Modulating the Growth Curve in Broiler Chicken Through Varying Balanced Protein Regimen (Conventional All Step-down)

In comparison with a random bred broiler chicken strain from 1957, the growth rate has been increased 4 folds and FCR has been reduced to half at 42 day of age in modern broiler chicken (Zuidhof et al., 2014). The breeding companies develop new strains, approximately every 5 years with the updated nutritional requirements (Brown et al., 2022). The nutrient requirements of broiler chicken change with age; hence rebalancing the diets, in demand to meet the changing nutrient requirements of broiler chickens according to their age are inevitable (Taheri et al., 2020).
       
Feed costs about 70% of commercial broiler chicken production. Selecting the appropriate level of dietary amino acids, therefore, may be of critical economic decision since protein (amino acids) are costly nutrients per unit feed (Maharjan et al., 2021). Globally, digestible amino acids and ratios to lysine (Lys) are used extensively in formulating poultry diets. The digestible Lys level in diet for each stage of production is critical in setting the minimums for the other essential digestible amino acids in the ideal protein concept as a balanced protein (Kidd and Tillman, 2016). Utilizing the correct amount of balanced dietary AA for poultry is, therefore, a high priority issue due to its significant impact on broiler chicken growth and meat yield.
       
In phase-feeding (PF) strategy levels of dietary amino acids (AA) are reduced gradually over time in order to decrease feed cost linked with excess dietary AA supply (Emmert and Baker, 1997). Reduced protein contents, however, during starter phase had negative effects on production performance. It is critical to optimize the requirements of AA at different time periods in broiler chicken for efficient protein utilization as ever increasing protein ingredient prices urge the needs of optimum balanced protein (BP) concept to fulfill bird’s requirements avoiding under formulation or over formulation. Since over formulation is costly and under-formulation may result in suboptimal growth and meat production leading to less economic returns. Knowledge of the period in a broiler chicken’s life when amino acid needs are greatest is critical so that amino acid requirements patterns may be adjusted in an efficient manner. The present study was, therefore, designed to evaluate the performance of the broiler chicken by modulating growth curve through varying BP regimen under conventional all step-down phase feeding.

This trial was carried out according to the guidelines of Animal Care and Use Committee, University of Veterinary and Animal Sciences, Lahore, Pakistan. In total, 1750 one-day-old straight run broiler chicks (Hubbard) were allocated to 10 experimental treatments, replicated 7 times with 25 birds each, in a completely randomized design. Three isocaloric experimental diets (Table 1); a high BP (HBP) with 1.2% standardized Ileal digestible lysine (SID Lys), a medium BP (MBP) with 1.1% SID Lys and a low BP (LBP) 1.0% SID Lys, were offered in starter (1-10 days), grower (11-25 days) and finisher (26-35 days) phases, as HHH (HBP for 1-35d), MMM (MBP for 1-35d), LLL (LBP for 1-35d), HHM (HBP for 1-25d and MBP for 26-35d), HHL (HBP for 1-25d and LBP for 26-35d), MML, (MBP for 1-25d and LBP for 26-35d), HMM (HBP for 1-10d and MBP for 26-35d), MLL (MBP for 1-10d and LBP for 26-35d), HLL (HBP for 1-10d and LBP for 26-35d)  and HML (HBP for 1-10d, MBP for 11-25d and LBP for 25-35d under conventional all step-down phase feeding. The birds were reared in environmentally controlled house using rice husk as a bedding material and offered ad-libitum feed and water throughout the trial. The birds were immunized against Newcastle disease virus (NDV) (Ceva-Phylaxia, Budapest, Hungry) in drinking water at day 2^nd, 14^th and 28^th day of age. The chicks were vaccinated on day 2^nd and 14^th against infectious bursal disease virus (IBDV) (Lohman Animal Health GmbH, Cuxhaven, Germany). The recommended management conditions were followed as per guidelines of Hubbard broiler chicken management guide.
 

       
The data of growth performance including body weight (BW), body weight gain (BWG), feed intake (FI) and FCR were collected at the end of each phase. The nutrient compositions of the experimental diets were analyzed with the procedure described by Hussain et al., (2020). At the 35^th day of experiment, three birds from each pen were selected randomly and 5 ml blood was collected by puncturing wing vein, serum harvested that was used to determine the concentration of glucose, cholesterol (mg/dl) and antibody titers against NDV and IBDV by procedures described by Waqas et al., (2019). The total protein (g/dl) (Biuret Method cat # 157004) and urea (mg/dl) were also measured (Berthelot Method cat # 10505). The selected birds were slaughtered by Halal method and carcasses were processed to determine weights of dressed carcass, breast meat, abdominal fat, thigh and drumstick and immune organs including thymus, spleen and bursa of Fabricius (Rajkumar et al., 2021). The values were expressed as percentage of live BW. The breast samples were used to determine proximate composition including DM, CP, ether extract and crude ash with the procedure described by Gnanaraj et al., (2020).
       
