Bidistilled glycerine in the diet of growing rabbitsABSTRACT
Two experiments were carried out to evaluate the nutritional value of bidistilled glycerin and the productive performance of rabbits fed diets that replaced soybean oil with different levels of glycerin. In the first experiment, 20 New Zealand red rabbits were used for the digestibility assay, with two treatments and ten replicates. Glycerin showed digestibility coefficients of 91.78% for gross dry matter and 93.36% for gross energy. In the second experiment, 80 30-day-old rabbits were distributed into five treatments (control diet and inclusions of 25, 50, 75 and 100% glycerin) in randomized blocks, with eight replicates and two animals per experimental unit. After the experiment, the rabbits were slaughtered to evaluate organ, meat and carcass characteristics. No influence of the inclusion of bidistilled glycerin was observed in any of the evaluated parameters. Glycerin showed high digestibility, with 83.35% digestible dry matter and 3936.92 kcal/kg digestible energy. Thus, double-distilled glycerin can be used as an energy ingredient in diets for growing rabbits.
Keywords:
biodiesel; co-products; rabbit farming; glycerol
RESUMO
Foram realizados dois experimentos para avaliar a glicerina bidestilada em dietas de coelhos. O primeiro estudo analisou seu valor nutricional, em que 20 coelhos da raça Nova Zelândia Vermelho, com 30 dias de idade, distribuídos em blocos ao acaso, com dois tratamentos e 10 repetições, foram utilizados em um ensaio de digestibilidade. Os resultados mostraram que a glicerina apresentou coeficientes de digestibilidade de 91,78% para matéria seca bruta e de 93,36% para energia bruta, indicando alta digestibilidade. O segundo experimento avaliou desempenho produtivo, características de carcaça e carne de 80 coelhos da mesma raça, distribuídos em blocos ao acaso, com cinco tratamentos: dieta controle e dietas com 25%, 50%, 75% e 100% de glicerina bidestilada e oito repetições substituindo o óleo de soja. Os animais foram abatidos ao final para análise de órgãos, carne e carcaça. Os resultados mostraram que a inclusão de glicerina bidestilada não afetou esses parâmetros, indicando que ela pode ser utilizada sem prejuízo. Além disso, a glicerina apresentou alta digestibilidade, com 83,35% de matéria seca digestível e 3936,92 kcal/kg de energia digestível. Assim, ela pode ser empregada como uma fonte energética eficiente na alimentação de coelhos em crescimento, contribuindo para uma dieta balanceada e nutritiva.
Palavras-chave:
biodiesel; co-produtos; criação de coelhos; glicerol
INTRODUCTION
The search for alternative sources capable of replacing soybean meal and corn has been the focus of numerous studies in the field of animal nutrition and feeding, aiming to increase production and strengthen rabbit farming. Expanding the range of alternative feed ingredients is one of the key demands to be addressed.
The pursuit of partial diesel replacement has intensified, mainly due to the growing need for the use of renewable energy sources in place of fossil fuels, with biodiesel emerging as a viable alternative for diesel-powered vehicles (Oliveira et al., 2017). However, biodiesel production generates by products that require environmentally and economically sustainable disposal routes, and some of these can potentially be repurposed for animal feed (Menezes et al., 2023).
Crude glycerin is a by-product of biodiesel production, obtained through the transesterification process (Bansod et al., 2024). It can be classified as low, medium, or high purity, which influences its glycerol content, as well as other specific characteristics such as odor, color, and levels of impurities (Schröder and Sudekum, 1999, as cited by Syperreck et al., 2024). However, the glycerin derived from biodiesel production is not yet subject to specific disposal regulations, and it is often discarded into the environment. This raises concerns among environmental protection professionals, as crude glycerin is frequently stored and accumulates at biodiesel plants, leading to large stockpiles of this by-product (Baú et al., 2024).
Given these issues, further research is needed to explore alternative uses for glycerin. One such possibility is its inclusion in animal diets, where it can serve as an energy source. Scientific studies investigating glycerin supplementation in rabbit diets are scarce, and its metabolic fate in rabbits remains largely unclear. Therefore, the objective of this study was to evaluate the effects of different inclusion levels of bydistilled glycerin in the diets of growing rabbits, replacing soybean oil, and to assess its influence on growth performance, carcass traits, and meat characteristics.
ETHICAL ASPECTS
The project was approved by the Animal Ethics Committee of the Federal University of Paraíba (CEUA/UFPB), under protocol number 051/2015.
MATERIAL AND METHODS
The experiments were conducted at the Rabbit Research Laboratory, affiliated with the Department of Animal Science of the Federal University of Paraiba. The first trial aimed to evaluate the nutrient and energy digestibility of bydistilled glycerin, while the second focused on assessing the effects of different inclusion levels of bydistilled glycerin on the performance and carcass traits of growing rabbits.
