In recent years, it is widely accepted that based on an ideal amino acid profile, a moderate reduction of dietary crude protein (CP), 20 to 30 g/kg, will not compromise broiler chicken performance.
These reduced CP levels are usually achieved by increasing feed grains such as wheat inclusion at the expense of soybean meal, relatively more starch replacing fat as the non-protein energy source. It is reported that increased non-starch polysaccharides (NSP) presented in wheat will enhance faecal bile acids loss, resulting in either increased bile acids pool or bile acids deficiency.
Traditionally bile acids are considered to be bio-surfactants, assisting intestinal digestion and absorption of lipids and fatty soluble vitamins such as Vitamin A, D, E and K, and thereby improving nutrient utilization. Recently it has been shown that bile acids act as regulators of gut microbiome and play a key role as signalling molecules by modulating cell proliferation, gene expression, lipid and glucose metabolism. To evaluate if the reduced CP diet could affect fat digestibility, the University of Sydney conducted a broiler chicken trial by adding the exogenous bile acids to both normal CP and reduced CP diets.
Standard, normal protein (SC) diets were formulated to meet or exceed the 2019 Aviagen Ross 308 nutrition specifications and reduced protein (RC) diets were formulated with a 3 g/kg CP reduction in the starter (0-14 d), grower (15-28) and finisher (29-42) phases. Another two treatments consisted of the SC and RC supplemented with the exogenous bile acids 200 mg/kg, respectively. All diets contained 1000 FTU phytase and 4000 U Xylanase.
Surprisingly, reducing 30 g/kg CP significantly depressed body weight gain and feed intake by 4.8 % and 2.8%, respectively (P < 0.01) although chicken performance fed on RP diets exceeded the Ross 2022 male performance objectives by 4.31% for weight gain (3361versus 3222 g) and was superior by 2.76% in FCR (1.475 versus 1.517). Figure 1 clearly showed that the current ideal amino acids profile resulted in significant lower plasma Valine, Isoleucine, Arginine, Leucine, tryptophan and histidine concentrations in RP diets, indicating that the current ideal amino acids profile could not support 30 g/kg CP reduction although it might achieve commercially satisfied performance.
Figure 1. Plasma AA concentrations in response to normal and reduced CP diets (log µg/ml)
Interestingly, the addition of the exogenous bile acids had no effect on crude fat digestibility but significantly improved crude protein and dry matter digestibility (Table 1) possibly due to reduced endogenous bile taurine loss. In Table 1, it is also shown that adding bile acids to the RP diet, significantly reduced mortality rate (P < 0.003).
Table 1. The effect of dietary treatments on the mortality and digestibility of fat, CP and dry matter (%)
In RP diets, due to other amino acids such as valine, isoleucine and arginine limitation, dietary methionine becomes more surplus but adding bile acids significantly increased plasma methionine concentration (Figure 2). Therefore, the efficacy of bile acids may be related to dietary methionine concentration which could convert to taurine. In general, in grower period (15-28), adding the exogenous bile acids to a RP diet achieved the best chicken performance and FCR was improved by 5 points (Table 2).
Figure 2. Plasma Methionine concentration in response to dietary protein and bile acids addition (µg/mL)
Table 2. The effect of dietary protein levels and bile acids on growth performance (15-28 days)