The horse is a grass-eating (herbivore) animal and its digestive system is adapted to rations consisting mainly of roughage, i.e. grasses and herbs, either fresh or as hay or haylage. Roughages are rich in fibre and the horse needs this fibre for both energy and as a structural component which ensures proper functioning of the digestive system. In simple terms, we can divide the horse's digestive system into two parts. In the stomach and small intestine, which comes first, we find enzymes that digest starch, sugars, proteins and fat. Further along, in the caecum and colon, a rich population of microbes (i.e. protozoa, bacteria and yeast) helps to break down the fibre component into short-chain fatty acids that can be absorbed and utilized for energy. In horses on forage-rich diets, a significant part of the energy needs are met by the acids produced from fibre. Although the cellulose in the fibre-fraction is composed of glucose molecules, microbial digestion transfers the glucose to short (volatile) fatty acids – which subsequently are absorbed. As a consequence, the products of fibre digestion do not affect the horse's blood sugar concentration and insulin secretion.
Fibre-rich diets require a long time for chewing. They keep the horse occupied and increase the time needed to consume a meal. This is important for the horse's comfort and well-being, and leads to increased saliva production, which in turn helps reduce the risk of gastric ulcers.
A horse ration must contain a certain amount of roughage and in PC-Horse we place that requirement at 1.5 kg per 100 kg body weight. When using haylage, with its higher water content, it is necessary to adjust the amount fed to reach the requirement for roughage dry matter. This is done easily through the formula:
hay (in kg x 85)
----------------------------------------------- = kg of haylage
dry matter content, % in haylage
Example: A horse weighing 500 kg has a requirement of 7.5 kg hay per day. How much haylage with 60% dry matter must be given in order to be equivalent to 7.5 kg of hay:
7.5 kg hay x 85
-------------------- = 10.6 kg haylage
The fibre content of forages will vary widely between plant species, and also from one stage of botanical development to another. When plants are harvested early in their growth, the fibre content is not high, but the nutritional value of the fibre is at its highest. A later harvest will have a higher fibre content, but at the same time increased amounts of lignin will cause a decrease in the nutritional value of the fibre. We must therefore consider carefully what are the forage qualities that will suit different types of horses the best.
In PC-Horse, we calculate the amount of fibre in the ration based on an analysis of the crude fibre content. In regular hay or haylage there is about 300 grams of crude fibre per kg dry matter (DM). It will be somewhat lower for early cuts and higher for the late cuts.
PC-Horse sets the lower limit for crude fibre in the daily ration to 3 grams per kg body weight daily, and the horse should preferably receive more than this. This minimum limit is roughly equivalent to the amount of forage mentioned earlier.
There are several methods available for the analysis of fibre in roughages. All recommendations given in PC-Horse are based on crude fibre analysis, and this is the only method that should be used when including analysis of fibre in the program. If selecting NDF (non-detergent fibre), a fibre analysis which is commonly used for cattle, you will get twice as high numbers as when using the crude fibre analysis.