To determine the nutritional value of bydistilled glycerin, a digestibility trial was conducted using 20 New Zealand Red rabbits, with an average initial age of 55 days and an average body weight of 1,149.6±2.7g. The animals were housed individually in metal cages equipped with nylon mesh to retain feces and allow urine drainage. The determination of the feed's nutritive value (Table 1) was based on a digestibility assay performed in accordance with the methodology described by EGRAN (1999).
Bydistilled glycerin replaced 10% of the reference diet on a natural matter basis, resulting in two experimental diets, which were dry-pelleted. Animals had ad libitum access to feed and water throughout the experimental period, with feed being provided once daily. The reference diet (Table 2) was formulated to meet the minimum nutritional requirements for growing rabbits, as recommended by De Blas and Wiseman (2020).
After an adaptation period to the diets and housing conditions, the feed intake and fecal output of each animal were recorded. The chemical composition of the feed and collected feces was subsequently used to calculate the apparent digestibility coefficients for each animal, as described by EGRAN (1999).
The digestibility trial lasted 12 days, consisting of seven days for adaptation to the facilities and diets and five days for total feces collection. During the assay, samples were collected, stored, homogenized, dried, weighed, and ground following the methodology proposed by Silva and Queiroz (2002). Feed samples were ground using the same procedure as the feces and stored in plastic containers for further analysics.
From the analyses of feed, feces, and bydistilled glycerin, dry matter (DM) was determined by drying the material for 12 hours at 105°C, according to Silva and Queiroz (2002). Gross energy (GE) of the feed and feces was measured using a PARR® bomb calorimeter.
Based on the analytical results, the apparent digestibility coefficients for dry matter and gross energy were calculated, along with the digestible energy of the bydistilled glycerin, using the equations proposed by Sakomura and Rostagno (2016). Apparent digestibility coefficients (%ADC) were calculated using the following formula:
In the performance trial, diets with different levels of soybean oil replacement by bydistilled glycerin (0, 25, 50, 75, and 100%) were evaluated. The diets were formulated based on the nutritional values obtained in the previous digestibility trial and were designed to be isonutritive, in accordance with the nutritional recommendations for growing rabbits proposed by De Blas and Wiseman (2020). After mixing, the diets were dry-pelleted and provided ad libitum.
A total of 80 New Zealand Red rabbits, with an initial average body weight of 499.6 ± 1.65g and 30 days of age, were used. The experimental design was a randomized complete block design with five treatments and eight replicates, each replicate consisting of a pair of animals (one male and one female) housed together in galvanized wire cages equipped with clay feeders and drinkers. Growth performance was monitored from 30 days of age until slaughter at 85 days. Feed and animals were weighed weekly to determine feed intake, weight gain, and feed conversion ratio.
At 85 days of age, animals were fasted for 12 hours, weighed to determine slaughter weight, and then humanely slaughtered. Slaughter procedures and evisceration were performed following an adaptation of the method by Retore et al. (2012). Carcasses were chilled (1-2°C) for 24 hours before undergoing quantitative assessment according to the NPPC methodology (NPPC, 1991). The carcass yield (CY), hot carcass weight (HCW), and cold carcass weight (CCW) were recorded. Carcass yield and yields of forelimbs (FMY), hind limbs (HMY), loin (LY), and thoracic-cervical region (TCY) were calculated relative to carcass weight.
Carcass yield (CY) was calculated relative to pre-slaughter body weight using the formula:
Hot and cold carcass yields were calculated as follows:
Commercial cuts were made according to the methodology of Blasco and Ouhayoun (1993). Thighs were sectioned at the seventh lumbar vertebra, followed by separation of the Longissimus lumborum and shoulder cuts.
The pH of the Biceps femoris muscle was measured in the hot carcass 45 minutes post mortem (pH₄₅), and again after 24 hours of chilling (1-2°C) (pH₂₄) using a portable digital pH meter (HI 99163, Hanna Instruments), following the recommendations of Bridi and Silva (2009). Muscle color was also measured 24 hours post-slaughter, based on the method described by Bridi and Silva (2009). Six surface measurements were taken from the Biceps femoris using a CR-400 portable colorimeter (Konica Minolta®) configured with D65 illuminant, 0° viewing angle, and 4-point auto average. The CIELAB system was used to report L* (lightness), a* (red-green component), and b* (yellow-blue component) values.
The collected data were tested for homogeneity of variances using Levene’s test (5%) and for normality of residuals using the Cramer-von Mises test (5%). According to Everitt (1998), data that met the statistical assumptions were subjected to analysis of variance using the PROC GLM procedure of the SAS statistical package (version 9.1; SAS Institute, Cary, NC, USA), with multiple regression analysis up to the third degree. Treatment means for the different levels of glycerin substitution were compared to the control group (0% glycerin) using Dunnett’s test at a 5% significance level.