Data were analyzed using analysis of variance (ANOVA) technique in a completely randomized design (Steel et al., 1997) using PROC GLM in SAS software (version 9.1; SAS Inst. Inc., Cary, NC). Means were declared statistically significant at P<0.05 and compared using Duncan Multiple Range (DMR) test (Duncan, 1955).

Growth performance
 
The impacts of the dietary treatments on growth performance are presented in Table 2. During starter phase, higher (P<0.05) average BW and BWG were observed in the HMM group. The average BW and BWG, however, were not influenced (P>0.05) by the dietary treatments in the grower and finisher phases. The FI was not affected (P>0.05) by the dietary treatments in the starter phase. The greater (P<0.05) FI, however, was observed in HHH and MLL groups in the grower and finisher phases, respectively. The HMM group had better (P<0.05) FCR in the starter, grower and finisher phases. These results are validated by the previous work in broiler chicken (Zarghi et al., 2020). The improved BWG in the broiler chicken fed HBP diets may partially be due to a higher FI resulting in more nutrients and fulfilling the energy needs (Nasr and Kheiri, 2011). Amino acids are major regulators of growth and protein metabolism. The broiler chicken fed diets deficient in Lys had reduced growth performance, protein accretion and breast muscle development (Tesseraud et al., 2009). It has been reported that increasing Lys (0.95 to 1.36%) improves FCR in broiler chicken (Tran et al., 2021).
 

 
Carcass characteristics and composition
 
The impacts of the dietary treatments on the carcass characteristics and composition are presented in Table 3. The higher (P<0.05) dressed weight was observed in HHM and HMM groups. The HHH group had heavier (P<0.05) breast and abdominal fat pad. The HHH group, likewise, had a higher (P<0.05) drumstick weight at par with HLL, HMM, MML and MMM groups. The HHH group had a greater (P<0.05) thigh weight at par with HHL and HMM groups. The HHH group had heavier (P<0.05) spleens, thymus and bursa of Fabricius. Meat production is the interaction of growth and feed efficiency. An adequate dietary level of limiting AA is needed to support optimum growth and carcass yield of fast growing broiler chicken (Handique et al., 2019). The diets with high Lys may affect carcass characteristics since Lys is an essential AA in protein synthesis. The results regarding carcass characteristics are in line with previous findings by Zarghi et al., (2020). These later authors reported that the increasing dig Lys levels (0.88, 0.94, 1.00, 1.06 and 1.12% dig Lys) in broiler chicken diets enhanced carcass and breast meat yield, whereas the abdominal fat was reduced.
 

       
The chemical composition of carcass including moisture, CP, crude and ether extract contents in the meat samples was not influenced (P>0.05) by the dietary treatments (Table 3).
 
Serum metabolites and antibody titers
 
The effects of the dietary treatments on the serum metabolites and antibody titers are presented in Table 4. The HHH group had higher (P<0.05) serum glucose, total cholesterol and urea concentrations. In agreement to current findings, it was reported that blood glucose was influenced by the supplementation of Lys above the recommended levels since it acts as a precursor of L-carnitine that has a glucogenic effect (Arslan, 2006). In contrast to the present findings, it was reported that cholesterol concentration was not influenced by Lys levels (Zhang et al., 2021). The greater total cholesterol concentration in the HBP fed group may be due to the availability of cholesterol in feed or synthesized in the body. The increase in cholesterol concentration in circulating blood could be due to endogenous synthesis of cholesterol in birds. The current blood urea findings are in contrast to those reported by Gong et al., (2005). The blood urea may be increased due to increasing CP levels in the diets.
 

       
The greater (P<0.05) antibody titers against NDV and IBDV were observed in HHH group. It has been reported that feeding of high density diets leads to greater development of immune organs (Attia et al., 2017). The greater weights of bursa of Fabricius in HHH groups in the current study might be the possible explanation of greater antibody titers since the bursa of Fabricius is the primary lymphoid organ responsible for the formation and conservation of the B cells in birds (Jankowski et al., 2014).

In conclusion, 1.2% SID Lys in the HBP diets improved the growth performance, carcass characteristics, selected blood metabolites and immunity in the broiler chickens. However, the nutrient density and phase feeding had no effects on carcass chemical composition.

None.