More on the caudal hoof - and the digital cushion

Everything about the way the hoof and the limb is structured is about dissipating ground reaction forces, ensuring traction, optimizing circulation and conserving energy.

The evolutionary advantages of the tough but lightweight single hoof are offset by the facts that:

• it has to fulfil all the loading and balance functions of a multi toed foot; 
• there is no contractile muscle tissue below the knee to buffer the bones and to aid venous return; 
• the joints of the digit are at a considerable distance from the muscles which act across them. 

To understand why the hoof and its limb have to function in a certain way, we have to understand the suspensory apparatus of the lower limb and its role in the stay apparatus; to understand that we need to understand the anatomy of the whole limb, and to properly understand that, we need some understanding of the anatomy and physiology of the whole horse.

It has become a barefoot mantra that the 'horse lands heel first', and as a result, the health of the soft horn/soft tissue structures of the heel is paramount. I agree with that but the heel first landing is important not just because the back part of the hoof is more flexible, it is also important because of bone alignment at the point the heel comes into land, which is important because of blood flow and muscle health.

At rest, the cannon should be vertical to the ground, and the pastern bones and pedal bone are offset to the vertical axis of the limb, ie the fetlock joint  is held in permanent extension and is able to achieve hyperextension by its suspensory apparatus. 

The tendinous interosseous muscles or suspensory ligaments  are strong enough to passively hold the fetlock joint in permanent extension, but elastic enough to allow it to hyperextend under load. At rest, the joint should be about 25% extended; at full load, it can extend to 180 degrees and even beyond.

The SLs are backed up by the digital flexor tendons, both of which have a check ligament that come into play to protect the contractile tissue of the muscles of the deep and superficial flexors tendons.

The deep digital flexor tendon (DDFT) is massive and has a fan shaped insertion point in the back  of the pedal bone. It is so over-dimensionalised not because it is needed to flex the lightweight pedal bone, or even because of its distance from its muscle, but because of its role in backing up the suspensory apparatus of the fetlock joint.

The coffin joint, between P2 and 3, has its own suspensory apparatus which gives the joint a greater mobility than the pastern joint (P1 and 2) but much less than the fetlock joint. Like the fetlock joint, the coffin joint has a sesamoid bone (navicular) which extends the bearing surface of the joint.

Both joints have an optimal alignment at rest, deviations from which can create a myriad of problems.

The hoof comes into land as the limb is being retracted, at which point the coffin joint should be fully extended with the digital arterial pathways fully open.

As the hoof engages the ground:

• the heels expand laterally and deform vertically; 
• the suspensory apparatus of the fetlock allows it to descend; 
• the suspensory apparatus of the coffin joint allows the two pastern bones to also descend, as a unit, with the fetlock;
• as the pastern bones descend, the angle of the back of the coffin joint begins to close and the digital arteries are compressed between bone and DDFT until, at full pastern extension, the blood flow into the hoof is completely closed off;
• after body weight has passed over the limb, the coffin joint is flexed and the limb is lifted off the ground;
• as the limb is extended, arterial flow is resumed.

Maximum fetlock extension is reached as the horse’s body weight passes over the vertical axis of the limb at which point the hoof should be fully and evenly planted with load dispersed across its volar surface and as a consequence, the whole bearing surface of the coffin joint. The extensor branches of the SLs are fully tensioned to stablise the coffin joint.

The venous plexuses are expressed by the reversible deformation of the hoof capsule and the mechanical regulation of arterial flow when the heart is pumping hard is a vital part of balancing hydraulic pressure in the hoof. 

Problems occur when the persistent alignment of the coffin joint mimics the degree of pastern extension that occurs at high impact. This can happen with a grossly steepened pedal bone where the palmar/plantar processes are elevated, and in a ground parallel pedal bone where the hoof-pastern angle is severely broken forward.

The role of the Digital Cushion

As the pastern bones descend, the digital cushion (DC) is engaged. This is a structure that is both misnamed and misunderstood.

It is widely argued that the DC aids shock absorption and also contributes to venous return by helping to mechanically express the palmar venous plexus.

Strasser argues that the DC’s effectiveness, both as a shock-absorbing cushion, and in transmitting ground reaction forces to the lateral cartilages, is negated by its ability to displace caudally because there is only thin skin at the back of the pastern above the heel bulbs. Because of its location and its structure, she argues that it is more akin to a sling, the role of which is to assist in arresting the downward pressure of the pastern bones under load and with the bars, to assist in controlling the lateral expansion of the heels.

