Nerida’s PhD Thesis
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ABSTRACT
Cereal grains do not form part of the natural diet of equines, however, results from an initial survey conducted, suggest that horses in the Australian thoroughbred industry are currently being fed on average about 7 kg of grain concentrate/day. Horses are not well equipped to digest the starch from cereal grains in the small intestine and as a consequence, the hindgut fermentation of starch, which may lead to hindgut lactic acidosis and diseases such as laminitis, is an evident problem in the thoroughbred industry.
Two main factors determine the extent of starch digestion that will occur in the equine small intestine. These are:
the attributes of cereal grains that determine starch digestibility; and
the ability of an individual horse to digest starch in the small intestine.
The general hypothesis for this thesis was therefore that both the attributes of cereal grains that determine starch digestibility and the ability of individual horses to digest starch in the small intestine will determine how much cereal grain starch may be fed ‘safely’ before problems with hindgut starch fermentation and acidosis will be experienced.
A series of experiments were conducted to test this hypothesis and to specifically examine the effect of grain species, grain processing and enzyme supplementation on small intestinal starch digestion in horses. Oats, barley, triticale, corn and rice were the cereal grains studied and expansion, extrusion, steam rolling and micronising were the processing techniques investigated.
Two in vitro assays were used to examine the rate and extent of enzymatic starch digestion and the fermentation characteristics of the cereal grains studied. The glycaemic and insulin responses were also used as virtually non-invasive assays for determining in vivo small intestinal starch digestion. Our major findings were:
1. triticale was the most digestible of the unprocessed grain species with 42% of starch digested during the 15 min in vitro incubation. Corn was the least digestible grain species with 14% of starch digested in vitro in the same time period;
2. grain processing improved the rate and extent of in vitro starch digestion of all grains. Extrusion appeared to be most effective, increasing the extent of corn starch digestion from 16% to 74% digested in vitro in 15 minutes and increasing the extent of rice starch digestion from 19% to 80% digested in vitro in the same time period;
3. grain processing increased the rate at which cereal grains fermented in vitro in equine caecal fluid with the fermentation medium of processed grains having an average of 10 mmol/L more lactic acid than the fermentation medium of unprocessed grains;
4. processed grains appeared to be more thoroughly digested in vivo in the small intestine with peak glucose and insulin responses an average of 1 mmol/L and 33 miu/mL higher for processed grains, respectively;
5. there was large variation between horses in their glycaemic responses to the cereal grain diets, possibly reflecting their ability to digest starch pre-caecally. However, although the glycaemic response was variable between horses, it was able to consistently rank horses in order of response to specific cereal grain diets and thus appears to be a useful tool for assessing a horse’s capacity to digest cereal grain starch;
6. the addition of a combination of α-amylase and amyloglucosidase to a steam rolled triticale diet, significantly increased peak glycaemic responses from 8.8 mmol/L (control diet) to 10.8 mmol/L, indicating that low glycanase concentrations in the equine small intestine are a major limiting factor of pre-caecal starch digestion in equines; and
7. equine α-amylase has a lower substrate specificity or activity than α-amylase derived from Bacillus licheniformis, digesting an average of 20.6% less starch during an in vitro starch digestion assay with a 15-minute incubation period. Thus a low activity of equine α-amylase may also be limiting starch digestion in the small intestine of the horse. Despite the differences between equine and bacterial α-amylase however, the assays were highly related (R2=0.91) indicating that the standard in vitro starch digestion assay may be used to reliably rank grains in order of small intestinal starch digestibility for equines.
Thus in support of the hypothesis presented, attributes of cereal grains, which are partly removed by grain processing methods and an apparent deficiency or poor substrate specificity of α-amylase in the equine small intestine, affect the extent of starch digestion that occurs, both in vivo in the small intestine and in vitro. These factors will therefore determine how much cereal grain may be ‘safely’ fed to horses without causing hindgut starch fermentation and its associated negative consequences.
From a practical viewpoint, the standard in vitro starch digestion assay and the glycaemic response appear to have application as feeding management tools, allowing the selection of grains with superior starch digestibility attributes and the characterisation of horses on the basis of their ability to digest starch in the small intestine, prior to feeding. If these tools, along with exogenous glycanase enzymes, can be used effectively to formulate diets for individual horses, many of the problems currently being experience with hindgut starch fermentation and the resulting acid accumulation may be largely avoided.
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About the Author
Dr Nerida McGilchrist
Dr Nerida McGilchrist is an Australian equine nutritionist with a PhD and over two decades of experience. As the founder of Equilize Horse Nutrition, and advisor to some of the world's largest nutrition companies, she’s built an international reputation for blending science with practical solutions. Now, she’s bringing her expertise to My Happy Horse to make advanced nutrition accessible to all.