Net Energy, the best energy system to predict pig performance
In animal feeds, energy supply represents a major part of the cost of the formula. Improving knowledge of energy utilization by the animal to better meet its requirements and having systems in place to evaluate for the energy content of raw materials and feeds are thus determining factors in least cost formulation. As for amino acids, the total amount of energy supplied by the feed, called gross energy (GE), is not available for the animal. The energy value of a feed (ingredient or diet) can be expressed as digestible energy (DE), metabolizable energy (ME) and net energy (NE). In pigs, after the different levels of energy utilization, less than 60% of the ingested GE can be used by the animal (Figure 1). In poultry, it is not possible to measure DE due to the impossibility to separate faeces and urine. The ME system is therefore directly measured and will be used for formulation because of the negligible part of the heat increment in the energy losses.
GE: gross energy; DE: digestible energy; ME: metabolizable energy; NE: net energy
Figure 1. Energy systems in pigs fed a standard diet based on cereals and soybean meal.
An energy system is a method for assessing the dietary energy content which corresponds to one or more stages in the utilization of energy by the animal. Systems of DE takes into account the digestive utilization of energy, and the ME system takes into account the energy losses in the form of urine and gases. The NE system takes into account the metabolic use of energy and the resulting heat production (see also Net Energy Evapig®). Research has shown that, in pigs, NE is the best estimator of the true energy of ingredients and complete feeds as DE and ME underestimate the energy value of feeds rich in fat and starch and overestimate that of feeds rich in protein and fibre. The NE system is also better for predicting pig performance and carcass quality, particularly when pigs are fed reduced protein or high-fat diets. For that reason, using NE for ingredients and for pig feed formulation is highly recommended.
Practical aspect of the use of net energy in pigs
The use of NE instead of DE or ME leads to changes in ingredients ranking. When dietary DE and NE values are expressed as percentage of a compounded feed (wheat: 67%, soybean meal: 16%, fat: 2.5%, wheat bran: 5%, peas: 5% …), the feedstuff ranking changes (Table 1). This different hierarchy has a huge impact on the daily feed formulation and on the usage and supply of the feedstuffs in a feed factory. For instance, the NE system allows being less dependant on imported protein sources feedstuffs like soybean meal.
Table 1. Ranking of feedstuffs depending on their digestible energy (DE) or net energy (NE) content. (for details, see EvaPig®)
Lowering the CP level of a feed will change its energy characteristic if the NE content is not controlled. Indeed, the reduction of the CP level in a feed, based on the same ME content does not take into account that the NE will be increased. In low protein diets some of the protein is substituted by starch and/or fat; this substitution has not a marked effect on the dietary ME content but will change the NE content of the feed. Indeed, in the NE system, the efficiency of ME supplied by starch (82%) is greater than that of digestible protein (58%). Therefore, there is an average increase of 0.5% in NE content for every point of reduction in dietary CP content. This effect is accentuated in case of substitution of some of the protein by fat, due to its higher efficiency than that of starch (90% vs. 82%) and its high energy content.
On a practical level, it is thus important to take into account this increase in NE associated with the reduction of dietary protein content in order to allow better feed utilization by the animal. Experiments carried out with low protein diets formulated with NE and with adjusted amino acid supplementation levels have shown that growth and carcass adiposity at slaughter are not affected even when animals are fed ad libitum (Figu