The DC is made up of horizontally arranged bands of fibrocartilage (FC) that  attach to the lateral cartilages and run above the frog corium and below the distal fibrous sheath of the deep digital flexor tendon (DDFT). There are tubular glands over the spine of the frog, ducts from which pass through the frog dermis to the frog surface. Strasser argues these are vital to frog health as they affect the pH of the pad and help make it resistant to pathogens.

In between the fibrocartilaginous bands is loose connective tissue (LCT).  The proportions of FC to LCT varies between hooves and Bowker argues that lots of fibrocartilage is evidence of a healthy hoof. He also argues that in a healthy hoof, the lateral cartilages are thicker and denser. it seems logical that this would be the case but there are many other factors at play.

Fibrocartilage is a specialist type of cartilage that contains Type 1 collagen as well as the normal Type 2 which is the main component of cartilage.  Type 1 is found in skin, tendon, vascular ligature, organs and is the main component of the organic part of bone.

The fibrous element in fibrocartilage confers flexibility and toughness, and the cartilaginous element confers elasticity.

The proportions of each element in any given digital cushion may vary according to such factors as age, breed and importantly, the terrain the horse lives on and how much movement it gets. The relative amounts of the two types of collagen may vary as well.

Cartilage has no blood vessels or nerves of its own. The cells that secrete the matrix of cartilage and become embedded in it obtain all their nourishment from blood vessels in the perichondrium (the connective tissue that envelops cartilage where it is not at a joint) and surrounding connective tissues – and it does so by diffusion of oxygen and nutrients through the matrix. It is because of this lack of a direct blood supply that cartilage heals so poorly and the lack of nerves is why a weak DC in itself will not cause discomfort.

With age, normal cartilage may calcify which inhibits the diffusion of nutrients through the matrix and may result in degeneration of the cartilage. I do not know if this happens to the cartilaginous components of the DC.

For diffusion to occur, cartilage (and tendon & ligament) need movement - preferably under load.The movement the DC gets is from the direct expansion and contraction of the heels, and to a lesser extent, from the upward pressure of the ground on the frog pad.

The horse evolved from a multi toed ancestor and it contains remnants of those toes in the splint bones, the ergots and the suspensory ligaments which, as their anatomical name indicates, are not true ligament. It is suggested that the SLs of certain breeds (eg standardbreds) contain a higher proportion of muscle fibres than other breeds; perhaps the same is true of the relative amounts of the different types of tissue and/or collagen in the digital cushion.

The truncated cone shape of the hoof, the attachment of the caudal (rear) part of the hoof to cartilage, the inward turning of the walls, and twin folds of the frog in its rearward third  - all enable the hoof to reversibly deform (expand) laterally under load, to compress, and for the heels to displace vertically.

The hoof has to have a balance between the load bearing properties of a vertical structure and the load spreading properties of a diverging one. 

Too vertical a cone concentrates ground reaction forces on a smaller area and is less capable of dissipating energy, ie it is more prone to concussive damage both of hoof and the structures it protects.

A too diverging cone will spread load over a wider area but be incapable of retaining vertical integrity and will expose the sole and pedal bone to the effects of both concussive and persistent loading.

In either extreme, the soft horn/soft tissue structures of the caudal hoof are permanently deformed, incapable of the desirable degree of reversible deformation that is the basis of healthy hoof mechanism.

In the narrow hoof form which cannot deform sufficiently under load, and especially if combined with a steepened bone alignment: 

• blood flow will be compromised; 
• the DC in particular will not get the movement it needs for diffusion to occur and atrophies; 
• the bars will deform and very likely become high pushed, compressing veins and arteries and causing discomfort; 
• the LCs will be deformed by the inward turning heels; 
• the laminar corium in the heels will be under a state of persistent pressure; 
• the frog corium will be starved of nutrients and the glands that maintain its protective acidic pH will not be able to function; 
• shock absorption will be reduced with consequent effects on the joints; 
• a steepened bone alignment will throw out the balance necessary for the optimal operation of the stay apparatus and may impair arterial flow to the frontal regions of the hoof in particular; 
• the abnormal angle of the pedal bone to the ground, combined with the mechanical effects of high heels or short toe, will force the horse onto its forehand, overloading the toe reducing toe growth, stressing the solar corium, allowing greater heel growth ......

