Determination of the Energetic Value of Corn, Soybean Meal and Micronized Full Fat Soybean for Newly Hatched Chicks1

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INTRODUCTION
The study of nutrition and metabolism of newly hatched broiler chicks has proven to be an interesting research area, which may allow the expression of the potential growth rate and meat production of birds by optimizing feed management. In the past decades, growth rates of broilers have increased and enabled to reach final bird weight earlier. Therefore, the dietary management of newly hatched chicks represents an important tool to improve broiler production indexes.
However, the immaturity of the digestive system of young chicks reduces its ability to absorb nutrients compared to older birds. According to Vieira & Moran Jr. (1999), the intestinal tract of birds is not completely able to digest dietary nutrients until two weeks of age.
During the early stages of broiler development, the ability of digesting carbohydrates and fat is decreased compared to other phases. Noy & Sklan (1995) showed that chicks at four days of age had reduced absorption rates of lipid and carbohydrate (85%) and protein (80%) when compared to older birds.
This lower nutrient utilization during early stages seems to result in decreased metabolizable energy (ME) values of the diets. Previous experiments showed that ME values are depressed between four and seven days of age of broilers (Murakami et al., 1992;Sulistyanto et al., 1998). Corless & Sell (1999) reported that dietary ME values obtained from birds with four, seven and 14 days old were lower than values calculated using the NRC (1994) tables.
The metabolizable energy values of corn-soybean based diets obtained for birds in the first, second and third week of age differed in about 200-250 kcal/kg when different ingredients for birds were evaluated (Mahagna et al., 1988). However, ME values of wheatsoybean diets differed in 500-1,100 kcal/kg. This low metabolizability observed for wheat-soybean diets was due to higher digesta viscosity caused by the non-starch polysaccharide content of the ingredient. Nutrient digestibility is even more impaired when non-starch polysaccharides are associated to an immature enzymatic system. Furthermore, the presence of oligosaccharides in soybean meal, such as raffinose and stachyose, is partially responsible for the reduction in the energy digestibility that is seen when considering this ingredient for birds (Leeson & Summers, 2001).
In birds, the ME value of a corn-soybean diet increases with age until 14 days, when it stabilizes. The reason for the observed increase in energy utilization using this type of diet is based on the increased utilization of starch from corn, fat from corn and supplemental fat, protein from corn and soybean and other carbohydrates from dietary components (Batal & Parsons, 2002). Therefore, even the usual components of broiler diets can have their utilization diminished during the first few days of age.
Dietary ingredients like soy protein concentrate and soy protein isolate are known to be better utilized by chicks when compared to ordinary soybean meal due to the removal of the oligosaccharide content. It is well established that these compounds act as antinutritional factors and reduce protein digestibility in birds (Parsons et al., 2000).
The use of micronized full fat soybean in pre-starter broilers diets is an alternative for the use of soybean meal. This ingredient is produced using selected soybeans that are heated, dehulled, ground and micronized. The process results in a product with small particle size (between 30 and 300 µm) and completely free from hulls, which results in an increase of nutrient utilization by broilers (Goldflus, 2001).
The intake of pre-starter diet is low and corresponds to only 3.5% of total feed consumption, which justifies the use of high-quality ingredients in feed production (Penz Jr. & Vieira, 1998). In order that high-quality and expensive ingredients can be used in pre-starter diets, the evaluation of the actual capacity of ingredient utilization is critical for the formulation of efficient diets and for the improvement of nutrient utilization by young birds.
However, the use of high-quality and expensive ingredients in the composition of pre-starter diets is extremely dependent on precise evaluation of the utilization capacity of the standard ingredients, such as corn and soybean meal. The knowledge of these values would allow nutritionists to produce efficient diets that improve nutrient utilization by young birds.
The objective of this work was to determine the metabolizable energy value and the metabolizability coefficient of gross energy for corn, soybean meal and micronized full fat soybean for newly hatched broiler chicks.

