Wet-Dry Cycles - Effect on the Hoof

Winter is coming and with it comes the rain and mud and boggy fields. The reality is in the wild horses would never choose to stand in saturated ground, they would roam and find more suitable grazing areas. As we enter these wetter months what should we know about the effects of the wet and often wet-dry cycles, on our horses feet?

To answer this we need to have a basic understanding of the composition of the hoof wall and then explore its response to hydration.

Fig.1 The composition of the hoof wall as it grows from the coronary papillae. Fig. 1 shows how the cells proliferate from the germinative layer, forming tubules in a bed of undefined structure or rather a series of (crystalline) tubules within an (amorphous) matrix.

The hoof is made from keratin cells that go through a cycle of programmed cell death, called apoptosis. This means that the external hoof wall is essentially dead tissue and as such has no ability to heal or regulate itself. It therefore becomes a product of its environment.

Under normal conditions the stiffness and hardness of the tubular areas are greater than in the inter-tubular areas (fig.1). This indicates that the tubules serve as reinforced structures to support the compressive load created between the weight of the horse and the ground.

However, studies have shown us that the mechanical properties of the hoof are directly affected by the moisture content. A certain amount of moisture is documented as being important for the flexibility of the hoof, keratin materials are brittle without water molecules to plasticize the material. However, excessive soaking has been said to cause the hoof to become too flexible affecting its load bearing capabilities and possibly leading to increased plastic (permanent) deformation over the hoofs natural elastic (temporary) deformation under load.

It is the reinforcing mechanism of the tubular horn that has been shown to be affected with over-saturation of the hoof. Studies have shown us that both the tensile strength and hardness in the tubular and inter-tubular areas decreased significantly after full hydration. Even more interestingly and perhaps relevant is that the stiffness and hardness of tubules become less than in the inter-tubular areas, which is due to the higher water absorption in the tubular areas, this means these structures lose their reinforcing characteristic. A study showed that in a fully saturated hoof sample, the tensile strength and hardness decreased by as much as 98%. In reality a hoof would probably not reach that level of saturation but it illustrates possible implications of leaving our horses out in boggy fields.

So what does this actually mean?

In essence the hoof does become softer and less able to support the weight of the animal above it, it is more likely to collapse/flare. Soft, shod hooves that are taken from a wet environment and hacked down the road may have their shoes become loose quicker, clenches may rise and the hoof may expand over the shoe. The sole may loose concavity and the foot become flatter due to its reduced strength and new more plastic deformation.

Often you hear the phrase “The shoe got sucked off in the mud” in my opinion this is not physically possible in a shoe fixture that hasn’t already been compromised, what may have happened is the hoof has become weakened, loosening the shoe and the mud has finished it off as the hoof/shoe attachment can no longer take the torque associated with every step. Even more likely the front foot got “held up” by the suction and didn’t get away quick enough for the hind to come down on top of it and remove the shoe.

Do these findings mean we should bring them in and out from the wet?

Well it would seem logical but anecdotally this wet-dry cycle predisposes the hooves to cracks, Ilka Wagner, DVM, MAgr, BS, of Equine Veterinary Services in Hearne, Texas, states that “The hoof expands with water and shrinks when it dries; that’s when you see a lot of the wall cracks. You want to keep a nice, even level of moisture in the feet.” The expansion and contraction of the hoof that occurs, even between the structures on a microscopic level mean the bonds are disrupted and more easily broken.

Winter 2019 was a particularly wet winter, the fifth wettest ever according to the Met office. That winter I had some unusual cases of lameness where the frogs became soft and almost wrinkled.

Fig.2 Sloughed Frog, winter 2019.

A study by Dr McConnico into flood injuries in the horse found sloughed frogs. Sloughing is the act of casting off dead tissue, as we stated before, the hoof wall is made from dead keratin cells and so is the frog, so prolonged water immersion causes the frog to slowly loose its layers. In Fig.2 you can see how the frog lost its layers until the frog was sensitive to the touch. As well as the effects on the hoof wall, we also have potentially damage to the frog. This of course opens the frog to the risk of thrush infections.

The structures of the hoof have different moisture contents.

