Nutritional Value of Glycerin for Pigs Fed a Mixture or on Top Diet

1. Introduction

The increasing demand for transportation fuels, coupled with decreasing crude oil reserves and a growing awareness of climate change, has increased global interest in biodiesel [1]. Biodiesel is produced by the transesterification of vegetable oils or animal fats with an alcohol present as a catalyst. This reaction yields a considerable amount of crude glycerin as a byproduct, approximately 10% by weight of the biodiesel produced [2].

Global biodiesel production reached nearly 59 billion liters in 2022, with the main uses being palm oil, soya oil, rapeseed oil and animal fats [3]. The availability of byproducts such as glycerin jumped from 290 K M³ in 2013 to 552 K M³ in 2022, with soybean oil being the raw material that accounted for 66% of the total [4]. The continued growth in biofuel production has led to a search for alternative value-added applications for the use of glycerin. The surplus glycerin production and increased cost of feedstuffs have increased the emphasis on evaluating its nutritive value for animal feed [5].

The utilization of glycerin in pig diets has increased due to its great advantages, but the success of its use is related to the knowledge of its characteristics and nutritional value. Studies using glycerin for pigs have been conducted, and glycerin has been shown to be a good digestible ingredient and source of energy [6,7,8]. A consensus has not been reached, as the metabolizable energy (ME) value ranges from 2531 kcal/kg [9] to 5509 kcal/kg [10].

In many countries, combining a food compound, such as feed, with water or industrial liquid byproducts has gained popularity as a feeding method. Its effects on digestibility and weight gain have been demonstrated [11]. In addition, the use of glycerin in diets may lead to operational difficulties in the preparation and supply of rations, with effects on animal performance. Reports of lump formation and poor fluidity [12] and low-quality pellets [13,14] have reflected 5% maximum inclusions for pellet durability [15], improvement in feed fluidity [16], and increased pelleting efficiency [17], thus limiting the use of higher glycerin inclusions. Thus, the inclusion of glycerin just prior to onset, called "on top", allows for greater inclusions; however, to the best of our knowledge, its nutritional value has few or no effects.

The method used to determine the energy content may influence the nutritional value of glycerin. Either the total collection method (TC) or the index method with chromium oxide (Cr) can be used. TC is one of the most commonly used methods for determining the digestibility of nutrients as well as the values of digestible energy (DE) and ME, but obtaining representative samples without contamination is problematic [18]. The index method requires including a certain concentration of indigestible compound in the diet and allows partial sampling but requires a precise chemical analysis of the indigestible markers [19].

Therefore, the objective was to determine the digestibility of glycerin by inclusion in the feed mixture and addition on top by the TC and Cr methods.

2. Materials and Methods

The experiment was carried out in the experimental warehouse of the Nonruminant Nutrition Research sector of the Institute of Agricultural and Environmental Sciences, Federal University of Mato Grosso (Sinop, Mato Grosso, Brazil, latitude -11° 86' 26" and longitude -55° 48' 49"). The research complied with the ethical principles of animal experimentation adopted by the National Council for Animal Experimentation Control and previously approved by the Ethics Committee in the Use of Farming Animals at the Federal University of Mato Grosso (protocol number 23108.700673/14-4).

Twelve genetically homogeneous barrows (42.63±4.23 kg) were distributed individually into metabolic study cages in a 2x2+1 factorial design, with two forms of inclusion of the ingredient test, two methods of evaluating digestibility and a diet base. Each pig was considered an experimental unit, with four replicates per treatment, since the methods of evaluating digestibility were evaluated.

The basal diet (BD) was produced from corn and soybean meal [20] (Table 1). To evaluate the forms of inclusion of the test ingredients, the isometric substitution of 10% BD by glycerin and a new mixture (MIX) was performed [18], and the 10% isometric substitution of BD for glycerin was carried out immediately prior to delivery to the animals, called "on top" (ONTOP) (Table 2).

In all the diets, 0.5% chromic oxide (Cr2O3) was added [18]thus allowing the evaluation of digestibility through the chromium indicator (Cr) and total collection (TC) methods. BD was 90.00, 89.76, 4.09, and 10.24 and 3943 kcal/kg, and MIX and ONTOP were 88.00, 91.07, 6.72, 9.69 and 3980 kcal/kg DM, OM, EE, MM and GE, respectively.

The glycerin used here was obtained from the production of biodiesel, with soybean oil as the raw material, with 0.49% humidity, 86.5% glycerol, 0.05% methanol and 0.30% total fatty acids according to the technical report provided by the manufacturer. The experimental period was eight days, three days of adaptation of the treatments to the cages and diets, and five days of feces and urine collection. During the adaptation period, the diet was supplied ad libitum, and orts were recorded for a later calculation of intake on the basis of metabolic weight (LW^0.75). To prevent losses and facilitate intake, the diets were weighed and moistened at a 1:1 ratio and supplied twice daily (07:30 and 17:00).