In the splayed hoof form:

• vertical integrity cannot be maintained; 
• the heels may expand too much under load; 
• the DC will be unable to get the stimulation it needs for nutrition and, as a result, will atrophy; 
• the pastern bones have nothing to dampen their downward movement; 
• the frog will be exposed to too much direct pressure; 
• the back of the coffin joint will be exposed to too much movement overstressing the suspensory apparatus; 
• the same applies to the SA of the fetlock joint and as a result the SLs may be damaged;
• the horse may try to  steepen its pastern angle by contracting its flexor muscles and, because that cannot be maintained for any length of time, by changing its shoulder/elbow angles which will impact on the muscles of the upper limb, shoulder, neck and back; 
• the entire volar surface of the hoof will be exposed to too much concussive impact and persistent pressure and of course bone yields to persistent pressure ......

This is not an exhaustive list of the worst case scenarios but it gives a flavour of the sorts of things that can - and do - occur. 

Some horses have one hoof of each kind; some have steep high heeled hooves on their load bearing front limbs and shallow low heeled long toed hooves on their hinds. 

What we must all agree on is that every part of the hoof has a role, and all are important.  The role of some parts of the hoof and the ill effects of them not functioning optimally are immediate and obvious; that of other parts may be less obvious and more gradual and subtle. 

The bottom line is  :

• the hoof is a unit ;
• the whole hoof is more than a sum of its parts, and
• the whole hoof is never truly healthy if any of its parts is unhealthy, and
• the whole body is never truly healthy if the hooves are unhealthy.

The Natural Horse

Horses are a subject that most non-horsey people find very boring and horsey people find endlessly fascinating. One area that always results in the temperature in the equestrian world being raised to boiling point is the natural versus unnatural argument.

The moment we domesticate an animal we change its natural way of being. It is obvious, to all but the irredeemably stupid, that, if we change an animal's natural way of being too much we will cause it harm. There has to be a balance between what the animal needs to be healthy in body and mind and what we want it to do for us. 

I have heard people, who seek to legitimate highly unnatural ways of managing animals, argue along the lines that, as very few people can achieve the 'natural' ideal of running horses in a herd on 100 acres of wilderness, then keeping them isolated in a stable is ok. An equivalent would be to argue that, as if you cannot keep pregnant sows running in a forest, it's ok to keep them in crates. 

We subject the horse to extremes, we ask it to run further, faster, jump higher than it would ever choose to do in nature. The horse is not stupid, given the choice, it will conserve energy and avoid dangerous obstacles and it will stick to known pathways if possible. In so doing, it obeys the evolutionary imperatives of conserving energy and staying with its herd. 

We want to keep horses in stables or small paddocks because it is convenient to us. These conditions mean their feet cannot become or remain as tough and well conditioned as they need to be to cope with what we want them to do and are prone to being worn or contused. But we want them to jump massive tracks on potentially slippery or hard ground - at our convenience, and, for the safety of the rider and animal welfare considerations we don't want horses falling and breaking a leg or neck too often, so we shoe them to 'protect' the hoof from wear and bruising, and add studs to improve traction. We do this despite the fact that shoeing - even the very best shoeing - invariably causes some damage to the hoof. The logic behind shoeing is that any harm the shoe does is offset by the harm it prevents.

Why not breed for good feet, condition the feet from birth and keep the use of the horse within the limits imposed by its biology?  

We know that at liberty a horse adopts display posture only briefly and can alter its posture the moment it needs to. We know it needs an extended neck and a dryish, closed mouth to be able to breathe efficiently. In advanced dressage, under conditions at which the physical demands on the horse are pretty extreme, tradition insists on an 'outline' that inhibits respiration at a point when the degree of musculo-skeletal stress demands respiration be at its most efficient. And, it insists on the use of a certain type of equipment - a double bridle - to achieve and maintain that outline, which also reduces the efficiency of respiration. 

Show me the advanced dressage horse that competes with the dryish, fully closed mouth of the horse exercising at liberty and I'll show you a hundred that are slobbering wildly and have their mouths open. The simple fact that people think the slobbering is desirable demonstrates a lamentable lack of understanding of the competing physiological processes involved in eating and exercising.

It is insane. And it comes from an era when the prancing, 'prettily curvetting', horse was a status symbol of the rich and powerful. You have only to look at most statues of kings, princes and military leaders astride horses and invariably the horse's mouth is open, its jaw is twisted and its eyes are wild - a picture of a horse in pain, or in fear of being in pain. To the educated eye it is ugly. It's like the terrified grimace of a young chimpanzee that is presented to a gullible human audience as a 'smile'. 

