Introduction
To turn poultry farming into a more profitable business, producers use strains with high genetic potential, favorable diet nutrient density, and phase feeding regimes. It is well known that nutrient requirements change as broilers grow. Thus, feeding a single diet during the entire productive period may not satisfy bird requirements, thus it is necessary to use various feeding regimes (Belyavin, 1999; Mehmood et al., 2014).
In Iran, most broiler producers are interested in rearing the Ross 308 strain, and they arbitrarily follow the National Research Council (NRC, 1994) or the Ross 308 catalogue (Anonymous, 2007) recommendations to formulate the diets. Over a period of six weeks they normally apply two or three phase feeding programs when they use NRC (1994) and Ross 308 catalogue, respectively. Phase feeding is described as an economically feasible program in which diets are frequently changed to meet nutritional requirements at different ages (Emmert and Baker, 1997). There are other benefits of phase feeding such as lowering nitrogen excretion to the environment (Pope et al., 2004) and the possibility of producing under hot environments (Pope and Emmert, 2002).
According to some studies, feeding programs that include more phases can lead to better final body weight of broiler chickens (Mehmood et al., 2014; Hauschild et al., 2015). However, no effect of phase feeding on meat quality of birds has been reported (Brewer et al., 2012). Dozier et al. (2006) did not observe any significant differences in growth response and meat yield of broilers with a three-phase or a four-phase feeding program with high or moderate nutrient density. Improvement in growth performance of broilers has been reported with increased dietary apparent metabolizable energy (AME) and amino acids (Basurco et al., 2015), especially lysine (Lys) and sulfur-containing amino acids (SAAs; Kidd et al., 1998; 2004; Corzo et al., 2005). Warren and Emmert (2000) did not show any improvement in broiler performance when they divided the starter and finisher periods of NRC (1994) into three periods. It has also been determined that if broilers are fed a NRC (1994) starter diet for 30 d and a finisher diet for 18 d in a 48 d raising period they will have the best productive performance and carcass traits (Roush et al., 2004). However, few studies have compared different feeding standards. Yan et al. (2010) used four feeding standards with different nutrient recommendations, observing remarkable differences in broiler performance. On the other hand, such findings were not obtained by Trevisan et al.(2014) who used five different feeding standards. Thus, this study was conducted to compare the effect of NRC (1994) vs. Ross 308 strain feeding standards on growth responses, carcass traits, nutrient digestibility and economic efficiency of Ross 308 strain broiler chickens.
Material and methods
Birds and diets
A total of 300 one-day old straight-run Ross 308 strain broiler chicks were used. The chicks were distributed into 10 deep-litter floor pens with equal group weight, and were randomly assigned to one of two dietary treatments with five replicates of 30 birds in each. The iso-energetic experimental diets were formulated according to the National Research Council (NRC, 1994) or Ross 308 strain catalogue (Anonymous, 2007) recommendations, and were fed to chickens in two (1-21 and 22-42 days of age) or three (1-10, 1124, and 25-42 days of age) feeding periods, respectively. The chemical composition of the corn-soybean meal based diets are shown in Tables 1 and 2. The trial was conducted in an environmentally controlled room. Feed and water were provided ad libitum except at weighing times, when feed was withdrawn for 4 h to ensure the emptiness of digestive system contents (Friesen et al., 1992). Feed consumption and weight gain were measured at the end of each feeding period and feed conversion ratio was calculated. Mortality of birds in each pen was also recorded daily. Light was provided for 23 h/day, while temperature was gradually reduced from the initial 29 °C to approximately 21 °C by day 21, and was kept constant until the end of the growing period.
1Supplied per Kg of diet: vitamin A, 18,000 IU; vitamin D3, 4,000 IU;vitamin E, 36 mg; vitamin K3, 4 mg; thiamin, 3.5 mg; riboflavin, 13.2 mg; calcium pantothenate, 19.6 mg; niacin, 59.4 mg; pyridoxine, 5.88 mg; folic acid, 2 mg; vitamin B12, 0.03 mg; biotin, 0.2 mg; coline chloride, 500 mg; antioxidant, 2 mg;
2Supplied per Kg of diet: Mn, 198.4 mg; Zn, 169.4 mg; Fe, 100 mg; Cu, 20 mg; I, 1.98 mg; Se, 0.4 mg.
