Evaluation of the metabolizable energy of glycerins with different compositions in broiler chickens

Brazil has a great potential for the production of biofuels. Besides the diversity of oilseeds for the production of biodiesel, the country possesses arrowhead technology and a high capacity manufacturing structure, so as to develop such production. Biodiesel is an alternative for petrol based fuels and can be produced from animal fat or vegetable oils, and in Brazil, there are several vegetable species with a high production potential. According to the National Oil Agency, in 2009, the Brazilian production of biodiesel was of, approximately, 1,608 million liters and as a byproduct around 160 million of liters of crude glycerin were generated (2011).

The rising interest in the use of crude glycerin as animal feed is due to its energy value. In real terms, the energy value of crude glycerin resulting from each industrial process should be determined according to its glycerol purity, since different impurities may be present in the product.  Such statement is valid as long as impurities are not constituted by free fatty acids, or even triglyceride residues, which obviously contribute to the energy of crude glycerin.

Dozier et al. (2008), determined the value of the apparent metabolizable energy corrected for nitrogen (AMEn) of glycerin for chicken in 3.434 kcal/kg, corresponding to 95% of gross energy, indicating that crude glycerin is efficiently used in broiler chickens. Lammers et al. (2008), working with laying hens, found the AMEn value of  3.805 kcal/kg, which is similar to the value of gross energy of glycerin used in the trial (3145 kcal/kg), once again, showing the high degree of energy use.

The metabolizable energy values of crude glycerin are quite similar when compared to the values of the apparent metabolizable energy of corn for pigs (3.340 kcal/kg) and poultry (3.381 kcal/kg) (Rostagno et al., 2005). This stresses the potential use of glycerin as a feed energy ingredient for such species. It is important to underscore that when producing animal feed for poultry and pigs, the metabolizable energy value of crude glycerin will be proportional to its level of glycerol, this is, the proportion of glycerol and the gross energy of glycerol should be considered as 4.320 kcal/kg, according to what is suggested by Lammers et al. (2008).

Barlet & Schenieder (2002), proved that the ME values of pure glycerol for broiler chickens, laying hens and pigs, varied according to its inclusion in the diet. The authors suggest that the reduction of AME is due to the renal reabsorption of glycerol, excreting the excess via urine. Following the same direction, Gianfelici (2009), reported that at the moment ingestion of increasing amounts of glycerol, there should be a level, as of which, the metabolizing capacity is surpassed, provoking an increase of glycerol in blood, which should be excreted via urine. In that study and with the inclusion of 15% of glycerin in the diet of broilers, there was an excess of excretion in water, hindering its use.

Thus, it should be underlined that the different energy values, verified in the literature, are mainly due to the different types of glycerin available in the market, with different levels of glycerol, water and fat.

The objective of this study was to determine the corrected metabolizable energy of glycerin of different compositions for broiler chickens.

Initially, 200 chickens were kept in an experimental corral, receiving a control diet and water ad libitum. Twenty one days after, they were weighted and transferred to metabolism cages. Groups were randomly assigned, with 5 treatments and 8 replicates of 5 birds each, with a control diet and 4 experimental diets (4 glycerins with different compositions). The trial was performed during the period of 21 to 29 days of age, granting four days for adaptation to the cages and experimental diets and four days for the total collection of excreta. The feed consumption and the amount of excreta produced were registered during the collection period. The excreta collection was done twice a day in order to avoid possible contamination and fermentation of excreta. Control diet was formulated with corn and soya meal, according to the recommendations of Rostagno et al. (2005) and the experimental feed had 90% of the control feed and 10% of glycerin. These glycerins had different compositions (Table 1). 1% of iron oxide was added to all feeds as an indicator of the beginning and end of the excreta collection.

Table 1. Glycerin composition

At the end of the experimental period, the amount of the consumed feed was quantified and the birds were weighted in order to determine performance. Collected excreta was conditioned in plastic bags and frozen at -18°C until the end of the collection period. Afterwards, excreta was defrosted, homogenized and weighted and a representative sample of each replicate was taken for their laboratory analysis. Such aliquots were pre-dried in air forced circulation stoves at 65°C during 72 hours. Subsequently, samples were crushed and conditioned in containers for further analysis of gross energy in a calorimetric pump (model Parr 1261) and of nitrogen by means of the method of direct combustion of Dumas with an automat ic devise LECO.

Analysis data, as well as feed consumption and excreta production, were used in the calculation of the apparent metabolizable energy (AME) and the nitrogen-corrected apparent metabolizable energy (AMEn), as well as for the coefficient of metabolizability of glycerin energy, according to the methodology of  Sakamura & Rostagno (2007).

Even though the experiment was not planned in order to determine the bird´s performance, none the less, weight gain (WG), feed consumption (FC) and feed conversion (FC) were evaluated. No significant differences were detected, presenting a normal performance. Average daily WG in treated birds was of, approximately 74 g.

Data of the apparent metabolizable energy (AME) and corrected apparent energy (AMEn), are expressed in natural matter in Table 2. A AME and AMEn varied according to the glycerin composition (P<0,05), and mainly with the concentrations of glycerol and fatty acids. Lammers et al. (2008), observed that ME may have a direct relation with the amount of glycerol in the samples and that the characteristics of the glycerin used are important.

The value of AMEn for glycerin A was of 3.145 kcal/klg, close to the one obtained by Dozier et al. (2008), of 3.331 kcal AMEn/kg in natural matter in chicken of 38 to 45 days of age. Such ME difference is probably due to the composition of the glycerins, since the one used by the authors had a higher amount of glycerol. Lammers et al. (2008), when working with laying hens of 40 weeks of age, found similar results of 3.800 kcal/kg for AMEn, using 10 and 15% of glycerol.

Glycerin B presented an AMEn of 5.026 kcal/kg due to the concentration of fatty acids in the composition. Furlan et al. (2010), when working with quails, found a value of 4.564 kcal/kg for non-purified glycerin.

The metabolizability coefficients of glycerins were: 91%, 81%, 76% and 85% for glycerins A, B, C y D, respectively. This proves that the composition of glycerin, mainly in relation to the concentration of glycerol and fatty acids, affects such a coefficient.

Such differences in energy values, which are verified in the literature, are mainly due to the difference in the composition of the glycerins present in the market, and to the lack of production standards of the different companies. For the registration of glycerin as a feed ingredient, the product should have, at least,  80% of glycerol, a maximum of 12% of humidity and 150 ppm of methanol.

Table 2. Values of apparent metabolizable energy (AME) and corrected apparent (AMEn) of glycerins, expressed in natural matter  

The composition of glycerin directly affects ME, as well as the metabolizability coefficient, but when presenting high levels of glycerol in the composition, it might be an interesting ingredient, due to its metabolizable energy.

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