The histomorphometric characteristics of the rumen, intestine, kidneys, and liver of lambs fed with CB levels were assessed in the present study. Our results indicated that the intermediate levels of inclusion of CB in the diet of lambs do not alter the histomorphometric characteristics of the gastrointestinal tract and hepatic or renal changes, which could compromise the animal's performance.
The inclusion of CB resulted in a quadratic effect for ADG, this result is possibly a consequence of the decrease in the DMI with the inclusion of CB in the diet. The estimated inclusion of 7.2% of CB in the diet sustained the same ADG when compared to conventional diets in tropical regions using corn and soybean meal, which suggests that the evaluated by-product is an alternative that can be used as a substitute for grains in low quantities.
The quadratic effect for ruminal papilla height and epithelium thickness was possibly caused by the decrease in DMI and non-fibrous carbohydrates by lambs fed with higher amounts of CB, which possibly reduced the fermentable substrates by the ruminal microorganisms and may cause less production of the short-chain fatty acids at higher levels of CB inclusion (Rezaei et al., 2014; Shi et al., 2020). This fact can be explained by the absorption of the short-chain fatty acids, which stimulates the epithelial metabolism of the rumen and induces the growth of the epithelium (Baldwin et al., 2004; Wang et al., 2009), with the development of the height of the papillae strongly influenced by concentrations propionate and butyrate, as demonstrated by Suárez et al. (2006a,b), when observing greater development of the rumen mucosa in calves that were fed diets that provided higher concentrations of propionate and butyrate in the rumen.
A difference of 4.9% (56.8 µm) was estimated between the height of the ruminal papillae for the inclusion levels of 0% and 7.2% of CB in the diets, however, a more impressive decrease was observed in the papillae height, where the inclusion of 14.4% of CB promoted 18.1% of difference (202.19 µm), compared to the inclusion of 7.2% of CB. Possibly, up to the 7.2% level of CB inclusion, the propionate produced through the fermentation of glycerol released by the hydrolysis of triglycerides in CB stimulated the development of the papillae, and thus, combined with the increased availability of lipids in the small intestine, sustained similar performance when CB was not included in the diet, even with the decrease of non-fibrous carbohydrates in the diet.
Maintaining the height of the ruminal papillae is an important physiological process because even with the decrease in DMI, the quadratic effect found for the absorption area ensured that the products of rumen fermentation, which are important sources of energy for lambs, were absorbed and supply part of the lamb’s energy requirements (Lesmeister et al., 2004). Since the NDF was constant between the levels evaluated, the decrease in the muscular layer of the rumen may be associated with an increase in the lipid content of the diets and a decrease in the DMI with the inclusion of CB, which resulted in a reduction in peristalsis and an increase in the duration of digesta retention (Owens and Basalan, 2016). This is because the intake of diets rich in EE stimulates the release of cholecystokinin (CCK), which in turn acts by reducing the motility of the rumen and small intestine (Reidelberger, 1994), and causes less hypertrophy and hyperplasia of the smooth muscle fibers that make up the rumen, leading to less development of the muscle layer compared to the 0% level (Suárez et al., 2006b). In other studies, the lower development of the rumen muscle layer was also associated with decreased ruminal motility when there were changes in the chemical composition of the evaluated diets (Wang et al., 2009; Lima et al., 2019).
The quadratic effect observed for epithelial keratinization possibly reflects the decrease in the passage rate at the highest levels of CB inclusion, which induced a lower flow of the digest over the ruminal papillae, decreasing the abrasive effect on them, which led to a lower turnover rate cell in the outer layer of the epithelium, causing the keratin layer of the ruminal epithelium to decrease (Silva et al., 2020b). In the small intestine, the decrease in the passage rate possibly caused a linear decrease in the submucosal layer with the inclusion of CB. In this layer are located the Brunner glands that secrete an alkaline mucus to neutralize the pH of the region, and to protect the mucosa of the duodenum against harmful agents (Verdiglione and Montesi, 2019). Possibly, a decrease in the passage rate through this organ led to a decrease of mucus produced, thus causing a decrease in these glands, and consequently reducing the thickness of this layer (Lang and Tansy, 1983).
The amount of goblet cells in the intestine is considered a parameter of intestinal health, being better with the increase of these glands (Bueno et al., 2012). These cells are responsible for the mucin production, mucus responsible for assisting in peristalsis, in the mechanical protection of the intestinal epithelium and acting against infectious agents of the intestinal mucosa, and being a component of the intestinal glycocalyx, which helps in the food digestion. The lower passage rate in the higher levels of inclusion of CB may have decreased the proliferation of these cells for reducing the need to protect the epithelium against abrasion caused by the passage of the digesta, and thus, lesser need for mucus production.
The increase in the inclusion levels of CB increased the energy density of the diets, and the lipids started to represent an important source of energy to supply the lamb's energy demand. This event is evident with the increase of the intestinal mucous layer. This morphological modification allows the greater digestive capacity of the small intestine since a greater thickness of the mucosa indicates a greater height of the intestinal villus, because of the energy content of the diet and decreased passage rate (Montanholi et al., 2013; Lima et al., 2018). Thus, there was a greater area of contact between villi and nutrients and favored digestion and absorption to its maximum extent (Van Soest, 1994; Yansari et al., 2004; Gabriel et al., 2008). Despite apparently pointing to an increase in metabolic efficiency, there was no increase in ADG with the increased inclusion of CB, as there was a decrease of 65.3% in IDM between levels 0 and 19.2% of inclusion of CB.
The absence of liver and kidney damage in the lambs fed with CB may have occurred due to the absence of toxicological properties and anti-nutritional factors of the coconut (Lima et al., 2015). Based on this parameter, the inclusion of CB in all levels evaluated herein did not damage the liver and kidneys of lambs.
Therefore, our results indicate that CB can be included in diets for lambs up to the level of 7.2% without causing changes in the histomorphometric characteristics of the gastrointestinal tract and changes in liver and kidney tissue that compromise animal performance.