Digestible energy.
Pigs require energy to maintain normal body processes, and to grow and reproduce. Feed ingredients that supply energy are major components of all pig diets, and the quantity of a diet voluntarily consumed by a pig depends on its energy content. Consequently, feed is the largest single economic input in pig production, and energy efficiencey is closely associated with production efficiency. More than 80 per cent of the energy content of conventional diets used in commercial pig production comes from carbohydrates. In Australia, wheat, barley and sorghum are the most important grains that supply energy that are fed to pigs. Fats and oils contain more energy than carbohydrates per unit weight but are less important diet constituents, except where they are frequently used to increase the energy density of the diets of weaners. Amino acids or proteins also may function as energy sources if they are included in diets in excess of a pig’s requirement for protein deposition.
Energy breakdown.
Information about the energy content of feed ingredients is needed to formulate diets with specified energy concentration for various stock classes (i.e. to achieve production targets). It is used to calculate the amino acid (and other nutrient) levels relative to energy concentration. Being able to assess the energy content of diets for stockfeed regulation purposes is also important. The energy content of a feed may be broken down into the following components (see Figures 1 and 2), some of which are more easily measured than others:
- gross energy (GE). This is energy released on combustion of a feed. It is determined in a bomb calorimeter, and indicates the potential energy in a feed but not necessarily the amount of useable energy
- digestible energy (DE). This includes faecal digestible energy (DE), which is the energy in feed after subtracting the energy lost in faeces. It is determined in pig metabolism trials. Digestible energy also includes ileal digestible energy (iDE), which is the energy in feed
- after subtracting the energy that passes the terminal ileum into the pig’s hindgut. It is determined in pig metabolism trials
- metabolisable energy (ME). This is the energy in feed after subtracting the energy lost in faeces, urine and gaseous emissions. It is determined in pig metabolism trials, though it is more difficult to collect samples
- net energy (NE). This is the ME value less the heat produced during digestion of the feed. Of all the definitions, net energy is most closely related to the energy available for production. It is highly desired by nutritionists but is the most elusive value to determine because many factors influence it.
Figure 2. The energy content of feed m ay be broken down into a variety of com ponents.
Why DE is used to describe feed energy.
NE is the ultimate choice to express feed energy content as it estimates the amount of energy available for production. However, it is affected by factors such as the diet and the production system and so it is not a convenient system to describe individual feeds. Metabolisable energy is closer to NE than DE, in that it takes into account the energy lost in urine. This amount is small, and it depends on the diet’s amino acid balance and the pig’s requirements (greater loss if surplus amino acids). To take this into account and aiming to standardise ME values, they are often corrected to a given nitrogen (protein) retention. ME is more difficult to determine than DE.
Sorgghum
The DE values quoted in the literature are faecal digestible energy. More recently there have been moves to improve digestible energy values by measuring them at the end of the ileum, as energy digested here is more efficiently utilised than in the hindgut and so may be more closely related to the actual energy available for production. However, the data available for various ingredients using this method is limited.
Determining faecal digestible energy.