We subject the horse to a grossly unnatural lifestyle. We may have selectively bred for size, performance, athletic ability etc but their fundamental species needs remain unaltered – as do our own. 

I make no apology for the following. It may sound emotional – it isn’t. I love horses but this is a fairly dispassionate list of what the horse's species needs are versus what we do it – to meet our needs, or what we are led to believe should be our needs.

What follows is not an exhaustive list although you may feel exhausted after reading it. 

The horse :

is a highly social and hierarchal herd animal -  it is often isolated and/or exposed to sudden changes in its herd environment;

is a prey animal whose defence is flight and which evolved to move 18 hours a day  – it is often confined to stables and small pens;

needs confidence in its ability to flee from danger and to stay upright –  the way it senses the ground and the way its feet cut into the ground are changed by shoeing it;

is a trickle feeder, grazing head -low 16+ hours a day – it is usually fed to human convenience and needs;  

needs high fibre and varied forage  - it is often fed on sugar rich, too easily available grass and legumes and grain based hard feeds that are often coated with sugar and treated with chemicals, and it is often denied high fibre food to prevent a 'grass belly';

controls its own worm burden by moving away from its droppings – it is often forced it to graze worm infested grass and then dosed with wormers, whether or not it needs them;

needs the pumping action of its feet to aid venous return – the normal expansion and contraction of its feet is often changed by the way we trim (or don’t trim), manage the lifestyle and by nailing or gluing on shoes;

needs to keep its spinal ligaments tensioned to allow locomotor muscles to function optimally, which requires head low posture – we often create a range of conditions in which it stands and moves head high and/or with a hollow back;

needs a balanced skeleton to maintain its stay apparatus – we often force skeletal imbalances on it with resultant soft tissue damage, by the way we balance its feet, shoeing, poor riding, discomfort from bits etc;

is more prone to over heating than cold and has an efficient thermo-regulation system - we often rug and clip it for our convenience and because we project our needs onto it;

needs properly aligned teeth for nipping and grinding – we often affect the wear of its teeth by what we feed it and how we keep it and we have its teeth balanced while it is in a head high posture which changes the alignment of the jaw;

has a highly sensitive mouth and muzzle - we often damage it through the use of bits, chains, straps, tie backs, tie downs etc;

requires a relatively dry, closed mouth, a fully closed gullet and full extension of its neck for ensure efficient respiration - we often force it into flexion which inhibits this, prevent it from fully closing its mouth by placing bits in it, stop it opening its mouth to evade the pain of the bit by strapping its mouth closed or tying its tongue down ......

And, in addition to all that - we often cut off its sensory whiskers, pull the hairs of its mane and tail out by the roots, wash the natural oils out of its coat, spray it with silicon based products to make its coat shine, expose it to toxins from artificial fertilizers, herbicides, pesticides and anthelmintics….

In short, we often create conditions in which the horse is exposed to psychological, digestive, muscular-skeletal, circulatory, metabolic stress - and all too often, when its system breaks down, we then subject it to a whole new load of stressors trying to cure it.

Having these sort of views about animal husbandry in general qualifies me to be called a’ tree-hugging, granola eating kook’ or a ‘rabid PETA whore’  by people with Over-Active Amygdala Syndrome (mostly, it has to be said, horse owning residents of the USA where the condition seems to have reached epidemic proportions).  

I don't expect everyone to do as I do, I just want to encourage people to stop and consider why they do what they do - not just do it because other people do it, or say to do it, or because it's easier, or because it's always been done that way. 

Getting a kick out of a horse

Some time ago there was a front page story in the Christchurch Press about injuries caused by horses  and the cost to the tax payer. The story was headlined  ‘Hellish toll of hooves’ but it failed to mention the main reason for the toll– ie the metal shoe that is so often nailed to the hoof.  

A horseshoe can best be compared to a knuckleduster on a human fist in that it adds weight and rigidity to a comparatively light and flexible structure.  In so doing, it hugely increases the ability of the hoof to inflict major cutting, crushing and cracking injuries to both soft tissue and bone.

More Accident Compensation Commission claims result from riding accidents, than for rugby.  Most involve riders falling from horses but the most expensive are those that require extensive and complex reconstructive surgery.  Being kicked or trampled in the face and head will almost always fall into that category.

