Dietary protein levels influenced the ability of the native chicken considerably in moderating their body weight gain. It could be seen from this experiment that increment in the protein level above the recommended level (20%) for commercial layer chicks increased body weight gain linearly up to CP level of 21.0% but not beyond. The increase in body weight could be due to better assimilation of protein from the diet utilizing the available energy that was kept constant (iso-caloric) at 2800 kcal/kg of feed. However, the birds registered a decrease in body weight gain in 21.5% CP group (1210.63 g) by a margin of 74.22 g (5.77%) over the CP 21.0% group. Possible reasoning for the reduction in the body weight gain in the group of birds with the highest CP (21.5%) might be due to unutilized excess protein due to non-availability of energy for this biochemical process as the ME used in this experiment was fixed (2800 kcal/kg of diet) and might be also due to sparing of energy to the process of excretion of nitrogenous compounds (NRC, 1994 and Barzegar, et.al., 2019). As observed in an 8-week growth study on Aseel chicken where Haunshi et al., (2012) reported no significant increase in the BWG by increasing the CP level from 16 to 20% pointing to slow growing character of Aseel chicken (10 g/day), this slow growing character of Aseel could be another possible reason for the birds not showing further weight gain upon extending the protein level beyond 21.0%, in our study. However, the present work showed better BWG in higher CP level (21.0%) with 11.47 g/day of daily gain, slightly higher to the level reported by Haunshi et al., (2012). Miah et al. (2016) used single CP level of 23% for an indigenous chick variety with two energy levels of 2800 and 3000 kcal/kg up to 14 weeks and reported a body weight of 758 and 768 g respectively with 7.74 and 7.84 of ADG (g) which were lower than the values recorded in our study. Perween et al. (2016) found 21.0% CP rather than 19.0%, had a positive effect on body weight (1403.60 g) in fast-growing dual-purpose breed Vanaraja at 8th week which required higher energy of 3000 kcal ME/kg. In another study with an improved chicken Rajasri, Deepak et al. (2017) reported higher BWG when dietary CP was increased from 16 to 20.0% in 8 weeks of age which had similar ME (2800 kcal/kg) followed in our dietary treatments. However, Batool et al. (2018) revealed that Mushki Aseel chicken required 17.29% CP with 1.30% lysine and 2760 kcal ME/kg to show an improved growth performance in body weight gain (372.15 g) from 0 to 6 weeks of age. Though there was variation in the body weight gain in different native chickens reported earlier, it could be noted that 21.0% CP was required for maximum performance in terms of body weight gain in Aseel type of chicken with 2800 kcal/kg for meat purposes when reared intensively.
The group with highest CP of 21.5% consumed numerically higher feed (5427.30 g) but resulted in lower body weight gain which may be clarified by a possible explanation that excretion of excess protein is an energy consuming metabolism which might be a possible reason for the birds to consume extra feed in an attempt to satisfy the energy requirement. Thus, the reason for the lower body weight gain in the end of the experiment could be the direct result of an imbalance in protein: energy ratio in the feed. However, there was no significant difference in feed intake among all the treatment groups. Such non-significant difference in feed intake due to varying protein levels was also reported earlier in native chickens (Elangovan et al., 2004; Haunshi et al., 2012; Chandra Deo et al.,2014; Liu et al., 2014; Hidayat et al., 2016 and Kamble et al., 2019) experimented with protein levels from 12 to 20%. However, Perween et al. (2016) observed a significantly lowered feed intake due to change in dietary protein levels with 19.0 than 17.0% CP in Vanaraja chicken.
As FE is a mere reflection of ratio of feed intake (g) to body weight gain (g), it was clearly seen that decrease in CP below the basal diet of 20.0% resulted in reduced FE (4.33–4.42) while the opposite was true in high CP fed groups (4.33–4.06); the positive effect was observed only up to CP of 21.0% (4.06). Earlier, Mandal et al. (2016) also reported an efficient feed conversion ratio of 3.55, 3.69, and 3.88 when fed with 20, 18 and 16% CP diet, respectively, to Aseel chicks (0–8 weeks). However, Haunshi et al. (2012) observed protein efficiency was better at 16% followed by 18 and 20% CP during the juvenile phase (0–8 weeks) of Aseel chicks.
In the present study, the PER, at end of 16 weeks in Aseel chicken, was significantly (P < 0.001) influenced on changing the protein level. It showed an increasing trend with an increase in protein level, but the increase was recorded only up to 21% level (1.05 to 1.25). It was evident that 21.5% CP diet did not improve the FE and PER values in native chickens with 2800 kcal ME/kg (FE, 4.48 and PER, 1.04).
Dressing per cent was influenced significantly by dietary protein levels in Aseel chicken. The maximum dressing per cent (70.61) was observed in the 21 CP % fed group and the lowest (68.07) was observed in the 19.0% CP group. Research works on influence of protein level on the dressing yield were scanty in indigenous breeds. However, Singh and Pathak (2016) reported lower dressing per cent (61.97) in Aseel birds at the age of six weeks.
There was not much influence on the protein content of feed on organ and abdominal fat per cent. Interestingly, Rabie et al. (2017) reported a significant decrease in abdominal fat per cent with increased dietary protein levels (from 18 to 22%) in broilers chicken which is a fast-growing meat bird with 57–60 g ADG which happens to be more than 5 times of indigenous breed’s ADG.
This study shows that the MSTN gene expression level is inversely proportional to the BWG, carcass weight and dressing per cent in Aseel chickens, as seen from the trend line (Fig. 2). It can be noted that the relative expression of MSTN gene was up-regulated in the low protein groups and down–regulated in high protein groups. The down-regulation was highest in 21.0% protein group. Bhattacharya and Chatterjee (2013) and Yang et al. (2013) also revealed a negative correlation between muscle mass and elevation of MSTN gene expression in broilers. Earlier, Yang et al. (2009) recorded elevated MSTN gene expression and decreased muscle yield in broilers fed with reduced protein (23 to 18.4% CP) diets and reduced energy (13.4 to 12 MJ ME /Kg) levels as well.