Introduction

The rising cost of fossil fuels (Benoit and Mottet 2023) and their capacity to generate polluting gases have motivated the interest for searching alternative energy sources, especially renewable ones. In this regard, biodiesel has an important place as a biofuel produced from vegetable oils or animal fats (Kumar Singh et al. 2024) through transesterification (Tang et al. 2024). Glycerin is the main byproduct resulting from biodiesel production (Bansod et al. 2024), so the development of biofuel producing industries has generated considerable volumes of glycerol, which can be used as an ingredient in diets for ruminants (Madrid et al. 2019) and non-ruminants, including pigs, laying hens and broilers (Tavernari et al. 2022). This contributes to improving economic sustainability of biodiesel industry and reducing the environmental impact caused by the generated waste (Garlapati et al. 2016 and Abdul et al. 2019).

Initially, glycerol was used in the treatment of bovine ketosis or pregnancy toxemia in sheep. However, its availability and the high price of cereals led to studies in which its effect as an energy component of the diet was evaluated. Generally, glycerol is used to replace corn grain, since both provide similar amounts of energy, so it can be an economically viable alternative in the formulation of rations for ruminants, especially when the price of corn increases.

Glycerol can be used pure or with a medium level of purity. With the latter, more discreet results are achieved, but without incurring the expense that the refining process entails. However, when using raw glycerol, the existence of some impurities must be considered, which can reduce the beneficial effects of the product and even compromise animal health. The objective of this review is to delve into some topics related to the use of glycerol in ruminant feeding.

Obtaining, properties and uses of glycerol

Glycerol can be obtained from complex lipids, by organic synthesis, through the fermentation of carbohydrates or from synthetic derivatives resulting from petroleum refining. Initially, the main way to obtain glycerol was saponification of fats in the soap manufacturing process, until the development of biofuel production companies began. According to Badia-Fabregat et al. (2019), for every 10 kg of biodiesel, approximately 1 kg of crude glycerol are produced through the transesterification of fats from vegetable or animal origin with methanol.

Small amounts of glycerol can also be obtained from marine microalgae, such as Dunaliella salina (Celente et al. 2022). Animals have an endogenous source from lipolysis of adipose tissue or hydrolysis of triglycerides from blood lipoproteins. Glycerol resulting from lipolysis follows the hepatic gluconeogenesis pathway and can provide up to 15-20 % of total glucose demands (Jeon et al. 2023).

Glycerol is a colorless, viscous, almost odorless liquid. It is soluble in water and alcohol and insoluble in ether and chloroform. Currently, it is used in the chemical industry for the synthesis of resins and esters (18 %), pharmaceutical industry (7 %), production of cosmetics (40 %), as a humectant and food preservative, in the preparation of dressings for salads, sweet toppings and frozen desserts (24 %) and others (11 %) (Cardoso et al. 2015). It has also been used in the manufacture of explosives (dynamite and nitroglycerin) (Wu et al. 2023). Its contribution to the pharmaceutical industry corresponds to its use as a component of capsules, anesthetics, syrups and antiseptics (Wan Azelee et al. 2019), while in the production of cosmetics, it improves softness, provides lubrication and has moisturizing properties.

Its gluconeogenic and antiketotic effect explains its use in the treatment of bovine ketosis (Mammi et al. 2021) and to prevent fatty liver syndrome (Zhang et al. 2023). It can be used in the treatment of pregnancy toxemia in sheep (Cal-Pereyra et al. 2015). It can be used as raw material for biopolymers, polyunsaturated fatty acids, production of ethanol, hydrogen and n-butanol (Garlapati et al. 2016), as well as in the production of biosurfactants (Trentini Volpato et al. 2022) and solketal (Kowalska-Kuś et al. 2020).

