Slower Growing Broilers Showed Higher Appetite for Alanine

A balanced broiler chicken diet requires essential (EAA) and non-essential (NEAA) amino acids to attain maximum growth. However, the optimal ratio of these two dietary AA groups may change with different growth rates but this is not fully understood (Heger, 2003). In addition, commercial broiler feeds are formulated based on average flock requirements of EAA, hence neglecting NEAA ratios and potential individual variations. Consequently, it is expected that commercial broiler diets for slower growing birds may contain excess EAA relative to NEAA, and vice versa for fast growing birds. Reportedly, when given adequate choices, chickens have the ability to select diets based on their nutritional requirements, particularly AA (Zuberbuehler et al., 2002). Therefore, we hypothesized that in an attempt to optimize their EAA/NEAA, slow and fast growers fed a commercial balanced diet will show higher specific appetites for NEAA than EAA.

A two-choice feeding regime was developed to test AA preferences in chickens, consisting of offering two identical feeders: one contained standard feed and the other standard feed supplemented with AA mix. Four treatments were used: T1 a control with no supplement (standard feed in both containers or feed: feed); T2 with an EAA mix (feed: feed + Lys, Met, Thr), T3 with a NEAA mix (feed: feed+ Ala, Asp, Asn), and T4 with a non-limiting EAA mix (feed: feed + Cys, Ser, His). These treatments were tested using 48 slow and 48 fast growers during two weeks (week 5 and 6). In addition, we compared the gene expression and protein synthesis profiles in proventriculus and duodenum between 6 slow and 6 fast growers selected from a flock of 600 at week 6.

In a two-choice model, if the bird consumes the same amount from each feeder, the preference value is 50% or not significantly different from 50% in a t-test. Results showed that the preference for NEAA (Ala, Asp, Asn) was significantly higher than 50% neutral value in slow growers (57.4%; P<0.05) but not in fast growers. In contrast, the preference for EAA (Lys, Met, Thr) was significantly lower than 50% (35.1%; P<0.05) in the same group. In addition, slow compared to fast growers showed a significantly (P<0.05) lower EAA/NEAA ratio (0.577 vs 0.595, respectively). The analysis of gut-expressed genes revealed that slow growers had an increased rate of Ala catabolism. α-gustducin, which is downstream signalling of Ala sensor in chickens (T1R1/T1R3), and AA transporters (SLC38A1 and SLC1A2) were downregulated in slow growers (P<0.05) suggesting a potential lower uptake of the NEAA tested from the gut content. In addition, the proteomic analysis showed a low level of glucose transporters (SCL2A2 and GLUT4) in slow compared to fast growers (P<0.05). Furthermore, the glycolysis rate in slow growers was downregulated. All of the glycolytic pathway described above resulted in a low production of pyruvate, the only substrate for biosynthesis of Ala in the cell (hence the main reason why Ala is a NEAA). In conclusion, Ala catabolism in slow growers is upregulated, whereas Ala biosynthesis, sensing, and transporting were downregulated. This contributed to develop an appetite and higher consumption of Ala in slow growing birds.