A horse’s kick can transfer a force of more than 10 000 Newtons to the body. Medical trauma personnel have likened the destructive potential of an equine kick to that of the impact of a small automobile moving at around 30 kms per hour. A kick can shatter bones and severely traumatize soft tissue externally and internally. Medical journals document people going into cardiac arrest after sustaining a kick to the chest.

A Swiss study of traumas inflicted by horses concluded that the equestrian community may underestimate the risk of severe injuries attributable to hoof kicks, especially while handling the horse.

The horse has evolved a highly specialised foot bone for its single digit. The equine pedal bone and its protective covering of horn has very high tensile strength but is very light. This conserves energy which, for a prey animal whose main defence is flight, is an important consideration.

A metal shoe can more than double the weight of the hoof capsule.  This has major consequences for all the forces that act on the horse hoof and limb – ground reaction force in particular.

The horse without shoes not only has a lighter, flexible hoof at the end of its very powerful hind limbs, it has a better sense of the velocity and therefore the strength of its kick. 

The horse may strike out with its front legs but it is the hind limbs which deliver most force. The hind limbs are the horse's engine – the propulsion unit, and they are also a major defence mechanism.

http://www.youtube.com/watch?v=cCG7HWdRtkQ

The worst kick is from one or both back legs – delivered when the target is right behind the horse and at enough distance that the limbs can reach maximum velocity. Being ‘double barrelled’ by a horse has a special meaning for all horsefolk. You don’t want to have it happen to you and if it does, you want to the horse to be shoeless.

A horse than means to hurt you will do so if it connects. And they can be astonishingly fast and accurate.

http://www.youtube.com/watch?v=XqZ4UUDoZGs

Leaving aside the extreme provocation of having a red hot branding iron applied to its rump, a horse that kicks without warning means to do harm especially if they kick out at head height. Such a horse is either feeling very threatened and wants to get in first, or it is being aggressively dominant.

However, most horses do a lot of threatening before kicking. There’s almost always a lot of horse talk going on that many humans are not aware of, choose to ignore or worse, answer back in a manner that escalates the potential for conflict.

Horses which are kept in highly stressful situations, are in pain or anxious about pain, and / or are badly trained are far more likely to kick without warning. I have never been kicked by one of my own horses yet I quite often trim their feet sitting on a box with the hoof in my lap.  They are barefoot, live in a herd and are well trained and at peace with the world.

Aside from kicking, there are stamping, trampling type injuries – ranging from the very common standing on a toe through the slamming of a hoof onto a foot to being run over by a fast moving horse.  A race horse can gallop at around 50 kms an hour– and it can weigh upwards of 500 kilos. Its kick may feel like being hit by a small car at 30 kms an hour – being trampled must feel like being run over – and over – by a small car.

Dr Hiltrud Strasser, an outspoken advocate of keeping horses barefoot, suggests that, such are the dangers of shod horses, people riding or working with them should have to wear protective head gear. Given the numbers of major facial injuries inflicted by horse’s hooves, perhaps this should be full face helmets. We have laws which require cyclists (motorized and non motorised to wear helmets, why don’t we have a law requiring the same of people who are riding and handling horses which have the equivalent of knuckle dusters attached to their feet?

Just a thought ….

Pigment in Hooves

Cheryl Henderson, a US based barefoot trimmer, has a theory that pigmentation increases as the hoof becomes healthier.  She goes as far as saying that the unpigmented inner zone of the stratum medium is 'the enemy of the hoof' because it is more prone to pathogenic action - being less tubule dense and more moisture rich.

Logically, that means Henderson sees the pigment melanin as conferring additional strength to the hoof.

This seems to run counter to formal scientific studies that have concluded the presence of dark melanin pigment does not confer a greater tensile strength or rigidity to the hoof, ie that a 'white' hoof is no weaker than a dark one.

But, Henderson argues that a so-called 'white' hoof (actually creamy coloured) is just as heavily pigmented as a black hoof but just contains pigments of a different colour, ie there are more melanin pigments in the equine hoof than the two that mainstream science has identified.

Some people, in supporting her theory, argue that, just as sunlight stimulates melanogenisis in the skin, a change in the way the hoof meets the ground might stimulate melanin production and, as this increases, the hoof becomes stronger and healthier.

