Feedlot finishing of goats is a growing practice, but the economic viability of this technology is compromised by the inclusion of ingredients such as corn and soybean, which have high acquisition costs for farmers. An alternative to minimize this barrier is the use of agroindustry coproducts as substitutes for those ingredients, such as crude glycerol (a glycerol-rich alternative), a coproduct from the production of biodiesel.
Glycerol is a component of normal animal metabolism found in the bloodstream and in cells and is easily employed by animal bodies , where it can be absorbed directly by the ruminal epithelium, metabolized in the liver, directed to gluconeogenesis, and converted into glucose [2, 3]. Nevertheless, glycerol is not used in the animal feed; instead, glycerin, a product resulting from the biodiesel process and rich in glycerol (1,2,3-propanetriol) and containing other components, such as lipids, salts, water, and methanol, is used .
In ruminants, glycerol is completely fermented by ruminal fermentation to volatile fatty acids, especially propionate and butyrate [5, 6], which decrease ruminal pH and cause negative effects on ruminal microbial protein synthesis, and ruminal fermentation [7, 8].
According to the Brazilian Ministry of Agriculture, Livestock and Supply [5, 9], the national glycerin standard for animal feeds is at most 150 ppm of methanol and at least 80% glycerol. Therefore, standardization of the process is essential for the use of glycerin as an ingredient in animal feed. However, rules to prohibit the marketing of glycerin that do not meet these requirements are lacking, and inspection of those levels is nonexistent. Reports on disorders caused by glycerol were not foundis very scarse in the literature for goats, but a little papers have shown some effects on metabolic status of beef cattle and dairy cattle [10, 11, 12], although the results are inconsistent. concerns exist around the use of glycerol residues in animal feeding.
The inconsistency in results may be due to the glycerol purity, duration of supplementation, the speed with which glycerol is fermented in the rumen, and the absorption of glycerol, which is metabolized in the liver, in the rumen epithelium . The inconsistency between experiments reveals that more experiments are recommended to validate or refute the importance of including glycerol as a feed ingredient or feed supplement in the diet of ruminants.
Diets rich in substances such as methanol can cause changes in ruminal physiology and, depending on the type of feed, can affect the microorganism population, feed passage rate, and nutrient motility and absorption speed. These factors can cause a series of metabolic disorders that can lead to animal efficiency production losses, particularly to economic losses to farmers [14, 15].
The evaluation of the metabolic profile of goats submitted to new dietary systems using clinical biochemistry, including the determination of serum concentrations of protein and energy profiles and of enzymes related to liver activity (such as alanine aminotransferase, aspartate aminotransferase, and gamma-glutamyl transferase) and urinary profiles because these indicators help diagnose metabolic disorders and other diseases.
In view of the above, this study aimed to assess the effect of the inclusion of crude glycerin in the diet of feedlot-finished goat kids on metabolic, protein, energy, and urinary profiles, as well as to provide a histopathological evaluation of kidney and liver tissues.