvements in hoof wall condition after eight to fifteen months of supplementation. The changes in hoof condition then remained constant over the remaining observation period, which was as long as three years in one study. Both studies concluded that biotin supplementation should be continuous in horses with severe hoof alterations. However, there was no difference observed in either study for hoof growth rate between the horses receiving no biotin compared to those receiving biotin supplementation.  In contrast, there has been one study in ponies that reported a higher hoof growth rate when very high levels of biotin were fed for five months. The study also reported that the hoof growth was much lower for the older ponies.  The most consistent response to biotin appears to be for improving hoof quality and not necessarily always changing growth rate.

Summary
It is evident that specific nutrients have been identified as critical for the hoof growth processes. Providing adequate balanced levels of these nutrients can influence hoof growth for maintenance, repair and prevention of future hoof problems. However, remember that the key is balanced intake. Feeding more is not always in the best interest of the horse. Feeding excessive amounts of specific amino acids disrupts the total amino acid balance. High selenium levels can actually be more harmful than beneficial for the hoof.  It has also been shown that we need to keep the zinc to copper ratio in the total diet within a 3:1 up to 5:1 range for optimum absorption and utilization. Since biotin is a water-soluble vitamin, excesses are not necessarily harmful, as the horse is very efficient at excreting excess biotin through the urine. There are reputable well-balanced complete feeds and supplements on the markets that provide adequate levels of these nutrients. Being informed on the nutrient function as it relates to hoof function can help you make sound feeding decisions.