Feces and urine were collected once daily, weighed and homogenized, and then, 200 g/kg samples were stored in a freezer (‒10°C). The urine was filtered as it was excreted using a filter tissue placed in a funnel beneath the urine collection box and then collected in plastic buckets containing 10 mL of 6 N HCl [21].

At the end of the collection period, the diet and feces samples were thawed, weighed, homogenized, and dried in a forced-air oven at 55°C for 72 h for analyses of dry matter (DM, method 934.01) [22], crude protein (CP, method 2001.11) [22], ether extract (EE, method 945.38) [22], ash (method 923.03) [22] and gross energy (GE) values of the feces, urine, diets and coproducts were determined using a bomb calorimeter (Parr 6400 calorimeter, Parr Instruments Co., Moline, IL, USA) (Table 3). The organic matter (OM) content was determined as the difference between the DM and ash contents. Analyses of the chromium content in the diets and feces were performed via atomic absorption spectrophotometry. The urine samples were thawed and homogenized for the determination of total nitrogen and gross energy. The digestibility coefficients (DC) of DM, ash, OM, EE and energy and DE and ME and the corrections for the nitrogen content (DEn and MEn) were determined [18].

The animals were weighed at the beginning and end of each period, and the feed intake was recorded to calculate the daily feed intake (DFI), daily weight gain (DWG), and feed conversion (FC) of each experimental unit.

The digestibility and energy data were performed according to the following model (1):

$$Yij=\mu +Ii+Mj+Ii x Mj+eij$$

(1)

where Yij = observation of the effect of the form of inclusion i by the digestibility assessment method j; μ = overall mean; Ii = effect of the form of inclusion i (i = mixture or on top); Mj = effect of the digestibility method k (k = total collection and marker); Ii×Mj = effect of the interaction between the form of inclusion i and the digestibility method j; and εij = random error associated with each observation.

The data were subjected to ANOVA using the GLM (general linear model) procedure of SAS software (Statistical Analysis System, version 6.0), considering the 5% probability by Tukey test for performance and the F test for DC and energy values.

3. Results

Compared with those fed diets containing glycerin, the animals fed a basic diet presented a greater DFI (P < 0.05), but there was no difference in DWG or FC among the treatments (p > 0.05) (Table 3).

The DM, MM and OM DC of glycerin were not influenced by the inclusion form of the test ingredient in the diet (p > 0.05), but ONTOP inclusion generated a greater EE DC (p < 0.05) (Table 4). Compared with those of the TC samples, the glycerin DC samples presented lower values when generated by the Cr method compared to TC (Table 4). These findings imply that the top form with a 10% inclusion level can be used without detriment to the nutritional value of glycerin for pigs if it is more feasible in terms of logistics.

The values of DE, DEn, ME, MEn, ME: DE, and MEn: DEn of glycerin were not influenced by the inclusion form (p > 0.05); however, for all these parameters, the values of the Cr method were smaller than those of TC (p < 0.05) (Table 5).

4. Discussion

The lower feed intake of the animals fed diets containing glycerin may be related to the characteristics of the test ingredients. Although glycerin has a sweet taste [15], this ingredient may influence feed intake due to impurities such as sodium, methanol and mineral matter, suggesting adjustments in diet composition, such as electrolyte balance [7,17,23].

In this sense, Hansen et al. [24] reported a decrease in the DFI of the animals as the glycerin level (0-16%) in the first week of supply increased, suggesting that the period of adaptation of the animals to the new ingredient was necessary. However, other studies [7,8] reported no change in the DFI when glycerin up to 15% was included in the diet.

The form of inclusion of the test ingredient did not influence the DCs except for EE. These findings imply that the top form with a 10% inclusion level can be used without detriment to the nutritional value of glycerin for pigs if it is more feasible in terms of logistics. Inclusions of up to 15% glycerin in diets for weaned piglets did not affect DM, CP or GE digestibility despite a linear increase in urinary production [25]. Evaluating inclusions up to 10%, Martínez-Miró et al. [1] reported no effect on the apparent total tract digestibility of DM, OM and CP or on the levels of IGF-1, serum protein, insulin, glucose, and albumin in finishing Iberian crossbred pigs. However, the potential of glycerol to promote the proliferation of advantageous bacteria at the gut level was investigated by Wei et al. [26]. They discovered that by altering the makeup of the fecal microbiota and metabolites, glycerol, when paired with other nutrients, may be a useful method of enhancing muscle redness in pigs.

Although glycerin can be used as a humectant and sweetener, providing flavor and color to food [27], previous studies have shown worse pellet quality [14], flowability in feeders [12] and animal performance [13] when glycerin inclusion is greater than 10%, probably due to difficulties in dosing and homogenization during mixing.

