In most mixed feeds for pigs, energy has moved ahead of protein as the most expensive component. Because of this, diets should be reformulated primarily on the basis of energy content in order to take advantage of the changing price of ingredients.

Traditionally, accurate energy evaluation has been done by looking at digestible energy (DE) and metabolisable energy (ME), with the latter taking into account losses of energy that occur in urine and methane. Energy is utilised differently according to its source and the weight of the animal. Net energy (NE) considers the amount of energy used in digestion and deducts this from ME to leave the amount available for growth and maintenance of the animal.

Multiple versions in Europe

How the net energy value relates to an ME value for the same feed ingredient depends on the material involved, with ME (and DE) tending to underestimate the actual energy content of fat/oils and starch while over-estimating the energy value of materials rich in protein or fibre. To implement NE in practice, nutritionists compile a database of the nutrients they plan to use and the NE content calculated using prediction equations. Historically, net energy systems have gotten a reputation as being complicated. Even today, there are multiple versions of the net energy system. While most references tend to refer to the original work done in France, different methods of energy evaluation also called net energy exist in other European countries such as the Netherlands and Denmark, although there is no valid reason why a single system of net energy should not be used throughout Europe according to animal nutritionists.

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The practical application of the French system was discussed earlier this year in the AminoNews customer magazine from the feed additives unit of Evonik Industries. Written by Dr Rob Payne of Evonik with Dr Ruurd Zijlstra, who chairs feed research at the University of Alberta in Canada, the report states that the question remaining for many is, how do individual operators, in their unique circumstances, implement a net energy system to successfully obtain the anticipated benefits on a practical level?

Experts estimate that feeding pigs is the single most expensive aspect of pork production, as much as 70% of total costs. But about half of those feed costs can be attributed to providing energy to the animal, thus making energy the most important nutrient from a financial standpoint. As such, it seems logical to investigate the energy systems used to best meet the energy needs of the animal. Advancements for other nutrients, including protein, have been explored and are now largely accepted.

But unlike trends for looking at protein and other nutrients, in regards to energy, many nutritionists continue to formulate diets using digestible or metabolizable energy systems (DE or ME) as opposed to more advanced systems, such as net energy (NE). Why? For some, it's the sheer complexity of energy, which is derived from numerous dietary sources. Another hurdle is the lack of data and research about energy contents of specific feed ingredients. For some, it's simply a matter of being more comfortable using DE or ME systems.

Benefits of using NE

The NE system was developed to provide more accurate estimates of the "true" energy in an ingredient (and subsequent diet) that is going to be available for a pig to use for maintenance and product formation (i. e. growth, gestation, lactation, etc.). The main difference between the NE system and the DE and ME systems is that the NE system considers the amount of heat lost during digestion and subsequent deposition of nutrients in protein and adipose tissue. This point is illustrated in Table 1, which lists the DE, ME, and NE of several commonly used ingredients. Some ingredients, for example, may seem very similar when looking at DE and ME. But take a look at NE, and you will find their capacity as an energy source is actually quite different.

The authors say that a simple, yet practical, example of what this means for diet formulation in Western Canada is shown in Table 2. Diets formulated using NE are typically lower in crude protein (CP) than those using DE or ME, because the heat lost during catabolism and excretion of excess nitrogen is considered in the NE system.

However, the problem of lower CP levels can be solved. The authors point out that by employing the ideal protein concept and accounting for the standardized ileal digestible amino acids in the feed ingredients, the levels of essential amino acids (Lys, Thr, Trp, Met, and Ile) are easily maintained.

Plus, there's an environmental benefit: lower CP means that nitrogen excretion is decreased. According to Canh et al. (1998), each percentage point reduction in CP results in a 10% reduction in nitrogen excretion from the pigs. The decrease in nitrogen excretion results in decreased ammonia emissions and odor in the barns, which leads to improved animal performance. Canh et al. (1988) also indicated that water intake of pigs is reduced as dietary CP is reduced, which leads to less slurry volume.

But the authors say the advantages can be economical as well, making the system especially appealing in times of high feed costs. Using an NE system, the diet cost is decreased both on a per ton and a per pig basis (Patience, 2005; Payne, 2006). Of course, diet cost benefits will depend on the prices of each feed ingredient at any given time, but even if these cost advantages only occur 50% of the time, the authors point out, those savings would certainly be welcome.

