In recent years, reducing crude protein (CP) in poultry diets has gained considerable attention as a strategy to lower feed costs and mitigate environmental impacts. Escalating prices of protein-rich ingredients, particularly soybean meal, have intensified the need for alternative feeding approaches, leading to renewed interest in reduced protein (RP) diets in modern egg production systems. Reduced protein diets are defined by a reduction in dietary CP while maintaining essential amino acids (AA) levels, primarily through partial replacement of soybean meal with crystalline AA. The effectiveness of RP diets depends on precise formulation, particularly the inclusion of essential AA such as arginine (Arg), which is critical for laying hens. Beyond AA supplementation, nutrient digestibility is a critical determinant of the efficiency of RP diets. Exogenous enzymes, particularly phytase, are widely used to enhance nutrient availability and AA utilisation, although much of the supporting evidence is derived from broiler studies. Similarly, carbohydrase enzymes, including xylanase and β-glucanase (XB), are incorporated into wheat-based diets to degrade nonstarch polysaccharides (NSP), thereby improving nutrient utilisation and gut health. However, their efficacy in laying hens remains inconsistent, highlighting the need for further investigation under RP feeding conditions.
To address these knowledge gaps, this thesis explores these areas over a general introduction (Chapter 1), a literature review (Chapter 2), and a series of three trials, comprising chapters 3, 4, and 5, and finally, a general conclusion (Chapter 6). Chapter 3 investigated the optimal inclusion levels of phytase and XB in RP diets for hens during the post-peak laying phase from 35 to 57 weeks of age. The study included eight dietary treatments comprising two CP levels (standard protein [SP; 16.5% CP] and RP [14.5% CP]), two phytase inclusion levels (60 and 120 g/ton, equivalent to 600 and 1,200 FTU/kg, respectively), and two XB inclusion levels (100 g/ton, providing 1,220 U/kg xylanase and 152 U/kg β-glucanase; and 150 g/ton, providing 1,830 U/kg xylanase and 228 U/kg β-glucanase). The results of the study in chapter 3 demonstrated significant improvements in apparent energy digestibility (P = 0.003) and protein digestibility (P = 0.013), along with a marked reduction in protein excretion (P < 0.001), indicating enhanced nutrient utilisation and reduced environmental impact.
Building on these findings, Chapter 4 examined the role of functional Arg sources in RP diets for older laying hens from 60 to 75 weeks of age. Eight dietary treatments were evaluated, including SP (15.8% CP), RP (13.8% CP), and six groups of RP diets supplemented with two inclusion levels (0.06% and 0.12%) of Arg, guanidinoacetic acid (GAA), or citrulline (Cit). The results of the study in chapter 4 showed that a moderate CP reduction of two percentage points, combined with higher Cit supplementation (0.12%), improved egg mass, egg weight, yolk weight, and bone strength, highlighting the importance of Cit supplements in maintaining performance under RP conditions. Finally, Chapter 5 focused on optimising AA balance and energy to protein ratios in RP diets. Nine dietary treatments were evaluated, consisting of three CP levels (17%, 15.5%, and 14%) and three dietary energy levels (90%, 95%, and 100% of the recommended apparent metabolizable energy (AME), corresponding to 2453, 2589, and 2725 kcal/kg, respectively). The results of the study in chapter 5 found that reducing CP from 17% to 15.5% while maintaining 100% AME improved feed efficiency and preserved egg quality in hens between 20 and 35 weeks of age. Collectively, these studies explored three important aspects of challenges within RP diets and determined the effects of enzyme supplementation, supplemental arginine alternatives, and optimal energy to protein ratios. This holistic strategy offers a practical pathway to maintain productivity while improving nutrient efficiency and decreasing nitrogen excretion, thereby potentially reducing the environmental footprint of egg production.