Most studies with glycerol are based on small proportions added to the diet, due to its gluconeogenic characteristics (Neiva et al. 2012 and Soares et al. 2012). However, higher quantities have been used as another component of the diet, because production volumes have exceeded utilization capacity (Donkin 2008). This can constitute a way to increase the biological and financial efficiency of biodiesel production and, at the same time, prevent it from being discharged into the environment, becoming another pollutant.

The increase in production volumes suggests a decrease in its price, which strengthens the idea of using it as a substitute for energy concentrates in the diet of ruminants (Khalid and Al-Anbari 2024). There are other properties described by Schröder and Südekum (1999), who refer to its easy absorption in the rumen and intestinal mucosa, as well as its antiseptic power, capable of sanitizing the ration, and its high palatability, which responds to its sweet flavor and its agglomerating effect because it is hygroscopic. It is a normal compound in the metabolism of ruminants, which is found in the blood as well as in the cells.

Metabolism of glycerol

The glycerol that reaches rumen can follow three destinations. It is estimated that 44 % of glycerol that reaches the organ is fermented, 43 % is absorbed through the rumen wall and 13 % passes to the digestive compartments after the rumen, although these proportions can vary (Krehbiel 2008). Excess glycerol can be absorbed by the ruminal and intestinal mucosa, which constitutes a direct gluconeogenic source for the ruminant (Ortega-Cerrilla et al. 2018). According to Hejna et al. (2016), as a result of the microbial fermentation of glycerol, several chemical compounds could be obtained, such as propionic acid, succinic acid, butanol, propanediol, dihydroxyacetone, among many others.

According to Cabrera-Cruz (2019), replacing glycerol with corn in the diet does not generate a negative effect on the ecology of the rumen, even when there is a modification in the synthesis of volatile fatty acids. Glycerol is capable of increasing total production of volatile fatty acids, in vivo (Rémond et al. 1993 and Wang et al. 2009a) and in vitro (Trabue et al. 2007). In addition to increasing, fundamentally, the production of propionic acid (Wang et al. 2009a and Chanjula et al. 2016). It can enter the glycolytic pathway and it transform into pyruvate, which generates propionate via two different routes: succinate or acrylate. This justifies the increase in propionate, by adding glycerol to the ruminant diet (Cardoso et al. 2015).

The inclusion of glycerol to the diet can also increase the production of butyric acid (Kupczyński et al. 2020), and propionic and butyric acid (Van Cleef et al. 2016 and Madrid et al. 2019) with decrease in acetic acid (Chanjula et al. 2016), which contributes to the decrease in the acetic:propionic ratio (Wang et al. 2009a). The majority of glycerol is fermented to volatile fatty acids through the glycolytic pathway, with little production of lactic acid (Trabue et al. 2007).

Considering that propionic acid and glycerol itself are potent neoglycogenic agents (McWilliams 2023), it is reasonable to use glycerol as an energy supplement for milk production in the transition period. It could even be more recommended than other energy sources because it has a metabolic advantage over its traditional counterparts, especially propionate and propylene glycol, because it enters gluconeogenesis at the level of phosphate-isomerase, metabolically closer to glucose (Wang et al. 2021).

Propionic acid and glycerol are absorbed and reach the liver via the portal vein for subsequent conversion to glucose (Arias-Islas et al. 2020). According to Lei and Simões (2021), propionic acid produced by ruminal fermentation is the main substrate for gluconeogenesis in high-producing dairy cows. Between 50 and 60 % of the total glucose required is obtained in this way. Propionic acid production in the rumen is greater in animals that consume concentrate than in those that consume forage. Therefore, in grazing animals, glycerol supplementation could increase energy efficiency (Huerta-Jiménez et al. 2018).

According to studies carried out by Rémond et al. (1993), the maximum disappearance rates of glycerol in the rumen, determined by in vitro fermenters, is 0.52 to 0.62 g.h^-1. Other data suggest that, with a dose of 240 g of glycerol, disappearance rates in the rumen are between 1.20 and 2.40 g.h^-1. In studies where levels between 15 and 25 % of glycerol were supplemented, most of it disappeared within six hours (Bergner et al. 1995). According to Donkin (2008), between 50 and 70 % of the glycerol disappears from the rumen within four hours.