There is no scientific evidence that melanogenisis is triggered by physical stimulation such as ground reaction forces. There is a substance called magnolignan which prevents the formation of melanin - thereby lightening the skin. If that was deficient in unhealthy hooves, a reversal of that deficiency might increase the amount of pigment being produced but that would simply make the hooves darker on the outside - the layer where the pigment naturally occurs.

Coat and hoof pigmentation and skin colour in horses is determined by the presence, the absence or the relative proportions of two melanin pigments  - eumelanin, which is brown/black but thought to always be black in horses, and phaeomelanin, which is reddish and, in humans, found in high quantities in the lips, nipples, genitals and in red hair. 

Most equine skins are black and it seems likely that this is the evolutionary 'default' setting which some humans seek to alter in their ceaseless search for novelty.

If the skin on the pastern is black, the hoof will be black. A 'white' hoof or white part of a bi-coloured hoof always grows from lightly pigmented or unpigmented skin on the pastern.  A horse may grow a black hoof below a white leg but the underlying skin of that leg will be black. 

True albinism (complete lack of melanin) is rare in horses.  I do not know if the 'pink' skin patches found on some horses are completely lacking in pigment or if they contain high levels of phaeomelanin.

Melanin is created in the basal layer of the epidermis by melanocytes. The enzyme tyrosinase plays an important role in melanin production; it is synthesized inside the melanocytes, matures, and transfers into melanosomes, where it produces melanin by binding with the amino acid tyrosine.

The epidermis of the horse has an average thickness of 0.053mm.

Colour, like beauty, is only skin deep -  actually only a fraction of a mm deep.

But in some hooves, most obvious in dark hooves, the pigmented horn seems to flow into the unpigmented layers, ie it does not form a discrete, uniform outer layer. Henderson sees this as evidence that the unpigmented horn has in some way colonised the pigmented layer, or the pigmented layer has in some way degenerated, making the hoof less resistant to pathogens and lacking tensile strength and abrasion resistance.

She does not consider whether the issue is the reverse, ie whether the way a poorly formed hoof meets the ground results in deformations of the superficial pigmented layer.

As the hoof form improves, is what is seen at ground level a more accurate picture of the anatomically normal distribution of tubular and intertubular horn and pigmented and unpigmented horn  - which had been distorted in a forward running / flared / contracted hoof?

The hoof wall proper (the stratum medium) comprises tubules - which are modified hair - around which is intertubular horn that is formed at right angles to the tubules. The bulk of the melanin in the equine hoof is in the intertubular horn and like the pigment of the skin, it lies in a thin superficial layer.

The tubule cortex comprises dense keratinised cells that surround a hollow medulla that contains cellular debris.  The cortex is lightly pigmented or or unpigmented and therefore does not confer colour to the hoof.

The distribution of tubules relative to intertubular horn reduces towards the bone - an arrangement that  is thought to confer a degree of shock absorption. The zones where different types of horn meet and merge are important and actually little understood.   The interface between hoof horn and the highly vascular inner structures close to the bone is arguably the most sensitive to concussive damage - hence the need for an increasing moisture content to the stratum medium. But, the inner structures are also vulnerable to any pathogens that have been able to breach the protective outer layers of horn so, as in all things, there must be a balance.

Pollitt and others argue that it is the intertubular  horn which confers the greatest tensile strength and rigidity to the hoof as the tubules are 3x more likely to fracture than the intertubular  horn.

But, given the need for the outer layers to be more abrasion and pathogen resistant, and the inner layers to be more flexible to help prevent transmission of ground reaction forces to the sensitive structures,  this seems counter intuitive.

What, to my knowledge, has not been tested is the relative abrasion and pathogen resistance of tubular and intertubular horn.  Nor do scientists typically assess the degree and type of deformities present in the specimen hooves they use.

As usual, more questions than answers.

Spring aggression

Just read a post about a horse suddenly becoming aggressive and there possibly being a link to restricted grazing.

Several things may be happening diet wise - what immediately springs to mind are that magnesium uptake may be inhibited by spring grass and the horse may need some extra salt. If there is too much clover it may be short of iodine, so supplementing with iodised salt may be a good idea.

It may be uncomfortable and is anxious because of that or it may simply have too much energy from spring grass and is looking for ways to release it.

Fructans are sugars the horse cannot digest well and the grasses which store sugars in the form of fructans do so in the base of the leaf. Horses forced to graze short, stressed grass may be getting more fructan than is good for them. The havoc this causes in the hind gut may be setting the horse up for laminitis which can have a slow and insidious onset. The horse may be feeling sore long before we realise what the problem is.

