Effects of Branched-Chain Amino Acids on Growth Performance in Broiler Chickens Offered Reduced Crude Protein Diets Based on Wheat or Sorghum

The development of reduced-CP diets in broiler chickens is gaining more interest as it would drastically lessen the dependence on imported soybean meal which is strategically important for non-soybean producing regions such as Australia and Europe. Modest reductions in CP are already being realised by inclusions of unbound (synthetic or crystalline) methionine, lysine and threonine, which have been routinely included in poultry diets for decades (Kidd et al., 2013). There is the potential to reduce dependence on soybean meal by up to 50% as nonbound (synthetic, crystalline) amino acids are effectively an alternative ‘protein’ source to soybean meal (Selle et al., 2020a). The branched-chain amino acids (BCAA), isoleucine, leucine and valine, merit attention because valine or isoleucine is probably the fourth limiting amino acid in typical broiler diets and leucine is involved in metabolic and physiological roles in addition to protein accretion (Duan et al., 2016). Therefore, the objective of this experiment was to compare growth performance in broiler chickens offered moderately reduced CP diets based on wheat or sorghum which are the most common feed grains in Australia.

This study fully complied with the guidelines (2019/1667) specifically approved by the Research Integrity and Ethics Administration of The University of Sydney. The experimental design consisted of a 2 × 3 factorial array of dietary treatments, which were offered to 216 offsex male Ross 308 broiler chickens from 7 to 28 days post-hatch. The dietary treatments comprised of two feed grains, wheat and sorghum and three regimens of branched-chain amino acid inclusions as shown in Table 1. All six diets were formulated to contain 187.5 g/kg CP, 11.0 g/kg digestible lysine, 20.0 g/kg glycine equivalents, 12.97 MJ/kg metabolisable energy with a constant dietary electrolyte balance (DEB) of 200 mEq/kg. The composition of diets is shown in Table 2. Phytase and xylanase were supplemented in all dietary treatments. Growth performance was recorded and dead or culled birds were weighed to adjust feed intake and FCR. The two-way ANOVA of experimental data was carried out by IBM® SPSS® Statistics 24 program (IBM Corporation. Somers, NY). Experimental units were cage means (6 replicate cages of 6 birds per dietary treatment) and probability levels of less than 5% were considered statistically significant by Student’s t-test.

The effects of dietary treatments on growth performance and relative abdominal fat-pad weights are shown in Table 3. The overall 3.09% mortality/cull rate was not related (P > 0.15) to treatment. There was a highly significant (P < 0.001) interaction between feed grains and BCAA for weight gain. The transition from standard to elevated BCAA inclusions depressed weight gain by 9.49% (1288 versus 1423 g/bird; P < 0.001) in birds offered wheat-based diets; in contrast, the same transition enhanced weight gain by 9.26% (1451 versus 1328 g/bird; P < 0.001) in birds offered sorghum-based diets. Feed grain had a significant (P < 0.001) effect on feed intake where wheat-based diets generated 9.74%. higher feed intake (2141 g/bird) compared to sorghum-based diets (1951 g/bird). A significant (P < 0.014) treatment interaction was also observed for FCR. Elevated BCAA inclusions compromised FCR by 8.33% (1.665 versus 1.537) in birds offered wheat-based diets but, in contrast, it did not have any impact on sorghum-based diets. Feed grains had a significant (P < 0.001) impact on relative fat-pad weights. Birds offered wheat-based diets had fat-pad weights of 5.68 g/kg whereas birds offered sorghum-based diets had substantially relative heavier fat-pad weights of 13.48 g/kg. The underlying mechanism for such differences is unclear; however it could be suggested that the fatty acid composition difference between the grains in conjunction with the added dietary oil may influence fat deposition through an imbalance of saturated fatty acids such as palmitic acid (PA) to polyunsaturated fatty acids (Arroyo et al., 2013; Cherian et al., 2002; Carta et al.,2017). In a similar preceding study (Greenhalgh et al., 2021), leucine concentrations were elevated from 110 to 150 relative to lysine in 190 g/kg CP, wheat-based diets. This transition numerically increased feed intake by 1.05% (2109 versus 2087 g/bird) but numerically compromised weight gain by 2.52% (1390 versus 1426 g/bird) and FCR by 3.28% (1.512 versus 1.464) from 7 to 28 days post-hatch. In the present study, the same leucine elevation significantly decreased weight gain by 5.62% (1343 versus 1423 g/bird) in wheat-based diets. In contrast, high leucine in sorghum-based diets significantly increased weight gain by 9.26% (1451 versus 1328 g/bird) in the present study. Thus, the findings in wheat-based diets were somewhat similar in both the previous and present studies but differed with the outcome in sorghum-based diets. However, positive responses to elevated leucine levels in maize-based diets have been reported (Yamazaki et al., 2006; Chen et al., 2016). Overall elevation in sorghum-based diets are beneficial compared to wheat based diets, highlighting the importance of the feed-grain in reduced-CP diets, albeit in the context of differing BCAA inclusion regimes.

ACKNOWLEDGEMENTS: The authors would like to acknowledge the financial support and guidance of The AgriFutures Australia Chicken Meat Program and the technicians that have contributed to the data used to produce this paper.