On the other hand, the studies carried out by Chanjula et al. (2016) reduced ammoniacal nitrogen levels in the rumen by including 6 % of glycerol in the diet. However, Correa and Moreno (2019) did not modify blood urea nitrogen content.

Effect of the use of glycerol on dry matter intake

Studies in which glycerol was used showed variable results in relation to dry matter intake (DMI). In some other, this indicator was not modified with the inclusion of glycerol in the diet (Moriel et al. 2011 and Van Cleef et al. 2014). However, Bodarski et al. (2005) reported increases in intake by approximately 2 kg of DM at 70 d, while Ogborn (2006) observed a depressive effect in the postpartum stage, Ladeira et al. (2016) in young bulls when they used 18 %, and Chanjula et al. (2016) used 6 % in goats.

DM intake was also modified in the work carried out by Shin et al. (2012) when using glycerol. In addition, they obtained an effect of the inclusion levels of the product, with the highest values of 5 % (28.40 kg. d^-1). Neiva et al. (2012) did not modify this indicator within the same category (cows and steers), but when comparing cows with steers they observed a reduction in intake for the latter, when glycerol was used at a rate of 6 and 12 % of the DM.

Use of glycerol in animal diet as energy additive

According to Donkin et al. (2009) the energy provided by glycerol is similar to that of corn starch, when used in dairy cows. Nevertheless, the energy value of glycerol depends on its purity degree, on the percentage it represents regarding total dry matter (DM) and on the starch content of the used concentrate. Schröder and Südekum (1999) determined net energy of lactation of glycerol and obtained values of 2.30 Mcal.kg^-1, when it is offered in diets with low starch content, and between 1.91 and 2.03 Mcal.kg^-1 in diets with high starch content.

The production of biodiesel generates byproducts with potential use in animal feed (de Souza et al. 2014). In this group, glycerol stands out, which can be used as an energy source (Soares et al. 2012). It has been used in diets for pigs (Martínez-Miró et al. 2021, Dahmer et al. 2022 and Li et al. 2022) and for broilers (Liu et al. 2020).

However, most of the studies carried out are aimed at feeding ruminant animals. Sotgiu et al. (2021) used it in sheep and Chanjula et al. (2016) in goats. Prado et al. (2015) and Ladeira et al. (2016) used it in bulls. Likewise, Moriel et al. (2011) applied it to the feeding of replacement heifers in breeds intended for meat production. Correa and Moreno (2019) studied its effect in Holstein cows.

There are several studies that include glycerol in the diet of high-producing dairy cows in the transition period (Bodarski et al. 2005, Chung et al. 2007 and Wang et al. 2009b). Some studies refer to its use with a high level of purity (Donkin et al. 2009 and Carvalho et al. 2011) or raw (Neiva et al. 2012). In other research carried out with Holando bulls, glycerol was used as a substitute for corn grain in the ration for dairy cows (Donkin et al. 2009 and Carvalho et al. 2011) and in fattening (Mach et al. 2009). It was also used in the production of rumen activators for the production of beef in feeding systems with fibrous materials (Iriñiz et al. 2011).

De Frain et al. (2004) studied different levels of glycerol inclusion in the diet, at a rate of 430 and 860 g.cow^-1d^-1. Bodarski et al. (2005) did it in doses of 300 and 500 mL, while Ogborn (2006) included 504 g.cow^-1d^-1. Chung et al. (2007) evaluated lower amounts, by supplying 250 g.cow^-1d^-1. Wang et al. (2009b) introduced 100 and 300 g.cow^-1d^-1, and Lounglawan et al. (2011) worked with 150 and 300 g.cow^-1d^-1.