MATERIAL AND METHODS
A metabolism assay was conducted in an environmentally controlled room at the Department of Animal Science, ESALQ/USP, Piracicaba/SP, Brazil. One hundred and ninety-two AgRoss male broiler chicks with one day of age were distributed in a completely randomized design with four treatments and four replicates of 12 birds per cage. The birds were raised on battery cages with automatic heating and a tray under the screen floor to collect excreta.
The treatments included a reference diet ( T a b l e 1 ) f o r m u l a t e d t o m e e t n u t r i t i o n a l r e q u i r e m e n t s o f b i r d s i n t h e e a r l y s t a g e s (Rostagno et al., 2000) and three test diets. Two of the test diets were composed of 60% of the reference diet and 40% of the tested ingredient (corn or soybean meal). In the third test diet, 20% o f t h e r e f e r e n c e d i e t w a s s u b s t i t u t e d f o r micronized full fat soybean meal.
Birds were fed the experimental diets from one to three days of age, corresponding to an adaptation period. Total excreta collection method was then performed for four consecutive days. Feed intake and excreta production were recorded daily. Iron oxide was added to the experimental diets to dye the excreta and establish the start and the end of the collection period.
The produced excreta were collected twice a day, identified and stored in a freezer until analysis. After thawing and homogenization, representative samples of each replicate were dried in an oven at 55°C for 72 hours and ground. Samples of dietary ingredients, experimental diets and excreta were analyzed for dry matter, nitrogen, total fat and gross energy. AME n values in the present study for young chicks were 150 and 300 kcal/kg lower than average values indicated in the literature for corn (3,360 kcal/kg) and for soybean meal (2,385 kcal/kg), respectively (NRC, 1994;Rostagno et al., 2000). However, micronized full fat soybean AME n was similar with values previously reported in the literature (4,068 and 3,950 kcal/kg). It should be noted that the reference values for AME n found in literature for those ingredients were determined using older birds. The N-corrected apparent metabolizable energy (AME n ) was calculated based on data obtained from analysis of experimental diets and excreta, as suggested by Matterson et al. (1965). AME n was calculated for each test ingredient based on energy values obtained for the experimental diets using the following formula: The metabolizability coefficient of gross energy (MCGE) was calculated based on gross energy (GE) and apparent metabolizable energy values (MCGE = AME n /GE x 100).
The MCGE ingredient data were evaluated using General Linear Model Procedure (SAS Institute, 1996) and means were compared by the Tukeys test.

RESULTS AND DISCUSSION
The values of dry matter (DM), gross energy (GE), crude protein (CP) and total fat (TF) of the studied ingredients (corn, soybean meal and micronized full fat soybean) are presented in Table 2. The nutrient composition for the test ingredients found in the present study are in agreement with the composition reported in nutritional tables.
The calculated AME n and MCGE values for corn, soybean meal and micronized full fat soybean are shown in Table 3, as well as comparative data found in the literature.   Sulistyanto et al. (1998) analyzed metabolizability of corn and wheat in broiler chicks aged one, three and 10 days and found that the energy values of these ingredients increased with age, although no significant differences were seen.
The results of AME n reported by Café et al. (1993) for micronized full fat soybean using broiler chicks from 13 to 23 days-old are only 108 kcal/kg higher (4,176 kcal/kg) than the value determined in this study (4,068 kcal/kg).
The MCGE values obtained in the present study for the three ingredients were statistically different (p<0.01). Corn was considered the most efficient ingredient on energy utilization followed by micronized full fat soybean and the least efficient was soybean meal. In agreement with Sulistyanto et al. (1999), corn also demonstrated better energy utilization for young birds compared to other carbohydrate sources like wheat and sorghum.
The calculated values of MCGE obtained from the nutritional data for older birds found in the Brazilian tables (Rostagno et al., 2000) were compared to those determined in this study using young chicks. It was noted that the ability of the younger birds to metabolize energy was similar for micronized full fat soybean (75.3% vs. 74.81%), lower for corn (85.7% vs. 81.6%) and even lower for soybean meal (60.7% vs. 49.7%).
The depressed values of AME n and MCGE found in this study for corn and soybean meal are probably related to the intrinsic characteristics of the feed ingredients and to the reduced digestion and absorption ability of young birds due to their immature digestive system. This demonstrates that metabolic aspects of young broiler chicks can regulate energy utilization of dietary ingredients.
Nevertheless, similar values observed for AME n and MCGE for micronized full fat soybean emphasize the importance of an effective quality control and normalization of micronized full fat soybean production, including the complete removal of hulls to insure the efficient utilization of the ingredients by young chicks. Pupa (2000) did not observe significant differences between MCGE for micronized full fat soybean when fed to swine at 22 days (76.7%) or at 33 days of age (80.0%).
The results reported in the present study confirm previous results described by Mahagna et al. (1988). Those authors associated the degree of energy metabolizability of soybean meal measured for young broilers with the increase of viscosity caused by this ingredient on the intestinal digesta. This fact can further reduce the ingredient digestibility when associated to an immature enzymatic system.
An accurate determination of AME n of corn and soybean meal for newly hatched broiler chicks corroborate the study of alternative ingredients with focus on appropriate and precise diet formulation. On the other hand, the results showed herein and in previous reports (Nir, 1998) suggest that AME n of feed ingredients described in the literature are overestimated for newly hatched chicks.
The reference diet produced for this experiment was formulated to achieve 2,950 kcal/kg but the diet actually offered 2,800 kcal/kg, based on the determined AMEn for corn and soybean meal. Therefore, when diets are formulated using alternative ingredients and energy contents that were calculated based on experiments with newly hatched chicks, it must be considered that the reference diet may not contain the expected energetic concentration.
The differences between calculated values obtained in this study and literature data indicate a deficiency on digestive and absorptive processes of birds from one to seven days of age and emphasize the importance of the study of ingredients for this phase with the objective of optimizing diet formulation.