Values of Metabolizable Energy and Metabolization of Nutrients for Slow- and Fast-growing Birds at Different Ages

Two trials were performed to determine apparent metabolizable energy (AME) and AME corrected for zero nitrogen balance (AMEn) values and coefficients of apparent metabolizability of nitrogen (CAMN) and of ether extract (CAMEE) of slow- and fast-growing broilers (Isa label and Cobb, respectively) fed four different diets between 10 to 17 days or 28 to 35 days of age. The method of total excreta collection was applied. The tested feedstuffs were corn, whole corn germ (WCG), sorghum, and soybean meal (SBM). A randomized block experimental design was applied, with two treatments (breeds) and four replicates of ten animals each in the first trial, and four replicates of six animals each in the second trial. No differences in CAMN values were observed between the breeds; however, Isa Label birds presented higher CAMEE for all tested feedstuffs. The AME and AMEn values obtained in Isa Label chickens fed the corn diet were 5.75 and 3.44% higher relative tothe Cobb birds, respectively. Breed did not influence AME and AMEn values of the other tested feedstuffs. Independently of breed, age influenced the AME and AMEn values of WCG, sorghum and SBM, suggesting that birds become utilize feed more efficiently as they age.


INTRODUCTION
In order to formulate economically viable feeds, it is important to know the nutritional value of feedstuffs, which requires determining their chemical composition, nutrient availability, and energy content and utilization by a given species (Generoso et al., 2008).Although feedstuffs are often evaluated for fast-growing chickens, few studies have been performed to determine feed nutrient utilization by slowgrowing chicken strains reared in alternative animal production systems.
Additional studies are required to elucidate nutrient utilization of protein-and energy-rich feeds used in slow-growing broilers, particularly because bird age, which is associated with the development of the gastrointestinal tract, may interfere with the nutritional evaluation of feeds, as mentioned by Calderano et al. (2012).
According to Mendonça et al. (2008), slow-growing broilers show increased energy metabolization capacity as they age.Therefore, the use of values of apparent metabolizable energy (AME) corrected for zero nitrogen balance (AMEn) derived from published tables to assess feeds may overestimate energy utilization by those birds during the early stages of life and underestimate it at older ages.
Considering that the growth rate of Isa Label chickens is slower than that of fast-growing broiler strains, their rates of digestive tract growth and enzyme production may be different from those of broilers selected to fast growth.This difference in gastrointestinal development may influence nutrient utilization, which has implications for the

Values of Metabolizable Energy and Metabolization of Nutrients for Slow-and Fast-growing Birds at Different Ages
formulation of feeds for slow-growing chickens, which feeds are commonly formulated based on nutritional data derived from fast-growing breeds.
The current study was performed to determine the chemical composition, coefficients nutrient metabolization, and levels of apparent metabolizable energy and apparent metabolizable energy corrected for nitrogen of selected feedstuffs included in the diets of slow-and fast-growing broiler chickens at different ages.

MATERIALS AND METHODS
Two metabolism assays were performed in the poultry sector of the Federal Institute Goiano (Instituto Federal Goiano),Rio Verde campus, state of Goiás, Brazil.This project was approved by our institutional ethics committee for animal research under the protocol number 012/2012.

Experimental Design, Animals, Housing and Diets
The assays evaluated a broiler strain selected for slow growth rate (Isa Label) and one selected for high growth rate (Cobb) during the periods of 10 to 17 days and 28 to 35 days of age.
Energy values and coefficients of apparent metabolizability of nutrients were determined for each evaluated feedstuff.A randomized block design was applied, in which each cage represented one block with two treatments (breeds).During the first trial, there were four replicates of 10 birds each, and during the second trial, four replicates of six birds each.Data were submitted to joint analysis of variance to verify the effect of bird age on the evaluated variables.
In order to determine the coefficients digestibility and metabolizable energy levels, a reference diet was formulated, and the four test feeds were obtained by substituting 40% of the reference feed by the tested feedstuffs.Corn, sorghum, soybean meal (SBM) and whole corn germ (WCG) were evaluated.Nutrient utilization was determined in four groups of 10 birds of each breed that were fed the reference diet.The ingredient composition and calculated nutritional values of the reference diet are presented in Table 1.
Birds were housed in galvanized-wire cages measuring 0.90 m x 0.60 m x 0.45 m.In the first experiment (10 to 17 days of age), 200 male broiler chickens from each breed, with initial average bodyweights of 130.33 ± 6.51 gand202.69 ± 10.13 g for the Isa Label and Cobb breeds, respectively, were evaluated.In the second experiment (28 to 35 days of age), 120 chickens from each breed, with initial average body weights of 632.0 ± 31.58 g and 1055.0 ± 50.26 g for the Isa Label and Cobb breeds, respectively, were used.
Cages were equipped with trough feeders and nipple drinkers.Water and feed were provided ad libitum for the entire experimental period.The feeders were refilled twice a day to avoid waste. 1 Compostion per kg of the product: manganese -75,000 mg; iron -20,000 mg; zinc -50,000 mg; copper -4,000 mg; cobalt -200 mg; iodine -1,500 mg and carrier q.s.p. -1,000 g.

