Order of Limiting Amino Acids in Wheat-Sorghum Based Reduced Protein Diets for Laying Hens

Reduced protein (RP) diets have received increasing interest in poultry nutrition due to the potential benefits in feed cost and environmental footprint (Liu et al., 2021). Understanding the order of limiting amino acids (AA) in RP diets is critical to ensure requirements are met cost effectively. To date, only the first three limiting AA (including lysine [Lys], methionine [Met], and threonine [Thr]) have been demonstrated in RP layer diets, while the order of the next most limiting AA is still controversial. Thus, it is unclear which AA should be given priority when formulating RP diets. Therefore, the limiting order of eight crystalline AA in wheat-sorghum-based RP laying hen diets were determined.

A total of 330 Hy-Line Brown laying hens were randomly assigned to 11 dietary treatments (30 replicates of a single bird per treatment) from 20 to 39 weeks of age. The first treatment (control diet) was a standard protein (SP, 17% crude protein) diet with a sufficient level of all AA and other nutrients according to the nutritional recommendations of Hy-Line Brown laying hens. The second treatment was an RP diet (15% crude protein) containing sufficient levels of Lys, Met, and Thr and deficient levels of eight essential AA (EAA) including tryptophan (Trp), valine (Val), isoleucine (Ile), arginine (Arg), leucine (Leu), histidine (His), phenylalanine (Phe), and glycine_equivalent (Gly). The third treatment contained sufficient levels of Lys, Met, and Thr and sufficient levels of eight essential AA (EAA). Then, each of these eight EAA was individually deleted from the RP-EAA diet by removing its supplemental level to generate treatments 4 to 11, following the deletion method described by Fernandez et al. (1994). Mash feed and water were provided ad libitum throughout the study. Eggs were collected and weighed daily, feed consumption recorded weekly and feed conversion ratio (FCR) calculated. Egg quality traits including egg shape index, eggshell reflectivity, eggshell breaking strength, eggshell thickness, albumen height, yolk index, yolk colour, and Haugh unit were measured at 29 and 39 weeks. Data were analysed using IBM SPSS statistical software with significance at 0.05.

The limiting order of AA were ranked based on egg mass and FCR, due to the commercial importance of these parameters. Hens fed the RP diet and RP diet deficient in Val or Ile had significantly lower egg mass (mean ± SEM: 53.7 ± 0.28 g, 54.0 ± 0.28 g, and 53.9 ± 0.28 g, respectively) compared to hens fed the SP diet (mean ± SEM: 57.5 ± 0.28 g) and RP diets deficient in His (mean ± SEM: 57.7 ± 0.28 g) and Gly (mean ± SEM: 58.4 ± 0.28 g) (P < 0.001). Moreover, hens fed the RP diet and RP diet deficient in Val had a higher FCR (mean ± SEM: 2.38 ± 0.01 and 2.33 ± 0.01, respectively) compared to those offered the RP-EAA diet (mean ± SEM: 2.20 ± 0.01) and RP diets deficient in Leu (mean ± SEM: 2.21 ± 0.01), Phe (mean ± SEM: 2.21 ± 0.01) and Gly (mean ± SEM: 2.19 ± 0.01) (P < 0.05). FCR was not affected by deficiency of Trp, Ile, Arg and His in RP diet (mean ± SEM: 2.26 ± 0.01, 2.31 ± 0.01, 2.26 ± 0.01, and 2.27 ± 0.01, respectively) compared to SP diet (mean ± SEM: 2.24 ± 0.01) (P > 0.05). Egg quality traits were not different between the dietary treatments at both 29 and 39 weeks (all P > 0.05).

Thus, for egg mass, Ile was the fourth and Val the fifth limiting AA after Lys, Met and Thr in laying hens fed on wheat-sorghum based RP diets. For FCR, Val was the fourth limiting AA and Trp, Ile, Arg and His may be considered as co-fifth limiting AA in wheat-sorghum based RP diets for laying hens. Overall, it may be concluded that Val and Ile are the most important AA, after Lys, Met and Thr; while Leu and Gly are the least important AA, in wheat-sorghum based RP diets for laying hens.

ACKNOWLEDGMENTS: The authors would like to acknowledge and thank Poultry Hub Australia for funding this project and their guidance, encouragement and support.