Amy F. Moss

Understanding the order of limiting amino acids (AA) in reduced-protein (RP) diets for laying hens will facilitate precise feed formulation and ensure that AA requirements are met cost-effectively. The order of the first three limiting AAs—lysine (Lys), methionine (Met), and threonine (Thr)—has been well established in RP laying hen diets. This study aimed to determine the priority order of eight additional limiting AAs (critically important AAs) when formulating wheat–sorghum-based RP diets for laying hens: tryptophan (Trp), valine (Val), isoleucine (Ile), arginine (Arg), leucine (Leu), histidine (His), phenylalanine (Phe), and glycineequivalent (Gly). A total of 330 Hy-Line Brown laying hens were randomly assigned to 11 dietary treatments (30 replicates of individual birds per treatment) from 20 to 39 weeks of age (WOA). Treatments were a standard-protein (17.24% CP) diet as the control (SP); a reduced-protein (15.00% CP) diet with sufficient levels of Lys, Met, and Thr but insufficient levels of the eight experimental essential AA (RP); a reduced-protein diet with sufficient levels of all essential AAs (RP-EAA); and eight subsequent dietary treatments of the RP-EAA diet with one of the experimental essential AAs removed: Trp (RP-EAA-Trp), Val (RP-EAA-Val), Ile (RP-EAA-Ile), Arg (RP-EAA-Arg), Leu (RP-EAA-Leu), His (RP-EAA-His), Phe (RP-EAA-Phe), and Gly (RP-EAA-Gly). Eggs were collected and weighed daily, and feed intake and feed conversion ratio (FCR) were calculated weekly. External and internal egg quality was measured at 29 and 39 WOA. Nutrient digestibility, serum uric acid concentration, caecal microbiota composition, and tibia parameters were measured at 40 WOA. Overall, hens fed the RP-EAA-Val, RP-EAA-Ile, and RP diets presented significantly lower egg mass compared to hens fed the SP, RP-EAA-His, and RP-EAA-Gly diets (p < 0.001). Hens fed the RP diet and RP-EAA-Val diet had a higher FCR compared to those offered the RP-EAA, RP-EAA-Leu, RP-EAA-Phe, and RP-EAA-Gly diets (p = 0.046). Lower protein intake and excretion were observed in hens offered the RP diets compared to hens fed the SP diet (p = 0.001 and 0.018, respectively). Based on the egg mass, Ile may be considered the fourth and Val the fifth limiting AA, after Lys, Met, and Thr, in laying hens fed wheat–sorghum-based RP diets during peak lay. However, if ranked based on FCR, Val may be considered the fourth limiting AA, followed by Trp, Ile, Arg, and His as the co-fifth limiting AAs. Leu, Phe, and Gly may be considered as non-essential AAs for laying hens fed RP diets.