Pros / Cons / Discussion
W.J.Kirkpatrick – General Manager, Sound Horse Technologies
6 September 2013
This paper is a brief discussion regarding the horse (currently found on every continent except Antarctica), the hoof and some shoeing products. It is not an in-depth research or reference paper nor is it intended to advocate a particular method of shoeing. It may, however, provide some useful basic information and raise some questions while suggesting some answers. For more information see: http://en.wikipedia.org/wiki/Horseshoe
The horse is an interesting herd animal with a long, recorded history of service to humans and an equally interesting history of evolutionary and breed development. As a herd animal, it is designed by nature to graze on a semi-continual basis and travel long distances in the search for nourishment [see Drs. Chris Pollitt & Brian Hampson re: The Australian Brumby]. While the horse is capable of running and jumping, the “design parameters” seem to support a contention that these capabilities are primarily to support the seeking of nourishment and to escape predators. The interaction of the horse with humans (domestication) has resulted in quite a bit of selective breeding to support specific tasks requiring specialization of strength, endurance, speed or a combination of these and other attributes.
Least well understood is the complete suspensory system that supports the horse. This remarkably durable animal ranges in mature weight from 800 pound ponies to a 2500 pound draft horses. The suspensory system (if we want to call it that) supports the weight of the horse as it stands, trots or gallops at up to 32-37 MPH (Thoroughbred) with each hoof hitting the ground every 0.4 seconds with a force of about 200 “g”. It is a wonderful design that utilizes tendons, ligaments, muscles & the hoof capsule to accelerate, drive and brake the animal, support the weight on an articulating limb AND, finally, absorb the high shock loads to which the hoof & leg is subjected while working, running and jumping.
The hoof is only one component of the suspensory system but it is the piece that is in contact with the ground. As a result, this organic hoof structure takes a pretty severe beating from the impact during each stride. The climate, soil, footing and impact conditions are a big part of the hoof wear as well as the work the horse is asked to do. The hoof is a complex, living structure that is designed to operate under high unit load and, frequently, high concussion conditions during a wide variety of sport or work challenges. The wear component – the hoof wall – is not a uniform material. It is an organic insensitive tissue of varying density with denser/dryer outer layers and less dense and more heavily hydrated inner layers close to the sensitive, supportive lamina tissue. This is interesting as the insensitive hoof wall, only ¼- 5/8 inch thick, is called upon to not only support the horse’s weight but to also hold the mechanical fasteners (nails) that may be used to retain horseshoes applied to protect the hoof or improve traction..
Early attempts to reduce the wear and damage to the hoof as a way to keep the animal in active / useful work utilized various hoof coverings made from an array of materials. The Romans are known to have used a leather covering, a “hipposandel”, that was more like today’s “hoof boot” than a horseshoe. The metal horseshoe emerged as a significant innovation in this continuing effort to protect the hoof. The application of metal horseshoes may have begun more than 2000 years ago with bronze shoes and nails found in tombs dating back to ~ 400 BC. The earliest written records of iron shoes of a “crescent shape” date back to ~ 1000 AD and by the 13th century it appears that metal shoes were available in quantity and were in reasonably wide use.
NAILS: The most common attachment for horseshoes was (and still is) a nail – a mechanical device. The nail provides a cost effective, generally reliable attachment that has changed little over 10+ centuries. While the metallurgy and design for nails has certainly changed, they still work by penetrating the hoof wall and relying on the integrity of this organic material (the hoof wall) to hold the shoe during work the horse is asked to perform. The problem with nails is that a hoof wall may be compromised – broken, cracked, shelly, thin or brittle – and may not have enough structural strength to hold the nail(s) for the entire shoeing interval. Additionally, when a shoe pulls (for a myriad of reasons) the clinched nail may break out a section of hoof and if the loose shoe stays on, it may cause additional damage. Having said all of that, the traditional method of nailing on horseshoes is remarkably effective for the vast majority of horses. This is a testament to the horse and the farriers who shoe the horse.
ADHESIVES: A relatively recent addition to horseshoe attachment methods is adhesive, aka: “glue”. Many “glues” have been used over the last century but the most common in use today are urethanes and acrylics. These materials were adapted from industry and then pioneered for use in the horse market in the late 1980s and both have seen reasonable acceptance from the farrier industry in the USA. They are not, however, going to displace nails in the foreseeable future as they tend to be used for shoeing or repairing the compromised hoof and require a specialized skill that some farriers may acquire.
Urethanes – this is a chemistry that generally delivers a relatively fast set time (about 30-60 seconds) and similarly fast cure. As with many products, there are pros and cons – the “pro” is speed; the “con” is sensitivity to moisture & surface contamination. Because of this, the farrier needs to be quite diligent in preparation assuring a clean, dry hoof for application of the urethane material. Urethanes are most commonly used for direct glue application of an aluminum shoe to a hoof, for hoof repair and as a “packing” for sole protection.
Acrylics – there are a broad array of acrylic adhesives and the one most commonly used by farriers is a modified acrylic, typically epoxy-methylmethacrylate. Use of these specific, modified acrylics is important as its maximum cure temperature is quite a bit lower than that of straight acrylic adhesives. The benefit of the typical acrylic used by farriers is that it is slower setting than the urethanes and provides some “open” or working time for the farrier – about 2-3 minutes at 70°F. The cure time is about 6-8 minutes, again 70°F, to achieve adequate strength. In addition to a longer “open” or working time, the acrylic adhesive is a more robust system and tends to be less sensitive to surface contamination than the urethane materials. Acrylics used in the equine industry are available in “slow set” and “fast set” and are used in hoof repair work as well as glue-on shoe applications.
