Dr Pat Harris, a leading authority on equine nutrition, discussed the nutritional aspects of laminitis at a recent meeting of the British Equine Veterinary Association.
The horse is designed for a forage-based diet. Today for our convenience, horses tend to be kept inside, fed on cattle pasture and given high-energy food because of the requirements for work she pointed out. Horses are greedy. If you offer them sweet feed they will eat it.
If you feed your horse twice a day, some of the carbohydrate will escape digestion in the small intestine and will pass unchanged to the large intestine where it will be fermented. This leads to cyclic changes in in the hindgut.
She pointed out that oats, the grain traditionally used to feed horses, is also the most efficiently digested in the small intestine. About 85% is digested before the hindgut. In contrast, only 21% of a barley meal is digested in the small intestine. Cooking and processing affect the digestibility of the grain. For example only 29% of maize fed as the whole grain is digested in the small intestine. If it is cooked before feeding, 90% is digested in the small intestine.
Does obesity play a role in laminitis? Certainly, increased weight bearing may make add to the problem. Dr Harris explained that she had been working with colleagues at the Virginia Tech. in the USA."We have been looking at the animal`s ability to cope with glucose and the effect when you give it insulin. We found that obese horses are resistant to the effects of insulin. Feeding diets rich in sugar and starch increased the insulin resistance." (Insulin resistance is thought to increase the risk of developing laminitis.)
So what can we do to help reduce the risk of laminitis? Dr Harris recommends that owners should monitor the condition score and weight of their animals regularly. "Avoid obesity, especially in ponies. But take care not to suddenly reduce the weight because of the risk of hyperlipaemia. Consider a fibre based diet for obese animals and those prone to laminitis. Avoiding grain-based diets rich in starch and sugar should help to reduce the risk of developing insulin resistance."
"Feed small grain meals - even to race horses. Feed no more than 0.5kg/100kg body weight of oats or processed grain. If more energy is required in the diet it is better to increase the number of daily feeds and not their size."
Currently there is much interest in fructans as a cause of laminitis, but high levels of other water soluble carbohydrates, including simple sugars, are also likely to be involved. Dr Harris explained that fructans are storage carbohydrates composed of one molecule of glucose with more than one fructose molecule. They are found in temperate pasture grasses and also in other plants such as dandelions. Horses do not have the enzymes needed to digest fructans in the small intestine. So the fructans pass into the hindgut unchanged. There, they are fermented rapidly, releasing large amounts of lactic acid, which makes the gut contents more acidic and disturbs the balance of bacteria and micro-organisms.
This change in the environment within the large intestine leads to the accumulation of various factors, which may be responsible for triggering laminitis. The overgrowth of organisms such as Streptococcus bovis leads to the release of MMP activators; while those bacteria that cannot survive the acidic conditions release endotoxins. The high lactic acid concentration damages the lining of the intestine, making it easier for these toxins to be absorbed.
The fructan content of the pasture is affected by a number of factors. Fructans are found in significantly greater amounts in the stem than in the leaves. They are present in highest concentration at times of restricted growth but continued photosynthesis, such as during a drought. The amount of fructan present in the grass also varies during the day. Generally there is less at night and early in the morning. Certain grasses, such as timothy have larger fructan molecules. These may be broken down more slowly in the hindgut and may therefore be safer than other grasses that contain smaller molecules. Levels of 14-20% fructan in fresh grass are not uncommon. Haylage is likely to contain less fructan than grass because it has already started to ferment. Hay is likely to have levels between those of grass and haylage.
So, to reduce the risk of fructan-induced laminitis, Dr Harris suggested measures that could be taken including:
* Choose pasture that is regularly grazed or cut (the grass stems tend to have high fructan content) and contains species such as timothy that produce low levels of fructans.
* Turn horses onto pasture late at night and bring them in before mid-morning, so they are grazing when the fructan content is lowest.
* Restrict grazing in spring and autumn when the fructan and water soluble carbohydrate levels are high.
* Don't use stubble grazing (i.e. after it has been cut for hay - because the stems will have a high fructan content.
* Don't turn horses onto pasture that has been exposed to frost and bright sunlight. (The sunlight produces energy, which the grass cannot use for growth because of the cold, and so it is stored as fructan).
Source: British Equine Veterinary Association meeting. London. December 2004