December 2013 - How do we express a horse's energy needs and the energy content of its feeds?
Everyone knows that sweets and fatty foods contain many calories and can easily cause obesity, while vegetables, fruits and whole meal bread contain fewer calories (less energy) per kilogram or gram. In terms of humans, we talk about the content of calories as a measure of the amount of energy in food.
The content of energy in horse feeds is more difficult to grasp. This is due partly to the fact that we can express the energy requirements for horses both in megajoules and feed units. In addition, energy demands and the energy content of horse feeds may also be calculated in different ways: i.e. as digestible energy, metabolisable energy, or net energy. We will return to this later, since it has practical significance, because the method of calculation of energy varies from country to country, and is in large part based on the traditions of different countries. As long as we calculate energy requirements and feed energy content in the same way, however, the horse will get the right amount of energy. We have taken this into account in the PC-Horse program.
Let us first look at the units used to specify an amount of energy. Calories are the old unit of energy, and should not be used anymore. These days we use instead the "Joule" as the unit of energy. The conversion from calories to joules is simple: 1 calorie = 4.18 joules. When energy content is presented in joules, the numerical value is therefore about 4 times higher than when using the calorie.
1 joule is small amount of energy. Since the horse is a large animal and needs millions of joules per day for maintenance and work, it is more convenient to use megajoules (MJ) than joules when calculating a horse's energy needs. A megajoule is equivalent to one million joules.
We will now try to explain the different calculation methods. Even if you do not need to understand in detail, it is important to be aware that there are different ways to calculate the energy, and that we often have to "translate" the energy content that is on a bag of imported horse feed to national units when comparing imports to other available feeds.
Digestible energy (DE)
A digestion trial is needed to determine how much energy a particular food will make available to the horse. In digestion trials we give a group of horses a known quantity of the tested feed for several days. We collect the manure, and determine the gross energy content of the feed and the manure by combustion. When we have measured the amount of energy supplied in the diet and the amount of energy lost in the manure, we can calculate the amount digested by the horse. This difference between the gross energy in the feed and the manure is called the digestible energy content of the feed, usually abbreviated DE. In the UK, the horse's energy requirements and the feed energy content is given as digestible energy (DE) and expressed in MJ. In the United States energy requirements are also calculated as digestible energy (DE), but the old calorie unit (in the form of megacalorie, MCal) is still used.
Metabolisable energy (ME)
In some countries, such as Sweden and Germany, the energy content of feeds is calculated as metabolisable energy and expressed as megajoules (MJ). In the calculation of metabolisable energy, one incorporates both the energy content of the manure and the energy which the horse loses in urine and fecal gas (methane) into the calculation. Therefore, the energy content of a given diet will have a lower numerical value (of MJ) when it is calculated as metabolisable energy than when it is calculated as digestible energy. We must therefore always check to see if the megajoules specified for a feed is calculated as digestible energy or metabolisable energy.
Net energy (NE or feeding units)
The net energy of a feed is the proportion of the gross energy that can be used for training and work, and for reproduction and growth, after subtracting energy losses (heat) that are incurred when nutrients are digested, absorbed, and metabolized in the body. The net energy value of a feed (in MJ) is therefore an even lower number than the metabolisable energy.
For practical purposes it is common to convert the net energy from MJ to a relative unit of energy, the "Feed Units Horse". This is an old tradition in countries that use the net energy system. In Europe this includes Norway, Denmark, Iceland, the Netherlands and France. When we calculate the “Feed Units Horse (FUH)”, we divide the net energy value by 9.414 MJ. Why divide by 9.414? This number represents the net energy value (MJ) in one kg of barley. When we express the energy content of a feed as FUH, we specify how much net energy a kilogram of the feed will give the horse, compared to one kilogram of barley. For example, if we have determined the energy value of a kilogram of hay to be 0.5 FUH, it means that a kilogram of this hay contains half as much energy as a kilogram of barley. In other words, we must feed 2 kg of hay to place the same amount of energy at the horse’s disposal as it gets from eating 1 kg of barley.
We have looked at several methods for the calculation of the energy content of feeds:
digestible energy (DE), metabolisable energy (ME) and net energy (NE).
Digestible energy and metabolisable energy requirements are expressed in MJ, while net energy is expressed in “Feed Units Horse”. In the Netherlands, the net energy unit is called VEP, and in France the UFC. As noted above, it is important to be aware that the horse's energy requirements must be calculated according to the same principles (i.e. digestible or net) and expressed in the same units (i.e. MJ or UFC) as the energy content of the feed.
Here is an example to help clarify the above point:
If you were to calculate your horse's needs in digestible energy, for example, by using the NRC calculations from the USA directly, and then feed MJ by MJ a concentrate where the energy is expressed as metabolisable energy, the horse will have too much energy and may easily become fat.
In the PC-Horse program, we can recalculate the energy content of feeds produced in different countries, so that the energy values will match those of your national system, whether expressed as net energy or digestible energy, and regardless of whether the units are MJ or UFC.