Over millennia, horses became exquisitely adapted to eat the coarse, woody grasses rejected by the ruminant animals with whom they shared the plains.
This adaptation to rough grazing with relatively low nutritional value is all inclusive, from the type and arrangement of a horse’s teeth and the nature of his gastrointestinal tract with its associated microflora, to the physiological and cellular mechanisms that handle nutrients entering his body.
Agricultural development has led to the introduction of high-sugar grasses, created to meet the huge metabolic drives of modern food animal breeds, such as the Holstein dairy cow. These grasses, ubiquitous on lowland pasture, no longer resemble the poorly digestible foodstuffs equines evolved to eat.
We should not be surprised that some horses and ponies struggle with their new staple diet — it could be likened to keeping Labradors in a sweet shop. The result is an unprecedented rise in equine obesity and metabolic disease.
Eating to excess
Horses are herd animals and derive many benefits from being turned out with companions.
This freedom not only enables a horse to express his natural behaviours, but also provides exercise and the opportunity to breathe air uncontaminated by the airborne particles inherent in the stabled environment.
While grazing allows something of a return to nature for this social, free-ranging animal, problems can arise with the type and amount of grass ingested. A big issue is that many horses and ponies enter spring in “summer condition”.
During the rapid grass-growth phase in spring, modern cattle pastures can exceed the energy content of many concentrate feedstuffs on a dry matter basis at a time when a horse’s adaptive physiology is driving up his appetite to encourage fattening ahead of the next winter.
Conversely, in winter, grass decreases in quality and quantity. In nature, this would be the time when an animal would start to mobilise stored fat, reversing seasonal fattening to restore a seasonally appropriate weight.
However, rather than accepting some winter weight loss, many owners offer hay, supplement with concentrates and provide insulative rugs. In the long-term, the horse grazing high-sugar pastures experiences accelerated summer weight gains without losing fat in winter.
Many owners fall into the trap of considering grass as we would lettuce — virtually calorie-free. The micro-organisms in a horse’s hindgut are in fact extremely efficient at breaking down plant fibres and metabolising plant cell content to maximise the release of highly digestible sugars.
Sugars, particularly glucose, drive the horse’s cellular machinery and can be stored in the liver and muscle as glycogen. Excess sugars are converted to fats and stored in adipose tissues.
While some glucose circulating in the bloodstream is essential, an excessive accumulation can be life-threatening. The horse has become supremely specialised in controlling glucose concentrations. The pancreas responds by secreting insulin to hasten glucose uptake, largely by muscle and adipose tissues.
Problems occur when tissues become resistant to the actions of insulin. Many studies have shown elevated blood insulin concentrations are linked to pasture-associated laminitis.
Not all animals grazing the same pasture are equally afflicted. Susceptibility may be a consequence of phenotype (native pony breeds are more at risk), management (obesity and lush grasses spell danger) or differences in gut micro-organisms.
The risk of grass intake as a trigger for laminitis is probably greatest where grasses are “stressed”, either through rapid growth early in the year, when grazed to the roots or following frosts. All have been associated with increased production of fructans, a specific plant sugar associated with laminitis onset.
But any consumption which exceeds the basic requirements of an already overweight animal will continue to drive the accumulation of fat.
Know the score
Grass quality varies with season. As a rule of thumb, grass alone should provide sufficient dietary energy for animals in light or even moderate work, year round, as long as availability does not limit intake. In these cases, where animals live at pasture, the only supplementation required might be a vitamin and mineral balancer. If grass supply is limited in winter, a conserved forage should be fed.
A pony on unrestricted grazing can consume up to 5% of his bodyweight daily as grass dry matter. On improved spring pastures, this can result in an intake of more than twice his daily energy requirements.
The best way to understand whether a horse is being over- or underfed from pasture is to have a clear and ongoing understanding of his body condition score. Ideally, he should maintain moderate condition year round. Some modest weight gain may be appropriate going into winter, as long as we let it return to moderate again ahead of spring.
Grass intake is notoriously difficult, if not impossible, to gauge. Since “poo picking” is routine practice for pasture hygiene, one of the easiest methods is to graze the horse individually and count the piles of droppings produced.
The obese horse producing 12 faecal pats daily when allowed ad-lib access to grass can be graze-restricted to limit faecal output firstly by a third. If there is no decrease in his body condition score after a few weeks, grass intake should be further limited. Decreasing faecal output to less than half (fewer than six pats) is not recommended without veterinary guidance.
A dairy cow grazing cattle pasture can produce 25 litres of milk a day from grass alone. The horse’s greater appetite means that a mare could theoretically produce 35 litres — energy which is otherwise available to the free-grazing horse to lay down fat. When we use these rich pastures for horses, we must intervene to safeguard health.
Managing grass intake
Stabling allows control over grass intake, but can backfire, because hungry horses will eat more voraciously and for longer once turned out. The same is true of short-term restriction using a grazing muzzle.
A dry, grassless paddock is useful to regulate forage provision. For owners who wish to keep horses at pasture day and year round, the judicious use of electric fencing to create “strip grazing” is currently the most practical solution.
Ideally, fences are moved twice daily to allow access to fresh grass, which is carefully calibrated against body condition score and faecal output. Our understanding of such intermittent caloric restriction on glucose/insulin dynamics remains limited, but this method seems to work well in practice.
A horse fitted with a muzzle needs constant monitoring to make sure he can still drink effectively and has not become caught up. The muzzle must be removed regularly to allow natural behaviour, such as mutual grooming, making this a high maintenance option.
Ref Horse & Hound; 4 April 2019
