There is growing interest among nutritionists in feeding reduced crude protein (CP) diets to broilers and layers. Current levels of CP are an economic consequence of balancing for amino acids. Dr Steve Leeson, Professor Emeritus, University of Guelph, Canada, at the recent EW Nutrition Poultry Academy in Jakarta Indonesia asked the obvious question – “what does ‘low’ CP really mean? To me it means no more than 2-3% less CP than you are currently using,” he said.

The formulation of these diets is typically based on decreases in soybean meal with increased feed grains, with concomitant increases in dietary starch and reductions in dietary lipid, together with increased inclusions of non-bound (crystalline and synthetic) amino acids to meet requirements.

Dr Leeson summarised the pros and cons of using low CP diets as follows:

Positives	Negatives
Reduced proteolytic bacteria – improved gut health	Reduced pellet quality
Reduced environmental pollution	Altered dietary electrolyte balance
Reduced water intake (impact on litter quality)	Diets often more expensive
Perhaps more sustainable	Often reduce growth rate and feed:gain
Improve dietary net energy	Questions regarding 100% utilisation of up to 25kg/tonne synthetic amino acids
Improved performance during heat stress	Increased abdominal fat deposition

Issues to be addressed with low crude protein diets

Protein versus amino acids

Diets are not usually formulated to CP levels, but it is still an important monitoring tool, as a measure of dietary nitrogen. Most broiler diets contain 22-17% CP, and most layer diets contain 18-14% CP. Dr Leeson observed that for some reason we rarely get adequate performance when broiler diets contain <19% CP in starter and <15% in finisher, and layer diets <13% CP, regardless of amino acid supply.

With high levels of crystalline amino acids (10-25 kg/tonne) the question is are these synthetic ‘non-bound’ amino acids used with 100% efficiency? Also, can the bird utilise a large influx of non-bound amino acids in the upper region of the gastrointestinal tract and the liver?

Non-bound amino acids do not require digestion, are immediately available for absorption in the small intestine and appear in the portal circulation more rapidly than protein-bound amino acids. Moreover, the digestion of ‘intact’ proteins, including soy protein, yields di- and tri-peptides or oligopeptides, which are absorbed more rapidly and efficiently than single or non-bound amino acids. The balance of non-bound to protein-bound amino acids increases as dietary CP levels are reduced. This may be an obstacle to the development of reduced-CP diets as the extent to which non-bound amino acids can replace intact proteins in broiler diets may be limited by more efficient intestinal uptakes of oligopeptides.

This raises the potential for encapsulated amino acids (or natural peptides) to slow digestion/absorption. Very few studies with encapsulated amino acids have been conducted. Most involve a simple mixture of amino acids coated with stearic acid or palm oil. These saturated fats are, however, poorly digested by young broilers.

Levels of non-essential amino acids

As CP is reduced, so are the levels of non-essential amino acids. Do we need a limiting value for non-essential amino acids, such as glycine and serine?

Energy level considerations

Broilers and layers still eat to their energy requirements, so changing the energy level of the diet, or utilisation of energy impacts feed intake, and hence intake of amino acids. For broilers, Dr Leeson recommended maintaining a ratio of energy to “protein”:

• 0-15 days – 0.42% digestible lysine/1,000 kcal nitrogen-corrected apparent metabolisable energy (AMEn),

• 15-28 days - 0.36% digestible lysine/1,000 kcal AMEn, and

• 28-45 days - 0.32% digestible lysine/1,000 kcal AMEn.

CP increases heat increment, which is wasted energy, and so decreases net energy (NE).

• Protein - +30% above field metabolic rate (FMR),
• Carbohydrates - +6% above FMR, and
• Fat - +4% above FMR.

The main practical difference when using NE values in formulation, rather than AMEn, is the reduction in CP level of diets, because of the energetic inefficiency of the utilisation of proteins. As CP is reduced, so effective NE capture is increased. Birds eat less since energy requirement drives feed intake. Therefore, should AMEn be reduced when CP is reduced?” asked Dr Leeson.

Low CP always increases abdominal fat deposition, the mechanisms of which are still not fully understood, but may be an NE effect. This discourages the acceptance of diets with reductions in CP.

Gut health benefits

Reduced-CP broiler diets increase flows of undigested protein into the hindgut to fuel the proliferation of potential pathogens, such as Clostridium perfringens. Protein (especially lysine and serine) is the main culprit. The reduced use of antibiotic growth promoters has increased interest in preventing necrotic enteritis, including the reduction of dietary CP.

As CP is reduced, there is less nitrogen flowing into the large intestine and caeca. Nitrogen is a driver of water intake, so low CP diets reduce water consumption, resulting in drier litter, and potentially less oocyte cycling.

Role of proteases

Regardless of the CP versus amino acid discussion, protease should lead to around 2-4% reduction in CP, depending on the inherent protein digestibility of the diets – the lower the quality of raw materials, the better the response.

Impacts on pellet quality

On average, each 1% reduction in CP, results in a 2% reduction in pellet durability index. Pellet quality is critical in modern production systems, especially at high stocking densities, during heat stress, and prolonged periods of darkness.

Electrolyte balance

Reduced CP invariably means reduced use of soybean meal (which is rich is potassium), can reduce dietary electrolyte balance (DEB) by up to -144MEq, or 50% of the normal level. Maintaining DEB with low CP diets is perhaps one of the main variables in the success or failure of low CP diets.

Dr Leeson added that he was “not a big fan of formulating to a DEB of 240MEq, rather he was more interested in sodium and potassium levels in feed.”

Future considerations

Reduced CP diets have the potential to attenuate environmental pollution from nitrogen also have the capacity to reduce the poultry industry’s dependence on soybean meal. Low CP diets are often quoted to reduce feed costs – but very rarely do so, even with the current cost of soybean meal, as it is still a cheap source of amino acids. Synthetic leucine and arginine are expensive. Dr Leeson, however, concluded that low CP diets are going to be more relevant in the future, perhaps even feeding protected amino acids, or natural peptides to overcome the inherently different digestive dynamics of ‘intact’ protein, and non-bound amino acids.

As non-bound amino acids become more economically feasible a greater array of these amino acids will be included in commercial diets at increasing inclusion levels and dietary CP and soybean meal contents will continue to decline.