COLIC ISSUE 42_Jerkins feature.qxd 31/05/2013 15:06 Page 2
Racehorses are at heightened risk of colic due to their high starch diet A high dietary intake of starch is a well established risk factor for colic. Researchers suggest that horses fed a daily concentrate intake of over 5kg, which would include most racehorses in hard work, are exposed to a six-fold increase in the risk of colic. The level of risk posed by a particular high starch diet will depend on the proportion of that starch that reaches the hindgut. Whilst high-energy concentrate feeds can be regarded as an occupational hazard for horses in training, there are a number of steps that can be taken to mediate the risk. With the advent of racing feeds with a high digestible fibre and oil content, the overall reliance on cereal starch can be moderately reduced. In addition, small meals and an insistence on feeding cooked cereal ingredients can reduce the amount of starch that escapes digestion in the small intestine. Traditionalists will be pleased to note that in the small intestine, oats have the most easily digested starch, when compared to other uncooked cereals such as barley, wheat, or maize. Research in Australia has shown that synthetic amylase and amyloglucosidase enzymes can be added to the horse’s diet to improve the efficiency of starch digestion in the small intestine. Unfortunately, there are no such additives approved for use in horses in Europe to date.
A pick of grass is a double-edged sward Limited access to grazing as experienced by most horses in training also increases the risk of colic significantly. The gentle exercise associated with grazing and the very high water content of grass means that impaction in grass fed animals is relatively rare, except where voluntary water intake is reduced during cold weather or snow. A recent change in type or duration of grazing represents a further risk. Grass contains fructan, a carbohydrate that cannot to all intense and purpose be digested in the small intestine but which can be very rapidly fermented in the large intestine. Fructan has the potential to abruptly change the population of microflora and the pattern of fermentation. This generally doesn’t occur in horses grazed for a consistent length of time, but could potentially be an issue for horses in training that are suddenly given access to pasture, perhaps as a result of an injury or layoff. It is difficult practically to introduce grazing gradually, as horses have a tendency to gorge
themselves when given the opportunity. The most realistic way of achieving this is to restrict both grazing area, perhaps using a playpen, as well as grazing time by gradually lengthening the time spend at grass over a period of one week.
Mature stemmy forage can be problematic One of the other main dietary components with a high risk factor for colic is forage. Even a horse in training receives a significant portion of its diet as forage. An increased number of bacteria capable of fermenting starch is inevitable in horses in training due to the high starch intake and the likely proportion that escapes digestion in the small intestine. These are generally lactate producing bacteria, i.e. they rapidly ferment starch to produce lactic acid resulting in a fall in hindgut pH. This hindgut acidosis is very inhospitable to the cellulolytic or fibredigesting bacteria. Mature stemmy forage with a low digestibility, whether hay or haylage,
“I learnt a lot from Alain, and his method of training. He is very patient, and knows exactly how to read horses and treat them as individuals” increases the risk of impaction. As guidance, low digestibility forage may be stemmy and contain many seed heads. On analysis this type of hay normally has a very low protein and high fibre content. Alfalfa, which is considered very digestible, can increase the risk of obstruction of the colon with enteroliths or stones if fed to excess. This is due to its high calcium content, but the quantities concerned are very high (typically 70% of the total diet). A sudden change of forage type, including simply switching from one batch to another, is risky, and measures to smooth the transition are sensible. Good hygienic quality of hay is also important and microbiologically, lowquality hay again is associated with an increased risk of colic. Measures such as laboratory analysis of moulds prior to purchase and steaming can help.
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