The mixing process is indispensable in feed manufacturing to support all nutrients in pigs. The use of liquid ingredients or mixing with water can interfere with the mixing, consumption and digestibility of animal diets. Thus, glycerin, which is a liquid ingredient, can improve the mixture when it is present in small proportions. In accordance with the findings of Brooks [28], the process of combining water with dry feed and subsequently providing it to pigs after a brief interval ensures a more consistent diet. This, in turn, accelerates the hydration process, particularly when the feed is finely ground, thereby supporting the optimal function of both digestive and in-feed enzymes.

The use of glycerin as a liquid ingredient, whether in a mixture or on top, provides new insights into enhancing the nutritional use of diets with different physical presentations. The organic matter digestibility and gross energy digestibility of pigs fed water-to-feed ratios of 2.1:1 and 2.7:1 was greater than those of pigs not fed water [11].

All the glycerin DCs presented lower values when generated by the Cr method than when generated via the TC method (p < 0.05) (Table 4). The DC and glycerin energy values obtained by Cr underestimated the values in relation to TC. This finding may be related to the nontotal indigestibility, recovery of chromium in the feces, quality of the analysis [19], quality of the mixture and variations in the diets [29]. Studies that evaluated glycerin [8] and dried distiller grains with soluble [30] also reported lower values of energy and DC determined with Cr than with TC. The results of this study are similar to those of Liu [31], who reported that the Apparent total tract digestibility (ATTD) values of GE, DM, OM, EE, CP and Neutral detergent fiber (NDF) were lower when the Cr method was used than when the TC method was used because of the recovery of markers in feces, necessitating further investigations.

Nevertheless, a study by Huang et al. [32] revealed that the DE, ME, ATTD of GE, ash, NDF and acid detergent fiber of experimental diets determined using Cr markers were greater than those determined using the TC method. In this context, Prawirodigdo et al. [33] reported a significant correlation (R² = 0.86, 0.88, 0.74) for the ATTD of nitrogen, OM and DM in experimental diets between Cr₂O₃, an indigestible marker, and total fecal collection. However, the authors reported that some results were smaller or larger depending on the basal diet and ingredient tested, so it appears that the amount and type of protein source can influence the reliability of the Cr₂O₃ marker, especially for the ATTD of N.

The DC and glycerin energy values obtained with Cr were lower than those obtained with the total collection method. Kavanagh et al. [34] reported that markers in feed could be lost at the mixing stage and sampling of feed, resulting in variations in marker analysis. Agudelo et al. [35] reported that the values of digestibility for DM and energy were lower for chromium oxide than for total collection and that the indicator method was less able to detect differences in ingredients with highly concentrated nutrients. In this sense, the results of Wang et al. [36] also revealed that the apparent digestibility of energy determined by TM was greater than that determined by Cr₂O₃ or TiO₂. The results of Jang et al. [37] indicated that Cr results in lower digestibility values than the TC method does and does not provide the same treatment difference as the TC digestibility for energy and nutrients that are not highly impacted by dietary treatment. In addition, the TC data seemed to have a lower coefficient of variation than the indicator method data for many components. Therefore, total collection has been considered the “gold standard”, as all voided nutrients are supposedly collected.

According to a recent review published by Zhang and Adeola [19], the markers should be 1) totally indigestible and nonabsorbable, 2) nontoxic to the digestive tract, 3) pass through the digestive tract at a relatively uniform rate with digesta, and 4) easy to analyze.

The energy value of glycerin is the main highlight of this ingredient. When values close to those of corn are presented, it becomes a potential substitute in diets for pigs. The glycerin values of GE (5397 kcal/kg), DE (3443, 3411 kcal/kg) and ME (3356, 3293 kcal/kg) determined in this study were close to 3387 and 3270 kcal/kg [8] but were lower than 5240, 5070 and 4556 kcal/kg [20] and 6500, 5839 and 5509 kcal/kg [10].

The difference in the DE and ME for glycerin in this study and others is related to the raw material, biodiesel process, glycerol and lower fatty acid contents [38]. Glycerol from glycerin is absorbed by passive diffusion and metabolized into glucose via phosphorylation to glycerol-3-phosphate by glycerin kinase [39], with excess being excreted in the urine [40] but no saturation up to 15% glycerin in the diet [41].

5. Conclusions

The glycerin from soybean oil had 3443 and 3411 kcal/kg of digestible energy and 3356 and 3293 kcal/kg of metabolizable energy for pigs, as determined by the total collection and indicator methods, respectively.

The inclusion of glycerin in the mixture or on top did not affect the digestibility coefficients or energy for pigs, but the values were lower when the addition of chromium oxide was used than when the total collection method was used.