GLUE-ON APPLICATIONS: There are primarily two (2) types of glue on application methods – direct glue and fabric-cuff systems (made by Sound Horse Technologies). The following is a very basic discussion of each product system. As with almost all methods & processes used by professionals, these techniques have many nuances and farrier experience frequently makes for significant performance variation (particularly in the direct glue applications). As mentioned previously, glue-on horseshoes are typically only used when there is a hoof issue that precludes the use of nails as an attachment method.
Direct Glue – This is the most widely used “glue-on” technique in the industry today. On first glance it appears to be the easiest as all that is required is glue & a horseshoe….. What could be easier? As with many things, first glances can be deceptive.
The first question is: “What glue do I use?” This is a particularly contentious issue among farriers but it appears that the battle may be going to the acrylics. As mentioned previously, the acrylics are less sensitive to surface contaminants and allow the farrier the time to apply, position & smooth the adhesive. In addition, the farrier will always need to do a bit more work at the heel of the shoes in order to preclude early failures. This work may involve mixing some chopped fiber (fiberglass or another polymeric material) into the adhesive used at the heel to generate higher bond strength.
One of the problems with direct glue METAL horseshoes is that the metal shoe, once glued to the hoof, will not move as the hoof capsule tries to flex & move, particularly at the heel. For comparison, if you look at the heel of any metal shoes that have been nailed onto a hoof, after about 3-5 weeks you will see a distinct dished area where both the heels have continually moved laterally using dust & grit to abrade and wear the metal. This is particularly problematic with some horses when the heel is constrained by the direct glue application method with a metal shoe. The glue & metal secures & immobilizes the heel by fastening it to the shoe in an effort to prevent a hoof to shoe failure (generally starting at the heel and propagating forward). Some horses (perhaps as many as 10+%) develop heel bruises in this type of a direct glue application. There is also a great deal of variability with success based on whether the shoes are steel or aluminum with the preponderance of “successes” being with the aluminum shoes.
NOTE: recent work on direct glue applications utilizing non-metallic (“plastic”) horseshoes with lateral / medial / vertical flexibility seems to minimize this heel bruising problem. This may introduce other problems due to a lack of shoe rigidity and (varying with design) ground surface grip. The farrier needs to be conscious of this in making shoeing decisions.
Fabric Cuffed Systems – The only supplier in this category is Sound Horse Technologies with the patented Series I as well as the Series II & III products. This shoe type was developed and patented at the University of Pennsylvania, School of Veterinary Medicine Large Animal Hospital at New Bolton by the Chief of Farrier Service – Rob Sigafoos, Mary Hazzard and Dr. Bill Moyer (now at Texas A&M). Commercialized more than 15 years ago, the shoe incorporates a fabric cuff that is glued to the outside of the hoof wall using modified acrylic adhesive. The fabric is captured in a ¼-inch thick urethane rim pad that serves several purposes. First, to securely bond the cuff to the metal shoe but provide a controlled release should the horse grab or over-reach thereby preventing a catastrophic hoof wall failure. Second, to reinforce a compromised hoof wall while serving to attach the shoe to the hoof. Third, the ¼-inch thick, urethane rim pad acts as a shock absorber to reduce impact concussion on an already compromised or sore hoof. While this system was originally conceived as a therapeutic shoe, it has proven well able to meet the rigorous performance needs of competitive horses in ALL areas of equine events.
The Sound Horse shoes, as designed with the polymer fabric cuff, are particularly effective as their attachment strength is more than 2x that of eight (8) clinched nails in a perfect hoof. Finally, inasmuch as there is no glue on the bottom of the hoof, the heels are permitted to move laterally & medially on the urethane heel flats designed into the shoe. This “natural hoof movement” coupled with the resilient nature of the urethane rim pad seems to complement the natural suspensory system of the horse. This is supported by the fact that the “black” rim pad material – 3x softer than the standard performance “blue” urethane rim pad – has proven particularly effective in getting profoundly sore horses (laminitic, arthritic, navicular) back to work. The shock attenuating property of the rim pad that minimizes impact concussion is coupled with the fabric cuff to reinforce a compromised hoof wall and appears to be particularly useful in therapeutic shoeing applications.
The Sound Horse Series I shoes are all open-heeled variants available in an array of forged shapes (supplied by The Victory Racing Plate Company in Baltimore, MD) or machined aluminum alloy for the specialty therapeutic shoes (Morrison Roller Motion). Where unique shoes shapes are required, the farrier / vet has the option of using the Series II plate & cuff assembly to build the therapeutic shoe OR they can use the Series III universal cuff for attachment to any shoe, any size, any shape, any material. Using these products, the farrier / vet is able to build the right therapeutic shoe for the horse and apply it with the confidence that it will stay attached for the entire shoeing interval regardless of the condition of the hoof.
The horse is generally willing to do the work that the owner/rider/trainer asks it to do. The demands on the horse, however, are quite a bit different from that which nature may have originally intended. While the horse can run and jump, we find that the rigor of work, training and competition are such that the hoof is subjected to concussion, impact & wear that are not what the horse would normally encounter in its natural environment. The farrier and the horseshoe is civilization’s response to the problems created for the horse in regular work. Recent innovations in shoes, materials and attachment methods are all efforts to help the horse better perform the required tasks while minimizing damage to the hoof.