RESULTS AND DISCUSSION
The values for dry matter (DM) and gross energy (GE), the apparent digestibility coefficients, as well as the digestible nutrient and energy content of the bydistilled glycerin, are presented in Table 3.
The bydistilled glycerin presented 90.82% total dry matter and 83.35% digestible dry matter, as well as 4216.93 kcal/kg of gross energy and 3936.92 kcal/kg of digestible energy on an as-fed basis (Table 3). The gross energy value observed was higher than that of corn-the most used energy ingredient in rabbit diets-which contains 3907 kcal/kg of gross energy (Rostagno et al., 2024) and showed a digestibility coefficient similar to that of corn energy in rabbits, which is close to 100% (Blas and Gidenne, 2020).
The bydistilled glycerin evaluated in the present study showed a high digestibility coefficient and an energy value comparable to corn (Table 3), making it a potentially viable raw material for use in growing rabbit diets. The high bioavailability of glycerin is likely due to its molecular structure it is not esterified to fatty acids and has a low molecular weight, which facilitates passive diffusion across the intestinal epithelium. Moreover, its high energy content makes it a suitable energy source in the diets of non-ruminant animals (Jesus et al., 2020).
Studies have shown that the absorption rate of glycerol in the intestinal lumen is approximately one-quarter that of glucose, without significant impact on their relative concentrations (Lin, 1977). Data presented by Retore et al. (2012) suggest that the gross and digestible energy values of glycerin are closely aligned, further indicating the ingredient’s high digestibility.
Jesus et al. (2020) state that, provided optimal inclusion levels are respected for each species, the use of glycerin as an alternative feed ingredient does not impair carcass traits or animal performance, and may be used effectively in non-ruminant nutrition. Similarly, Syperreck et al. (2023) reported that glycerin inclusion in animal diets has beneficial effects without compromising nutrient digestibility, while also improving short-chain fatty acid profiles and increasing blood glucose concentrations.
Numerous studies have assessed the effects of glycerin in animal nutrition, with observed variations largely attributed to animal species, inclusion levels, and the purity of the glycerin used (Menezes et al., 2023). According to Delgado and Galindo (2024), glycerin use in animal feed is promising when considering its economic value, nutritional profile, and animal acceptability, in addition to its wide availability, which helps to avoid direct competition for this resource.
Retore et al. (2012), studying semi-purified vegetable and mixed glycerin in the diets of growing rabbits, reported similar values to those found in the present study: 4089kcal/kg and 4048kcal/kg for gross and digestible energy in vegetable glycerin, respectively, and 3751kcal/kg and 3697kcal/kg for mixed glycerin.
These findings support the feasibility of glycerin use in animal nutrition, provided that its energy value and inclusion rate are carefully evaluated, considering the compositional variability of this ingredient (Luciano et al., 2017). As noted by Leite et al. (2017), the higher fatty acid content in vegetable glycerin may explain its elevated digestibility coefficients. The difference in gross energy observed in bydistilled glycerin compared to other types of glycerin can be attributed to its higher glycerol concentration (Jesus et al., 2020).
Based on these results, it can be inferred that animal species, glycerin quality, and inclusion levels directly influence the digestibility of dry matter and energy and their subsequent utilization (Menezes et al., 2023).
No significant effects (P > 0.05) were observed for the performance variables analyzed. The lack of differences in feed intake, weight gain, feed conversion, and slaughter weight among growing rabbits suggests that the nutritional quality of the diets was maintained as bydistilled glycerin was incorporated, indicating good acceptability of this energy ingredient by the animals (Table 4).
The results (Table 4) suggest that increasing glycerin levels in the diets did not affect feed palatability. Leite et al. (2017) noted that glycerin is a viscous liquid, soluble in water or alcohol, and possesses a naturally sweet taste, which may enhance diet palatability and intake.
These findings support the feasibility of glycerin use in animal nutrition, provided that its energy value and inclusion rate are carefully evaluated, considering the compositional variability of this ingredient (Luciano et al., 2017). As noted by Leite et al. (2017), the higher fatty acid content in vegetable glycerin may explain its elevated digestibility coefficients. The difference in gross energy observed in bydistilled glycerin compared to other types of glycerin can be attributed to its higher glycerol concentration (Jesus et al., 2020).
Based on these results, it can be inferred that animal species, glycerin quality, and inclusion levels directly influence the digestibility of dry matter and energy and their subsequent utilization (Menezes et al., 2023).
No significant effects (P > 0.05) were observed for the performance variables analyzed. The lack of differences in feed intake, weight gain, feed conversion, and slaughter weight among growing rabbits suggests that the nutritional quality of the diets was maintained as bydistilled glycerin was incorporated, indicating good acceptability of this energy ingredient by the animals (Table 4).