Experimental Procedure and Chemical Analysis
Energy values and coefficients of apparent metabolizability of nitrogen (CAMN) and of ether extract (CAMEE) were determined using the method of total excreta collection (Sakomura & Rostagno, 2007).Birds were submitted to four-day adaptation period of to the cages and to the experimental diets before the period of four days of excreta collection, performed daily at 8:00 am and 3:00 pm.An aluminum tray lined with a plastic sheet was placed under each cage for excreta collection.
Excreta samples, tested feedstuffs, and experimental feeds were analyzed in the laboratory of Animal Nutrition of the Federal Institute Goiano to determine dry matter (DM), nitrogen (N), and gross energy (GE) contents.Tested feedstuffs were also analyzed for crude fiber (CF), ether extract (EE) crude protein (CP), ash (A), and nitrogen-free extract (NFE) contents according to the protocol of Silva & Queiroz (2002).Total amino acid levels of the feedstuffs were analyzed by high-performance liquid chromatography (HPLC) at ADISSEO, a company that provides technical animal nutrition support.

Calculation of Variables
The variables AME and AMEn were calculated using the equations described by Matterson et al. (1965).The following equations were applied to calculate CAMN and CAMEE: CAM of a nutrient (%)= Nutrient intake -Nutrient excretion x 100 Nutrient intake The AME, AMEn, CAMN, and CAMEE values of the experimental diets were evaluated for each rearing period.

Statistical analyses
To verify the effect of bird age on the coefficients of nutrient metabolizability, and AME and AMEn values were assessed by conjunct analysis of variance using the Sisvar software (System for Analysis of Variance, version 5.3, Ferreira et al., 2011).The differences between breeds and age means were determined by the Tukey's test at 5% probability level.

RESULTS AND DISCUSSION
Small variations were observed in the energy content of the tested feedstuffs (Table 2) relative to literature reports (Brito et al., 2005;Generoso et al., 2008;Mello et al., 2009).Total amino acids content of the tested feedstuffs (Table 3) were consistent with those observed by Freitas et al. (2005) and Rostagno et al. (2011).According to Calderano et al., (2010), variations in the chemical and energy composition of feedstuffs may occur for several reasons, such as differences in cultivar, as well as growth, soil, and weather conditions and processing to which the source material is exposed.
The CAMN of the feedstuffs was not influenced by broiler breed or age, except for WCG.Isa Label birds presented CAMEE values11.74%,18.76% and 46.54% greater than those obtained Cobb broilers for corn, WCG, and SBM, respectively (Table 4).Duarte EF, Minafra CS, Café MB

Values of Metabolizable Energy and Metabolization of Nutrients for Slow-and Fast-growing Birds at Different Ages
Chickens fed WCG between 28 and 35 days of age presented with 13.63% and 22.78% higher CAMN and CAMEE values, respectively, when compared with chickens fed with the same feedstuffs between 10 and 17 days of age (Table 4).According to Brumano et al., (2006), young broilers present lower nutrient digestion and absorption capacity compared with older broilers because their digestive system is still under development.On the other hand, older broilers have a fully developed digestive system, with a larger digestive tract and higher enzyme production and gastric secretions, resulting in better feed utilization.
The effect of age on nutrient utilization was most obvious in the birds fed WCG.As show in Table 2, WCG presented the highest ether extract content (10.03%), and excessive availability of lipids can be a limiting factor for the digestion of WCG during the first days of the broiler's life (Maiorka et al., 2004).
Significant differences in ether extract metabolizability were observed between breeds, except for sorghum.According to Bertechini (2006), several factors related to the chemical composition of a given molecule and the digestive maturity of the bird can influence lipid absorption.
We did not observe any significant interactions between breed and age for any of the tested feedstuffs in relation to energy utilization (Table 5).As the birds aged, AME (kcal/kg) and AMEn (kcal/kg) of WCG, sorghum, and SBM values increased, except for corn, which values were not affected by age.
Based on studies on other feedstuffs, we expected an age-dependent increase in corn energy metabolization in the studied breeds.However, Mello et al. (2009) observed that age-dependent changes in AME content were not observed in corn, wheat bran, or poultry by product meal despite an age-dependent increase in AME and AMEn values detected in other feedstuffs (sorghum, soybean meal, and blood plasma).These results suggest that the energy use of birds of different ages changes according to how feedstuffs are processed by the digestive enzymes at the different stages of the bird's life.
When fed the corn diet, Isa Label chickens presented AME and AMEn values 5.75% (177.98 kcal/kg) and 3.44% (105.94kcal/kg) higher than those determined in Cobb birds, respectively.The AME and AMEn values of the other tested feedstuffs were not influenced by breed (Table 5).Energy utilization of WCG, sorghum, and particularly SBM was not statistically different between breeds, however, slow-growing birds presented higher corn energy utilization.Isa Label birds tended to show higher AME values (71.6, 42.57, and 85.10 kcal/kg) than Cobb birds, which accounted for increases of 2.56, 1.39, and 3.85% for WCG, sorghum and SBM, respectively.Santos et al. (2005) reported that there is large difference in the maximum growth rate between Cobb and Isa Label breeds.The authors verified that the inflection point of the growth curve corresponding

Table 1 -
Ingredient composition (%) and calculated nutritional levels of the reference feed

Table 2 -
Chemical composition and gross energy values of the tested feedstuffs.

Table 3 -
Total amino acid content of the tested feedstuffs.

Table 4 -
Coefficient of apparent metabolizability of nitrogen (CAMN, %) and ether extract (CAMEE, %) of the tested feedstuffs fed to slow-growing (Isa Label) and fast-growing (Cobb) broilers.Means followed by different lowercase letters in the same column and uppercase letters in the same row differ byTukey's test at 5% significance level. 1 Coefficient of variation.