1Supplied per Kg of diet: vitamin A, 18,000 IU; vitamin D3, 4,000 IU; vitamin E, 36 mg; vitamin K3, 4 mg; thiamin, 3.5 mg; riboflavin, 13.2 mg; calcium pantothenate, 19.6 mg; niacin, 59.4 mg; pyridoxine, 5.88 mg; folic acid, 2 mg; vitamin B12, 0.03 mg; biotin, 0.2 mg; coline chloride, 500 mg; antioxidant, 2 mg;
2Supplied per Kg of diet: Mn, 198.4 mg; Zn, 169.4 mg; Fe, 100 mg; Cu,20 mg; I, 1.98 mg; Se, 0.4 mg.
Nutrient digestibility and apparent metabolizable energy (AME) determination
To measure digestibility of nutrients and AME of diets, two birds from each replicate (one male and one female) were placed in battery cages. After a one week adaptation period they were fed 0.3% chromic oxide as an indigestible marker in the diets from 40 to 42 days of age. Excreta samples were collected twice per day for three consecutive days and were frozen at -20 °C for subsequent analyses. Excreta samples were oven dried (60 °C, 72 h) and ground prior to analysis. Gross energy of milled feed and excreta samples was measured in an adiabatic calorimeter bomb (Parr 1341, Moline, Illinois, USA). Dry matter, organic matter and ether extract were determined using standard procedures of AOAC (2000), and the chromic oxide content of samples was measured according to the method by Fenton and Fenton (1979).
AME of the diets was determined according to Sibbald and Slinger (1963), and apparent digestibility (AD) of nutrients was measured according to Saha and Gilbreath (1993) using the following equations:
Carcass traits
At 42 days of age, two birds (one male and one female) from each pen (replicate) were selected, weighed and killed by decapitation. Birds were immediately dissected and eviscerated, then edible parts of the carcass including breast and thighs and digestive enclosed glands including liver and pancreas as well as abdominal fat were weighed.
Nutrient consumption and economic evaluation
Apparent metabolizable energy corrected to zero nitrogen (AMEn), crude protein, lysine and SAAs consumption per each Kg of live weight gain were calculated to evaluate the efficiency of dietary treatments. Survivability percent and feed cost per Kg of live weight gain were determined, and European efficiency factor (EEF) was calculated using the following equation:
Statistical analysis
Data were subjected to statistical analysis using the t-test procedure of SAS (SAS Institute, 2002) (Statistical Analysis System Institute, Cary, NC, USA). The significant differences between mean values was also examined by t-test (p<0.05). Percentage values were transformed to arcsine √n before statistical analysis.
Results
According to the data presented in Table 3, the Ross 308 strain recommended diet improved weight gain (WG) and feed conversion ratio (FCR) form 1 to 10 days of age without significant effect on feed intake (FI; p<0.001). The reason to report data in this period is feeding each group of chickens with their own starter diets. However, to compare the effect of feeding standards on growth response, data were reported according to NRC (1994) feeding periods (1- 21 and 22-42 days of age). During the starter period, birds fed the Ross 308 diet consumed more feed and had higher feed conversion ratio compared with those on the NRC 308 diet (p<0.05). Dietary treatments did not significantly affect WG. Through the grower period (22-42 days of age) and also the entire experimental periods, none of the growth parameters were affected by treatments, although WG of birds on Ross 308 diets tended to be significantly higher (p = 0.0627).
a ,b Mean values within rows with different superscript letters are significantly different (p<0.05).
The amount of AMEn, crude protein, Lys and SAAs consumed per Kg of live weight gain of broilers during different feeding periods showed that birds on the Ross 308 diets had higher energy, crude protein and Lys (p<0.05), as well as SAAs (p<0.0001) consumption when compared with those on the NRC (1994) diets from 1 to 21 days of age. However, similar results were observed, except for SAAs, from 22 to 42 days of age and also throughout the whole experimental period (Table 4).
a,bMean values with different superscript letters within rows are significantly different (p<0.05).
†Sulfur containing amino acids.
As shown in Table 5, at the end of 42 d growing period, the relative weights of dressed carcass, breast and thigh meat as well as abdominal fat and enclosed glands of digestive system (including pancreas and liver) were not affected by the dietary treatments.
