Understanding digestible energy
What is digestible energy?
Horses require energy for maintenance, to maintain their body at rest, as well as for any additional activity such as growth, gestation, lactation and physical work. Diets tend to be formulated initially to meet the energy needs of the horse and then adjusted regarding protein, minerals and vitamins. Therefore, horse feeds cannot be properly formulated without knowledge of their energy contents.
What is energy? Although energy is supplied to the horse via its diet, fundamentally it is not a nutrient but rather the capacity to do work. Food energy comprises the potential chemical energy of carbohydrates (including sugars, starches and fibres), fats and proteins. Part of it can be converted to other body chemicals and mechanical work (useable energy), with the remainder lost as waste energy (mainly in the form of faeces, urine and heat). Although the thermochemical calorie (cal) has traditionally been used, the most common unit to measure energy is now the joule (J) (4.184 J = 1 cal).
Energy Categories In the animal feeding industry energy can be broken down into four categories: gross energy (GE), digestible energy (DE), metabolizable energy (ME) and net energy (NE). The heat produced from the complete combustion of food is expressed as gross energy (GE). The GE is not completely utilized by the animal since some of the feed leaves the body as faeces. The GE minus the energy lost in the faeces is called digestible energy (DE). Energy is also lost in the urine and through gases; this energy subtracted from the DE is the metabolizable energy (ME). Furthermore, energy is released in the form of heat, called heat increment, as food is ingested and absorbed. The remaining energy, GE minus energy lost in faeces, urine, gases and heat increment, is called the net energy (NE). There are two main systems used to express horses’ energy and nutrient requirements; the DE system and the NE system. Different countries use different feeding systems; in Australia the energy content of horse feed is expressed as digestible energy (DE).
Measuring energy in horse feed Unfortunately it is not possible to directly measure the amount of useful energy contained in feed as you can with other variables such as protein and minerals. This is primarily because all food calories are not the same; they differ chemically and these chemical differences affect the mechanism and efficiency of digestion and metabolism, and therefore health and performance. While there are several equations used to estimate DE, we still lack a good means of accurately and easily predicting the energy content of different feeds. There is no regulatory body suggesting equine feed manufactures use one particular method or equation. This can potentially be very misleading to the end user, making it virtually impossible to compare energy contents of feeds between different manufacturers. DE values should therefore not be relied upon when making decisions about what feed is right for your horse and should only be viewed as an estimate at best. Other balances relating to energy sources in the diet should be considered when selecting feeds for individuals including:
- Sufficient fibre to maintain normal gut and digestive function and limit behavioural disturbances.
- Adequate fat to maintain the required energy without negatively affecting palatability and gastrointestinal function.
- Highly digestible carbohydrate to help maintain ample energy without overloading the digestive capacity of the horse or causing metabolic disturbances.
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