Fig. 3 Illustration of the regions of the hoof wall and the moisture contents.

This illustration shows us the moisture contents of the different regions of the hoof as ideals. Having outlined that the hoof becomes softer with hydration and that wet-dry cycling can lead to cracks we can establish that hoof horn is at its strongest and most wear resistant when it has a stabilised moisture content of around 25%, we can extrapolate to the other structures being at their optimal functionality at their own prospective hydration levels.

Fig. 4 The effects of the wet dry cycle.

An important word in that sentence is stabilised, quick changes in hydration levels of the hoof will cause disturbance of the amorphous matrix surrounding the tubules (Fig.4), but as we have seen, prolonged water submersion will have detrimental effects on the composition of all the keratin structures of the hoof.

Fig. 5 The disruption of the hoof wall matrix allows for bacterial infiltration.

As the foot becomes softer with high hydration levels, and wet dry cycles can affect the cohesion of the layers and horn tubules and inter-tubula horn, we often see increased "flaking" of the hoof wall, made worse with the influx of bacteria that are now able to penetrate the compromised hoof wall (Fig.5) predisposing to white line disease. This is especially the case if the temperature remains mild. Bearing this in mind calls for stricter cleaning regimes and anti-bacterial products can be usefull for stemming the spread of bacterial infections.

In an ideal world, through wet spells, and especially when the ground becomes saturated, alternative turn out options would be indicated. This of course is not practical in a high percentage of the horse owning population, nether the less, the issue should be considered and planned for as autumn approaches.

Something to also take into consideration is that these effects will not be the same for every horse, just like with many other issues affecting the hooves, the horses individual hoof composition and conformation will affect how much it is affected by the issues we have discussed. For instance a thoroughbred with inherently weaker structures, less tensile strength and hardness to its hoof may experience more profound effects then a much stronger footed animal.

In conclusion, the wet weather that we are about to experience will have consequences to your horses’ feet, how badly will depend on the intensity of the conditions and the individuals genetics. Interventions that may seem logical, like bringing them in onto a dry bed and quickly drying their feet may actually be contraindicated in favour of keeping their feet at a more constant state of hydration. There are products out there that can help shield the feet and help repair them after the effects of hydration cycles, but what would be ideal, although I appreciate not always possible, is that the horses have the ability to remove themselves from boggy ground or are kept on a surface with good drainage just as they would naturally do.

References

A. Kitchener, J.F. Vincent Composite theory and the effect of water on the stiffness of horn keratin

J. Mater. Sci., 22 (1987), pp. 1385-1389

Glenn D. Ramsey, Peter J. Hunter, Martyn P. Nash,

The influence of tissue hydration on equine hoof capsule deformation and energy storage assessed using finite element methods, Biosystems Engineering, Volume 111, Issue 2, 2012, Pages 175-185, ISSN 1537-5110,

https://doi.org/10.1016/j.biosystemseng.2011.11.010.

J.E. Bertram, J.M. Gosline Functional design of horse hoof keratin: the modulation of mechanical properties through hydration effects

Journal of Experimental Biology, 130 (1) (1987), pp. 121-136

F. Lambert The role of moisture in the physiology of the hoof of the harness horse

Veterinary Medicine Small Animal Clinician, 61 (1966), pp. 342-347

Rebecca S. McConnico, Flood Injury in Horses, Veterinary Clinics of North America: Equine Practice, Volume 23, Issue 1, 2007, Pages 1-17, ISSN 0749-0739,

https://doi.org/10.1016/j.cveq.2006.11.002.

Wei Huang, Nicholas A. Yaraghi, Wen Yang, Alexis Velazquez-Olivera, Zezhou Li, Robert O. Ritchie, David Kisailus, Susan M. Stover, Joanna McKittrick, 2019, A natural energy absorbent polymer composite: The equine hoof wall, Acta Biomaterialia, Volume 90, Pages 267-277,

M.A. Kasapi, J.M. Gosline, Micromechanics of the equine hoof wall: optimizing crack control and material stiffness through modulation of the properties of keratin, J. Exp. Biol. 202 (1999) 377–391.