Donkin et al. (2009) used glycerol levels between 5 and 15 % of DM. Carvalho et al. (2011) included it at 11.50 and 10.80 % for prepartum and postpartum, respectively. Similar levels, 0 to 12 %, were used by Mach et al. (2009) in the diet. However, other studies surpassed them. D'Aurea et al. (2017) applied up to 20 % in combination with urea to evaluate some rumen parameters and the performance of the microbial mass.

Neiva et al. (2012) added up to 24 % in diets for dairy breed steers and cows. Van Cleef et al. (2014) included up to 30 % in the feeding of confined Nelore bulls.

According to Ortega-Cerrilla et al. (2018), the results obtained with the use of glycerol depend on base diet quality, purity degree and inclusion level. Precisely, the latter is one of the main questions generated by the use of said product. Generally, it is used in quantities close to 10 %. According to Donkin (2008), glycerol should be used at least 10 % of DM in diets for dairy cows. Shin et al. (2012) stated that they were successful in using diets for dairy cows, whose glycerol content was between 5 and 15 % of the total DM. However, the use of 15 % glycerol in dairy cows in mid-lactation may be accompanied by a temporary decrease in food intake (Donkin et al. 2009).

Effect of glycerol on diet digestibility

Studies of Schröder and Südekum (1999) demonstrated that the effect of glycerol on diet digestibility is determined by the amount of starch contained in it. Generally, when high starch diets are used, cell wall digestibility is reduced.

Other studies report that there was no modification in fiber digestibility (Hess et al. 2008) with the use of glycerol. Wang et al. (2009a) managed to increase the digestibility of organic matter and crude protein with the increase in glycerol supplementation up to an average level of 200 g.animal^-1d^-1, but with the increase of glycerol levels, digestibility decreased slightly. In studies conducted by Van-Cleef et al. (2014) also demonstrated an increase in the digestibility of crude protein with a reduction in the digestibility of neutral detergent fiber, as a result of a decrease in the digestibility of hemicellulose, when using 30 % of glycerol in the diet. Chanjula et al. (2016) reduced diet digestibility by using 6 % of glycerol.

Effect of glycerol on live weight and body condition

Several authors found no differences in live weight and body condition (Carvalho et al. 2011, Moriel et al. 2011 and Shin et al. 2012). However, Bodarski et al. (2005) observed a positive effect of glycerol on body condition at the end of the evaluation period. Wang et al. (2009b) reported its effect on live weight gain, as do Donkin et al. (2009) by providing 10 and 15 % of glycerol in the diet. Neiva et al. (2012), Van Cleef et al. (2014), Chanjula et al. (2016) and Ladeira et al. (2016) did not modify weight gain with the use of glycerol.

Effect of glycerol on meat quality

In the studies carried out by Lammer et al. (2015), carcass quality was not affected when using glycerol in pigs. However, the level of monounsaturated fatty acids in adipose tissue increased by increasing the content of this product in the diet.

There were no major changes when including glycerol in cattle. Elam et al. (2008) verified that the addition of up to 15 % in crossbred heifers does not affect intramuscular fat deposition and meat yield. The area of Longissimus dorsi (LD) muscle and its fat content were not modified in the studies carried out by Mach et al. (2009), who used 12.1 %. Similarly, Prado et al. (2015) reported no damage on the area and composition of the LD, the same with the thickness of the back fat and some indicators of meat quality (marbling, texture and color).

Nevertheless, Van Cleef et al. (2014) increased fat levels in carcass and Ladeira et al. (2016) favored the marbling of meat with the use of glycerol.

Effect of glycerol on milk production

According to Ogborn (2006), the effects of glycerol on milk production are observed when levels higher than 6 % are used. This may explain why this indicator is not modified in the studies carried out by Chung et al. (2007) and Lounglawan et al. (2011). However, Bodarski et al. (2005) increased milk production with small doses of glycerol, which could be related to the increase of DM intake, although Shin et al. (2012) increased dry matter intake, without modifying milk production.

