Grassland management
Getting the best out of your pasture In the second part of our series we look at the nutritional content of grass for horses and how grassland management can improve this Words: Dr Simon Daniels
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rass plants contain, protein, fibre, sugar, fats and vitamins and minerals. The extent of these nutrients will vary greatly with season, soil type and management. Plant protein is found inside the plant cells and is made up of sequences of amino acids. Some amino acids can be synthesised within the body but others must be sourced within the diet and are essential nutrients for normal body function. Lysine is an essential amino acid that is also the first limiting amino acid, meaning it is the first to become deficient. Therefore, when considering protein content in the diet, we have to consider if we can meet the horse’s amino acid requirements from our grass. The next consideration for protein within grass is the digestibility; the horse can only make use of protein that is digested and absorbed within the small intestine. As the plant matures and the cell wall becomes more lignified, then the amount of protein digestion in the small intestine is reduced. In more mature lignified grass, the protein is liberated from the plant cells in the horse’s hindgut where billions of bacteria, fungi and protozoa that we collectively term as microbiota ferment the plant material. However, protein that is digested in the hindgut is not bioavailable to the horse; instead, the microbiota make use of this protein themselves. So as the growing season progresses the protein digestibility within the horse decreases. Fibre is essential for the horse’s diet and fibre digestibility is also influenced by the grass growth cycle. Fibre is structural carbohydrate, meaning is it made up of chains of beta glucose molecules. Now this may initially
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sound confusing, as we tend to think of glucose as a soluble sugar. Soluble sugars, e.g. glucose, maltose and sucrose, can be made of alpha glucose molecules and this reflects their biochemical structure. So, glucose is a simple sugar, maltose and sucrose and examples where two glucose molecules or a glucose and a fructose molecule are joined together by a bond. Similarly, starch, the storage molecule of cereal plants, is where lots of alpha glucose molecules are joined together by chemical bonds and for starch this is what makes the more simple sugars a storage molecule (as mentioned in the first article) converting glucose to glycogen within our bodies.
“The digestibility of fibre is also influenced by the grass growth cycle” However, fibre is made by joining beta glucose molecules together – this makes short storage molecules such as fructans or more significant structural carbohydrates like cellulose. The key thing to grasp here is that to digest fibre, we need to break the glyosidic bonds between these beta glucose molecules to be able to make use of these carbohydrates. However, mammals do not possess the enzymes, principally cellulase, that is needed to break down the bonds between the beta glucose molecules which form cellulose. Microbiota that reside in a
horse’s hindgut and in the rumen of cows and sheep do produce cellulase and these microbiota can therefore break down fibre through fermentation. As I mentioned in part one, not all fibre can be digested and the younger the plant in the growth cycle, the more fibre can be digested. We measure this in feeds and forages through acid detergent fibre (ADF), which provides an estimation of the lignin content of the diet and neutral detergent fibre (NDF), which also gives an indication of the digestibility of the plant cell walls and tends to link to the amount an animal will eat. For both of these markers the lower they are the more digestible the fibre and the more the animal will eat. Of the cell wall contents, the most digestible component is hemicellulose; other digestible fibre sources are pectins, found between two plant cells. Pectin acts as a glue to stick cells together. Within the horse there is virtually no fibre digestion in the stomach and small intestine, however the fibre is fermented by microbiota in the hindgut to produce volatile fatty acids which the horse can metabolise into useful energy. You will note that earlier I defined fructan as chains of glucose and fructose molecules linked with beta glycosylic bonds. Lots of fructans entering the hindgut and those with longer chains can be problematic to the horse. Fructans are readily fermented by microbiota in the horse’s hindgut by ‘sugar loving’ saccharolytic fermenters. Fructan is converted into a volatile fatty acid that feeds into the glucose pathway but it is also converted into lactate. The production of significant amounts of lactate drops the pH in the horse’s hindgut ecosystem, altering