Implementing the system

Once a decision is made to look into using NE, the next question is, how to proceed? Drs Payne and Zijlstra say that a serious downfall of any energy system, including NE, is that most nutritionists have been, and still are, using the same energy values for their ingredients as they have been using for years. These energy values may have been developed within each company over the years or they could simply be average values from reference tables, such as those in the NRC (1998) or Sauvant et al.(2004).

Of course, this may be done for the NE as well, but it is not the best route to take since every change in the crude nutrient (protein, fiber, fat, etc.) profile means a change in the energy available from that ingredient. To make the task easier, the authors have developed a detailed guideline of how to proceed with implementing an NE system that could potentially be used in grow-finish diet formulations:

  1. Identify all energy-containing raw materials that would potentially be used in grow-finish diet formulations.
  2. Collect all necessary raw materials for pre-determined length of time.
  3. Analyze all raw materials for their crude nutrient content. These analyses include but are not limited to: dry matter, crude protein, ether extract, crude fiber, acid and neutral detergent fiber, starch, and sugar.
  4. Calculate DE, ME, and NE values for raw materials based on raw material analyses using currently available NE prediction equations.
  5. Compare calculated DE, ME, and NE values for raw materials with values currently being used in formulation software.
  6. Update nutrient matrices for energy-containing raw materials in diet formulation software.
  7. Insert NE values in grow-finish diets, and then reformulate all diets using current energy system (DE or ME).
  8. Based on calculated NE from reformulated diets, remove former energy restrictions (on DE or ME) and place new nutrient restrictions on NE.
  9. Re-optimize all diets to balance on their NE content.

Why start with the grow-finish diets? The researchers point to a number of reasons, including the diets in these phases typically contain the least amount of ingredients and these diets make up the bulk of the feed that a pig will consume over its lifetime. Furthermore, while the concepts of NE certainly apply to all phases of growth, it is conceivable that each phase of growth would require a different set of mathematical equations as the animal's ability to extract nutrients, including energy, change as the animal grows.

This concept is evident with the work of Noblet et al. (1994) as they suggest one set of NE equations for growing pigs and a completely different set of equations for breeding sows. The idea is that the animal's ability to utilize nutrients differently as it grows applies to not only energy, but all nutrients.

As with other nutritional advancements, such as digestible amino acids, the understanding of energy and NE is ever-evolving (De Lange and Birkett, 2005), but that should not be a reason to rule out using NE in today's commercial production scenarios, remind the authors.

Once the energy-containing feed ingredients have been identified, then the next step towards creating an NE database would be to collect each ingredient over a defined period of time. Ideally, this collection would be in conjunction with an on-going quality control sampling protocol, such that it is as seamless as possible. As each ingredient is collected, it should be analyzed for its macronutrient composition, including, but not limited to crude protein, fat, and fiber, moisture, ash, acid and neutral detergent fiber, sugar, and starch. The reason for this is that the two most-widely used NE systems, which were developed by the French (Noblet et al., 1994; Sauvant et al., 2004) and the Dutch (CVB, 2003), are both solidly based on the macronutrient composition of the feed ingredients.

Incorporating nutrient values

After analyzing the ingredients, the next step would be to incorporate the crude nutrient values into the NE equations so that a prediction of the NE content can be made.

Concurrently, it would be beneficial to also calculate the DE and ME contents for each ingredient. The calculated DE and ME content of each feed ingredient would then be used as a means of verifying the calculated NE values and, perhaps more importantly, to verify the DE and ME levels that are currently being used for each ingredient in the formulation software.

Next, the newly calculated NE values should be incorporated into the formulation software, and if not already present, NE iis to be added nto each grow-finish diet matrix. Rather than jumping directly into NE at this point, it seems logical to continue for a period of time formulating diets on a DE or ME basis with NE in the matrix, so that the resulting NE diet values can be monitored.

Finally, once the nutritionist has become comfortable with the NE levels, then the nutritionist should make the switch. Undoubtedly the NE levels of each diet will be smaller than what they were on a DE or ME basis, but remember, one of the greatest advantages of NE is that it accounts for all of the energy lost due to metabolic processes, thus the energy provided via NE is closest to exactly what the animal will have for maintenance and growth.

Combined with digestible amino acids and the ideal protein concept, an NE system will allow the nutritionist to formulate diets that provide the animal with the energy and amino acids that it needs for efficient and predictable growth and carcass performance.

Additionally, by improving nutrient utilisation and efficiency, these systems promote better environmental stewardship for more sustainable pig production. While NE may not be the final advancement to be made in energy evaluation systems (De Lange and Birkett, 2005), it is definitely a start in the right direction.