A feed’s faecal-DE value is determined with pigs kept in metabolism cages. Over the first 10 to 14 days, the pigs are allowed to adjust to the test diets, then over the next five to seven days, the feed intake is kept constant and the faeces produced during this period are collected. The feed samples being tested are either fed as the whole diet, e.g. wheat or barley, or as additions or substitutions to a basal diet e.g. of protein concentrates as these cannot be the entire diet. At the end of the collection period, the faeces are mixed, sampled and analysed for GE. The feed is also analysed and its DE is calculated as follows:
DE (MJ/kg) = (GE in feed (MJ/kg) - GE in faeces (MJ/kg)) ÷ total feed intake (kg)
Factors affecting the DE content of feeds
Digestible energy is relatively independent of many factors and those that do occur are generally ignored in diet formulation. The major limitation of DE as an indicator of energy for production is that the relationship between DE and NE is not constant. Fibre is not digested in the small intestine but passes to the large intestine where micro-organisms convert part of the fibre to volatile fatty acids, which are then absorbed. Digestion in the large intestine is less efficient (approximately 20% efficiency) than direct absorption of energy from the small intestine (approximately 80% efficiency), thus DE can over-estimate the NE content of high-fibre diets. There are also differences in the efficiency of NE usage even with nutrients that are absorbed in the small intestine, for example, energy in the form of fats and oils is more available to the pig than energy from carbohydrates. Other factors affecting feeds’ digestible energy content include high dietary fibre levels, which can reduce the digestibility of energy and other nutrients. There is a very small increase in digestibility with age, and the grind of cereals and pelleting may also have an influence on an ingredient’s digestible energy, with finely ground feeds having a higher energy digestibility than the same ingredient more coarsely ground. All these limitations of DE values are generally ignored in formulating diets. However, in the future, computer models that simulate metabolic processes could be used and there may be more use of NE values of feeds.
Predicting energy content.
Determining the energy content in feeds involves considerable time and resources, and it is not practical to suit commercial pig production systems. Accordingly, considerable research has been directed into estimating and predicting the energy content of diets and raw materials. With raw materials, four main approaches have been used to predict or estimate energy values. 1. Reference tables: based on direct measurements of energy values. They are of most use when the variation in energy concentration is low for a particular class of raw material. If variation is high, further subdivision on particular attributes such as protein level and plant species, may make them more useful to characterise their energy values. 2. Prediction equation: based on digestibility coefficients. They are of most benefit on single raw materials where the influencing factor on digestibility has been defined. 3. Prediction equation: based on chemical analysis. They are of most benefit where energy variation is wide i.e. in weather-damaged grains.
Barley
4. More recently near infrared (NIR) methods are being developed as a rapid and non-destructive means to measure the energy value of feed ingredients. Cereal grain DE values may be predicted with this technology however more research and validation are required to assess its potential with other feed ingredients.
DE content of feeds.
Most types of feed are relatively constant in their DE value (see Table 1). Thus table values can be used with some confidence especially for cereals and for proteins. However by-products and meat meals are more variable and frequent prediction equations are used. Table 1. DE content of Australian feeds (air-dry basis, 90% dry matter) (from SCA 1987)
There is a need for continuous updates on the DE, ME and NE content of new sources of raw materials. It would be best if the range in values could be determined for each class of ingredient. It can be seen that more emphasis should be given to providing information on the energy content of feed ingredients rather than mixed diets as the former is needed as a basis of formulating diets. Until there are developments in quantitative fibre determination and greater understanding of the factors influencing energy utilisation, tabulated values for DE and ME are more applicable than prediction equations for feed ingredient energy value assessment. Recent studies have shown that even small variations of DE in some cereals can have substantial affects on returns to producers. For this reason much emphasis is being placed on new technologies such as NIR to better and more rapidly assess grain nutritive worth. Similarly, processing techniques to reduce particle size, pelleting and in-feed enzymes are being examined as a means to improve the DE value and yield of grain ingredients. With the characterisation of feedstuff components into total, digestible or available nutrient levels, the feed formulator is in an improving situation of precision to ensure that the diet nutrient supply meets the nutrient need of different classes of pig. Hence efforts to achieve optimised feed efficiency with minimised feed cost and waste are more successful.
References.
SCA (1987) Feeding standards for Australian livestock Pigs (CSIRO) Standing Committee on Agriculture - Pig Subcommittee. Batterham E S (1990) Prediction of the dietary energy value of diet and raw materials for pig In ‘Feedstuff evaluation’ edited by J Wiseman and D J A Cole. Butterworths London. Batterham E S (1990) Digestible energy. In ‘Pig Production in Australia’ edited by J A A Gardner, A C Dunkin and L C Lloyd. Butterworth. Kopinski J S (1997) Characteristics of cereal grains affecting energy value PRDC/APL report DAQ35P.
Author:
John Kopinski and Alison Spencer