Both NRC (1994) and Ross 308 strain recommended diet had similar AME and apparent digestibility of dry matter, organic matter, and ether extract (Table 6). Feeding the dietary treatments to birds during the 42 d growing period also resulted in similar cost of FI per Kg of live BW, survivability rate, and European efficiency factor (Table 7).
Discussion
Although in both feeding standards (NRC (1994) and Ross 308 strain catalogue), the requirement for crude protein and amino acids decreased with increasing age, nutrient density recommended by both standards was different, and nutrient density of the diet formulated according to Ross 308 strain catalogue was higher than that of diets formulated according to NRC (1994) recommendations, as shown in Tables 1 and 2. It is evident that broiler growth performance improves when nutrient density of diet increases (Corzo et al., 2005; Dozier et al., 2006; Brickett et al., 2007; Yan et al., 2010). Furthermore, it has also been reported that WG and FCR is affected by decreasing crude protein (Bregendahl et al., 2002; Si et al., 2004; Faria Filho et al., 2005; Hernández et al., 2013). In most studies, improvement of growth performance due to high protein has been more pronounced in young broilers (Bregendahl et al., 2002; Faria Filho et al., 2005; Yan et al., 2010). In agreement with those studies, in the current study, WG (p<0.001) and FCR (p<0.001) improved during 1 to 10 days of age by feeding the Ross 308 diet as well. However, by decreasing dietary crude protein from 23.174 to 20.486% in the grower period of Ross 308 strain (Table 2), such improvement was not observed. Apparently, if that reduction in dietary crude protein did not happen or was less than that, the improvement in growth performance would be followed. But, because of a direct relationship between crude protein intake and nitrogen excretion by broiler chickens (Faria Filho et al., 2005; Hernandez et al., 2013) this suggestion will be questioned in terms of environmental considerations.
It is well known that broilers consume the diet to meet their need for energy (Pesti and Smith, 1984; Plavnik et al., 1997). In this study, as diets were formulated iso-energetically, the probable reason for higher FI and FCR with the Ross 308 diet (during 1-21 days of age) is an inadequate supply of nutrients such as amino acids. A 2.6% decrease in crude protein and similar reduction in amino acid levels when compared with the starter diet of Ross 308 strain may cause such higher feed intake, as birds had to consume more feed to meet the requirements and maintain their growth pattern at the starter period. It has been shown by Lipstein et al. (1975) that broiler chickens eat the amount of feed to meet their requirement for protein and/or essential amino acids.
Furthermore, in the study by Kidd et al. (2004), when amino acid density of broiler diet was lowered from 15 to 28 days of age, FCR was affected, similar to our findings. It is remarkable that chickens on the Ross 308 diet consumed the amount of feed to have the same energy to protein ratio as those on the NRC 308 diet (13.91 Kcal/g vs. 13.98 Kcal/g, respectively; Table 4). No significant difference (p>0.05) in growth parameters was observed. Such finding has been reported by Dozier et al. (2006), who used three or four feeding phases and did not show any significant differences in final body weight of birds. It has also been shown that using five different feeding standards to feed broilers for 49 days resulted in similar WG, FI and FCR (Trevisan et al., 2014).
The higher consumption of energy and nutrients through 21 days by birds on the Ross 308 diet as compared with those on the NRC (1994) diet was due to significantly higher FI in this period (Table 3).
Although chickens on the Ross 308 diet had higher SAAs consumption/Kg of live BW for the 22-42 days period and also throughout the experimental period, their growth performance and carcass, breast and thigh meat yields was similar to those fed diets formulated according to NRC (1994) recommendations (Table 5). In agreement with these results, Si et al. (2004) did not observe any improvement in growth performance by increasing the SAAs content of the diet.
The finding on abdominal fat was not in agreement with other reports (Kidd et al., 2004; Corzo et al., 2005), which noted that high amino acid content in the diet induces lower abdominal fat in chickens.
In conclusion, as birds on the NRC (1994) or the Ross 308 catalogue diets had a similar growth response at the end of the feeding trial and since there was no significant difference in economic efficiency between both standards, none of them appeared to have any advantage over the other. However, due to fewer phase feeding of the NRC (1994; 2 vs. 3) and higher SAAs consumption of the Ross 308 diets per Kg of live weight gain, feeding of Ross 308 chickens with the NRC (1994) recommended diets is suggested.
Acknowledgements
The financial support of the University of Mohaghegh Ardabili (Iran), to carry out this study is acknowledged by the authors.