A global social and moral problem to meet the food demands of population growth and the environmental sustainability of animal production is the high competition between food and feed (Herrero et al., 2015; Makkar, 2018).
Most monogastric animal diets contain soybean meal as the main source of protein (García-Rebollar et al., 2016). In the United States alone, livestock, poultry, and fish farms will consume more than 370 million tons of soybean meal in 2030, with 9 times more in the use of pesticides and causing greater deforestation for its cultivation (Pagano, 2016).
Another protein source is fishmeal, which due to its high nutritional quality (El Sayed, 2020) has become the most expensive product for animal feed; and although soybean, corn, and fish-based feeds have high nutritional value, on average, the caloric and protein efficiencies of these diets are 7–8% and lead to large energy losses (Shepon et al., 2016).
One strategy to reduce food-feed competition is to reduce the proportion of components for human use in animal feed (Schader et al., 2015) with non-conventional alternatives, such as by-products and agro-industrial waste (El Sayed, 2020; Schader et al., 2015), many of them with inadequate disposal processes that pollute the environment (Caro et al., 2014; Sivaram and Barik, 2019). These wastes, after proper processing, could be reincorporated into the production chain, making production systems more sustainable (Schader et al., 2015; Sivaram and Barik, 2019; Omoloso et al., 2021).
Among these strategies, the production of unicellular proteins has been proposed from residues, protein isolates and hydrolysates, co-products of the biofuel industry (Makkar et al., 2018), earthworm meal and other residues (Castro-Bedriñana et al., 2020), other residues from the animal industry that are incorporated into other productive subsystems must be identified, such is the case of the leather industry, which generates different residues (Kanagaraj et al., 2015; Omoloso et al., 2021). For every ton of bovine leather, 200 kg of usable leather is obtained, the waste includes 250 kg of solid waste free of contamination and without chemical products, 200 kg of tanning waste and 50 tons of wastewater (Hüffer and Taeger, 2004) that must be properly treated.
After receiving the skins, the cheeks can be obtained free of toxic substances, to be transformed into a meal with economic and environmental value as sustainable industrial practice, and reduce the negative impact of its disposal on the environment (Omoloso et al., 2021; Sivaram and Barik, 2019; Kanagaraj et al., 2015).
Bovine cheek meal can be used as a source of protein in the diets of small monogastric species such as the guinea pig, a species native to South America widely used for centuries by its population as a source of animal protein (Dunnum and Salazar-Bravo, 2010), a species that is gaining importance in the world for its lean meat, rich in protein, low in saturated fat and cholesterol, rich in calcium, iron, zinc, and vitamins of the B complex, and in several countries guinea pig blood is being used for the treatment of some classes of tumors or neoplasms since it contains the enzyme alpha asparaginase effective for the treatment of some lymphocytic leukemias (Avilés et al., 2014). Additionally, its contribution of tryptophan and phenylalanine favors the synthesis of antibodies and improves the response immune function of the body and would help in the rehabilitation of patients with various diseases, including COVID-19.
To develop scientifically balanced diets, it is necessary to know the nutritional value of feeds, so the objective of this study is to determine the proximal composition, digestibility, and metabolizable energy content of bovine cheek meals through indirect in vivo digestibility tests, comparing the effect of their inclusion at 15 and 20% in the reference diet, using adult male guinea pigs as an animal model.