The owner may know that the horse doesn't need extra food and in spring, but the animal's instincts are telling it to eat.  When we restrict grass in spring we have to compensate for that with high fibre substitutes to keep the horse's gut and mind happy - and healthy. We do not know enough about the interaction between the neuro-endocrine and peptidergic nervous systems to discount subtle imbalances within and between them as a reason for changes in behaviour.

The horse is instinctually primed to eat 16+ hours a day.  To get the energy it needs from the type of food it evolve to eat, it has to eat little and often. It is a trickle feeder - small stomach, long hind gut.

When it is eating it should be in a calm, relaxed frame of mind, head down, respiration slow and regular, heart rate low - and, producing saliva which contains enzymes that start off the digestion process and buffer the stomach and gut. The hormonal and peptide feed back systems maintain that frame of mind - all is well, keep eating.

If the horse is anxious (feels it is being deprived of food, feels threatened by others, is in discomfort etc) it is in an excited state. The pituitary gland is releasing factors that trigger the adenals to release adrenaline and cortisol in preparation for fight or flight. Heart rate and respiration go up - and critically, the mouth is dry because the horse has a relatively dry mouth when in flight mode.  Never underestimate the importance of saliva to a healthy digestion.

It's our fault that the horse gets access to soft leafed, NSC and legume rich pasture and doesn't get adequate movement so the evolutionarily driven need to eat almost constantly becomes a serious health issue. We need to ensure that the horse has grass substitutes - not fed to our convenience in a big pile of hay, but available in small amounts, almost all the time and in a way that encourages movement.

The basic advice always comes back to this - look to what nature's arrangements are and in what ways and to what extent your management of the horse differs from them. The horse cannot change its nature. It can and does accommodate itself to us to an extraordinary degree and it is a highly adaptable animal, BUT -  all animals have aspects of their physical and psychological being that are non-negotiable ie they are essential to the animal's wellbeing.  The adverse effects of these conditions not being met appropriately may be spectacular and sudden in onset, eg anaphylactic shock; or subtle and slow in onset, eg chronic laminitis, gut imbalances.

There is always a social component in the horse's behaviour because it is a highly social animal. It may be anxious about its place in the herd; it may be driven by its surplus energy to challenge other more dominant horses; it may be a social misfit that is always a nuisance in a large group but fine with one companion.

Horses reestablish herd bonds and hierarchy every day. We impose a huge stress on some horses when we keep them in a constantly shifting herd structure. Some cope very well - usually the calm confident types that every other horse wants to be with; and some go into extremely excited states - which, depending on a number of factors, may manifest as fearful nervous behaviour or overt aggression.

Finally - in trying to establish what the problem is for a given horse, be systematic. Eliminate one possible cause at a time so you can be reasonably certain what the trigger was.

Reading a Hoof

Learning how to 'read' a hoof is a useful skill. These are the severely compromised hooves of a 9 year old 15.2 hh thoroughbred.

 Left fore                    

Long toe;  long walls; stretched white line; long collapsed and contracted heels and bars; bars laying over and fused with sole; flared quarters, worse laterally; very little concavity; thin sole; frog load bearing along inner third; bulbs compressed; central frog sulcus a shallow cleft; no structure to digital cushion. 

                                               

Right Fore 

Medial heel contracted and medial side of P3 narrowed as result; lateral heel much longer, grossly flared and under run; bone loss along distal margins laterally and at medial toe; lateral bar long and fused completely with sole; medial bar wedged under frog; sole corium in medial heel/bar triangle deformed by inward turning heel, in lateral HBT subjected to persistent ground pressure transmitted by overlaid bar and heel horn; rearward third of frog and bulb folded inwards; medial lateral cartilage deformed inwards, lateral pulled under by collapsed under run heel; degenerated digital cushion.

Left hind

Long toe; long, collapsed heels and bars; vertically compressed bulbs and frog sulcus a cleft turned in on itself; weak DC; thin walls. This was the horse's least deformed hoof.

Right hind

Long toe; rotated pedal bone (broken forward HPA); severely contracted medial heel; thin walls, heels and bars; medial bar pushed up and wedged under frog; lateral heel forward and long; gross flare to quarter; bone loss; quarter crack; frog folded in on itself and collapsed forward. (I had already removed a section of medial wall and exposed the frog apex before these shots were taken and sadly there are no lateral or dorsal views of the hinds.)