29 Jun 2024
Investigations into reducing dietary crude protein (CP) in chickens began in the early 1940s, marking nearly a century of exploration in poultry nutrition research. While advances have been made, many hurdles identified in the 1940s still challenge researchers today. Early research established that diets with increased protein resulted in chickens having over twofold lower body fat content. However, low CP-amino acid fortified diets failed to produce birds with performance equal to higher CP-fed control birds. Decades later, despite industrial growth in broiler production, it was widely accepted that dietary inclusion of amino acids beyond sulfur-containing amino acids and lysine was not economically sustainable. Towards the end of the twentieth century, the adoption of the third feed grade amino acid, L-threonine (Thr), was aided by advances in ideal protein formulation tools and expressing amino acids on a digestible basis. Currently, L-valine (Val), the fourth limiting amino acid in most broiler diets devoid of meat meals, is being used.
Key Developments
The approach of U.S. nutritionists on the use of feed grade amino acids in least-cost poultry diets from 1950 to present shows that formulation adoption of a synthetic methionine source with crystalline lysine, threonine, valine, isoleucine, and arginine can allow for over a 50% reduction in soybean meal and over a 20% reduction in crude protein in a broiler grower diet. Production of poultry is projected to more than double from 82 to 181 million tons from 2005/2007 estimates to 2050, with significant increases in developing countries.
Current Research and Future Directions
Utilizing the ideal protein concept allows for precision feeding, requiring a keen understanding of dietary lysine needs over time and continuous assessment of additional amino acids. Recent low CP research indicates that moderate reductions in CP maintain performance and processing yields, while greater reductions inhibit performance and increase fat deposition. Reduced CP diets alter the proportion of corn, cereals, and feed-grade oils, leading to increased inclusion of crystalline amino acids and starch.
Intestinal uptake of glucose and amino acids are crucial to broiler performance. Starch and protein digestion, absorption of glucose and amino acids from the gut lumen, and their transition across enterocytes to portal circulation are essential considerations. Starch is more rapidly taken up than protein in broilers fed sorghum-based diets, and diets with decreased CP and increased starch can flood the small intestine with glucose, competing with amino acids for absorption.
Recent focus on amino acid digestibility in reduced CP diets shows increased ileal digestibility coefficients due to reduced endogenous amino acid flows. Reduced CP diets, supplemented with all essential amino acids, can increase intestinal permeability and reduce performance. Research is needed to determine the role of individual amino acids on gut health and overall bird performance.
Threonine and glycine linkages are vital in low CP diets, particularly in no-antibiotic and all-vegetable-based feed ingredients. Threonine is crucial for mucin integrity, which is essential for nutrient absorption and gut health. The interplay between glycine and threonine, especially under varying CP conditions, warrants further research.
Branch-Chained Amino Acids
L-Valine and L-isoleucine are increasingly used in broiler diets, but potential branched-chain amino acid (BCAA) antagonisms need to be addressed. Balance among the BCAAs is critical to avoid antagonistic effects, primarily mediated by leucine.
Phenylalanine and Tyrosine
Dietary levels of phenylalanine (Phe) and tyrosine (Tyr), previously of little concern, are now recognized as crucial for broiler performance and health. The interconnected relationship between Phe and Tyr requires careful consideration in diet formulation.
Research on histidine (His) responses and minimum requirements in least-cost formulation is needed. Histidine’s role in maintaining growth and carcass yield in low CP diets suggests unexpected yield losses at processing when His levels are marginal.
Glutamine and Intestinal Health
Understanding the bird’s intestinal amino acid needs in low CP diets is essential. Reduced CP diets, while increasing diet AME values and amino acid digestibility, may negatively affect gut health. Glutamine (Gln) supplementation can aid in mucosal repair and support gut integrity, suggesting that non-traditional levels of amino acids may be needed in reduced CP diets to maintain gut health.
Precision feeding using the ideal protein concept, with a focus on key amino acids, digestive dynamics, and gut health, is essential in developing sustainable low CP diets for broilers. Future research should continue to address the complexities of amino acid interactions, gut health, and performance outcomes in reduced CP dietary formulations.