Milk production was also increased in experiments carried out by Khalid and Al-Anbari (2024) and Correa and Moreno (2019). Meanwhile, Donkin et al. (2009), Wang et al. (2009b) and Carvalho et al. (2011) did not report modifications.

Bonis et al. (2022) obtained an increment in milk production of 155.91 % in multiparous Siboney cows from Cuba, which were grazing pitilla grass (Sporobolus indicus (L.) R. Br) when they used glycerol, obtained from the Jatropha curcas biodiesel manufacturing process.

Effect of glycerol on milk composition

Milk components are generally not affected by the use of glycerol (Carvalho et al. 2011 and Kupczyński et al. 2020). However, there may be some modifications in fat and protein. Bodarski et al. (2005) obtained increases in protein with the increase of glycerol in the diet, while Wang et al. (2009b) recorded a decrease.

Another indicator that can vary is fat content. Shin et al. (2012) confirmed the higher values of fat quantity and concentration, when using 5 % of glycerol. The same occurred in the studies carried out by Correa and Moreno (2019). However, the incorporation of glycerol in the diet reduced the proportion of fat in milk, as reported by Lounglawan et al. (2011). In studies by Donkin et al. (2009), a decrease in urea content was recorded.

Main limitations of the use of glycerol

The main limitation of glycerol derived from biodiesel industry is its methanol content. Other impurities, such as soaps, sodium and diethylene glycol, can also have a negative influence. Methanol and diethylene glycol are potent tissue toxicants. However, a dairy cow of 600 kg live weight is capable of consuming 7.44 mg of methanol, that is, 1.24 % for each kg of live weight and converting it into H₂O and CO₂. This is because, under normal conditions, methanogenic bacteria in the rumen transform it into methane (Soares et al. 2012). The toxic and limiting effect of methanol intake is more frequently verified in monogastric or pre-ruminant animals (calves).

There are limits for the proportion of methanol in glycerol, which will be used in animal feed. Studies carried out by the US Food and Drug Administration (FDA - USA) indicate that methanol levels greater than 150 p.p.m. can be considered unsuitable for animal feeding. Higher levels have been established in Germany, where a maximum limit of 5,000 p.p.m was defined (Sellers 2008).

Methanol is metabolized in the liver, transforms into formaldehyde, formic acid and finally CO₂ and water. The metabolism of formic acid is slow, so it accumulates in the body and produces metabolic acidosis (Soares et al. 2012). The effects related to methanol poisoning manifest themselves with damage to the optic nerve, neurological and renal disturbances, as well as degeneration of liver fat.

According to Soares et al. (2012), heavy metals and sodium level could also limit its use in the diet. Excess sodium reduces intake and animal yield. In addition, the incidence and severity of udder edema increases, mainly in prepartum heifers.

To avoid the harmful effects of impurities, some authors recommend purification of the product. However, this process has a high cost (Chol et al. 2018), so it is necessary to assess whether it is profitable and to what extent it is more feasible to purify. Evaluations of this glycerin, tested by Schröder and Südekum (1999) and Thompson and He (2006) indicated contents in the order of 63 to 76 % of glycerol in crude glycerin of low purification. The glycerol content increases to 85 % in medium purifications, with a significant reduction in the methanol content, which ends up being less than 0.50 % and can reach 99 % of glycerol, when the purification process continues (Schröder and Südekum 1999).

General considerations

Currently, biofuel production companies are the main source of glycerol. Its low cost, high palatability, gluconeogenic effect and energy content are some of the properties that support its use as animal feed. Several researches demonstrate promising results with the use of this by-product as an alternative energy source for feeding ruminants.

The inclusion of glycerol in ruminant diet could improve the value of meat and milk fat by increasing the anti-cancer, anti-diabetogenic and anti-dipogenic properties due to the presence of conjugated linoleic acid. These reasons place it as a product with functional characteristics. The development of future research that demonstrates this will allow the diversification of its use and commercialization.

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Author notes

*Email:alvaro85.del@gmail.com

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