The results (Table 4) suggest that increasing glycerin levels in diets did not affect feed palatability. Leite et al. (2017) noted that glycerin is a viscous liquid, soluble in water or alcohol, and possesses a naturally sweet taste, which may enhance diet palatability and intake.
According to Retore et al. (2012), semi-purified vegetable glycerin can be included in rabbit diets at levels up to 12%, and semi-purified mixed glycerin up to 9%, without negatively affecting growth performance or carcass weight in growing rabbits, in addition to contributing to reduced production costs.
The energy available in the diet is derived from its composition, including fat, carbohydrates, and protein. The energy released through oxidative metabolic processes is used to support muscular activity, maintenance, and other metabolic functions (Fernandes and Toro-Velasquez, 2014).
Thus, the lack of variation in the evaluated parameters confirms that the bydistilled glycerin provided a sufficient metabolic energy supply in the diets, due to its high content of fatty acids and glycerol. This is a crucial aspect, especially considering that the period between weaning and 50 days of age is when rabbits exhibit their most rapid growth and, therefore, have the highest nutritional requirements.
Regression analysis indicated no significant effect (P ≥ 0.05) of the inclusion level of bydistilled glycerin on the variables hot carcass weight (HCW), cold carcass weight (CCW), carcass yield (CY), foreleg yield (FLY), hind leg yield (HLY), loin yield (LY), and thoracic-cervical region yield (TCRY) in growing rabbits (Table 5). Similarly, Dunnett’s test showed no significant difference (P ≥ 0.05) between the glycerin inclusion levels and the control diet (0% glycerin). This response suggests that the nutritional values used for glycerin were appropriate, given that the diets were isonutritive and the coproduct did not contain compounds detrimental to rabbit performance.
Results found by Verussa et al. (2017), in studies with growing pigs, indicate that the use of glycerin at levels up to 15% of semi-purified glycerin (78.50% glycerol; 11.99% moisture; 6.0% sodium chloride; and 0.08% methanol) had a positive effect on the animals, leading to improvements in body weight and daily weight gain. The inclusion of different levels of double-distilled glycerin did not cause any changes (P>0.05) in the weights of the heart, liver, kidneys, or fat (Table 6). These results show that there were no harmful effects from the double-distilled glycerin, thereby preserving the nutritional quality of the diet and the proper functioning of the analyzed organs.
Retore et al. (2012), when evaluating the inclusion of different levels (3, 6, 9, and 12%) of semi-purified vegetable and mixed glycerin in rabbit diets observed a linear increase in kidney yield in animals fed increasing levels of both glycerin types.
Glycerol is absorbed through passive diffusion across the gastric and intestinal cell walls and is subsequently transported to the liver, where it is metabolized. In the liver, glycerol can be utilized either for lipid synthesis or as an energy source (Martínez-Miró et al., 2021; Jesus et al., 2020).
None of the qualitative meat characteristics (Table 7) were influenced by the inclusion of bydistilled glycerin. The assessment of meat color involves calculations based on the lightness (L*), red-green component (a*), and yellow-blue component (b*), such as chroma (C*), hue angle (H*), and total color difference (ΔE). According to Domínguez et al. (2019), changes in meat color are among the primary indicators of reduced sensory quality, as they are closely associated with texture, odor, and flavor. Such changes typically signal the onset of lipid oxidation processes, which may compromise the nutritional value of the meat.
The pH values of meat can range from 5.7 to 5.9 and play a critical role in the meat processing industry, as they are directly related to quality parameters and shelf life. In addition to pH, other factors influencing consumer acceptance include water-holding capacity and color (Kindlein et al., 2016). These authors, who evaluated different dietary fat sources in broiler diets, also reported no significant effects on meat lightness or final pH.
The promising results of the present study suggest that further research is warranted to better elucidate the nutritional value of glycerin in rabbit diets. Special attention should be directed toward characterizing the composition of distilled glycerin, as crude glycerin often contains high methanol concentrations. According to Clasen et al. (2015), crude glycerin may exceed the recommended limit of 150 ppm of methanol, rendering it unsuitable for use in animal feed.
CONCLUSION
Bydistilled glycerin demonstrated high digestibility, with 83.35% digestible dry matter and 3936.92 kcal/kg of digestible energy for growing rabbits. Based on these findings, bydistilled glycerin may be used as an energy ingredient in diets for growing rabbits.
ACKNOWLEDGMENTS
The authors would like to thank the Graduate Program in Animal Science at the Federal University of Paraíba for the opportunity to conduct this research, and the National Council for Scientific and Technological Development (CNPq) for providing the scholarship support.
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