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Branched chain amino acids and low crude protein

Protein sources, such as soybean meal, are regarded as the most expensive component in broiler diets and if market prices for these sources shoot through the roof, poultry producers need to adapt their strategies to maintain profitability, broiler performance, and meat yield. One adaptation strategy used by nutritionists is to reduce the crude protein (CP) levels and include commercially available synthetic amino acids in the diet formulation, given that they are available at viable prices (Miranda et al., 2015).

Isoleucine is included in the group of branched-chain amino acids among valine and leucine (Dong et al.,2016). Some authors classify valine and isoleucine as equally fourth-limiting amino acids in broiler diets (Baker 2009), whereas others classify valine as the fourth-limiting amino acid, followed by isoleucine as the fifth-limiting amino acid (Miranda et al., 2015). It is important to note that these two amino acids should not be considered independently from one another. The ideal ratio between these two amino acids must be maintained to maximise growth when diets are formulated on a decreased CP basis. This was demonstrated in the study by Miranda et al. (2015), which reported reduced growth performance in broilers that were fed diets that did not have the ideal valine-to-isoleucine ratios, irrespective of using a lower CP diet when compared to those fed the ideal valine to isoleucine ratios (Miranda et al., 2015).

Why consider isoleucine?

Researchers have reported on numerous beneficial functions of isoleucine, including improving microbial populations, antioxidant capacity, and intestinal immune function. In addition, isoleucine may also play a role in the regulation of gene expression of antioxidant enzymes and tight junctions within the intestine. Not only does the benefit of individual amino acid supplementation result in low-cost formulation, but it may also become increasingly important in poultry production systems in tropical and subtropical regions, where heat stress is an ongoing challenge (Dong et al.,2016).

Isoleucine supplementation has been investigated for multiple livestock species. When formulating laying hen diets with a reduced CP content, it is of key concern to egg producers that isoleucine might become marginal in these diets, causing the immune function of the hens to be compromised (Dong et al.,2016). For growing pigs, isoleucine is usually classified as the seventh-limiting amino acid (Van Milgen et al., 2012). In their meta-analyses, Van Milgen et al. (2012) inferred from several studies that supplying isoleucine at 10% lower than that recommended by the National Research Council (NRC) for growing pigs resulted in reduced feed intakes and decreased average daily gains. The authors had also noted that should high concentrations of amino acids such as leucine, valine, phenylalanine, and histidine prevail in the diet, it may lead to competition for isoleucine transport across the blood-brain barrier or for catabolic pathways, which may essentially increase the dietary isoleucine requirement.

The ideal protein concept

The ideal protein concept was originally implemented in the formulation of swine diets; however, has assisted in the determination of requirements for broilers, layers, and turkeys. In addition, it has assisted the ordering of limiting amino acids. How has this been done? Since lysine is the first-limiting amino acid in poultry diets, researchers have rationed other amino acids to lysine, creating an ideal pattern of amino acids for broiler chicks in research. By knowing the digestible amino acid requirements and the common amino acid digestibility values of popular feed ingredients for poultry, knowledge has been added to the amino acid nutrition and metabolism in poultry. These amino acid ratios become increasingly important when considering the age of the bird, the gender, and the criterion of response (Baker, 2009). However, determining exact amino acid requirements become a challenge owing to the interactions between essential amino acids, between essential and non-essential amino acids, and between amino acids and other nutrients (McDonald et al., 2011). Not only does the ideal protein concept allow for more nutrient-accurate diet formulations, but it also aims to reduce the quantity of excess amino acids absorbed relative to lysine. This, in turn, holds several benefits including preventing excess oxidation, reducing metabolic costs, and improving amino acid balance (Miranda et al., 2015).