In vitro and in vivo investigations on the use of yellow mealworm (Tenebrio molitor) as a novel protein feed ingredient for fattening lambs

The quest for novel, alternative, and sustainable protein feed sources, including insects, has gained attraction by the feed industry. Here, two experiments (Exp.) explored the impacts of Tenebrio molitor meal (TMM) as a protein feed ingredient for fattening lambs, as compared to conventional plant-based and animal-based protein sources, namely soybean meal (SBM) and fish meal (FM), respectively. In Exp. 1, in vitro rumen gas kinetics and fermentation profile of three experimental diets [1- SBM at 150 g/kg dry matter (DM); 2- FM at 50 g/kg DM; and 3- TMM at 60 g/kg DM] were assessed using three fistulated lambs. In Exp. 2, twenty-four male Suffolk lambs [3 months of age; 24 ± 1.3 kg body weight (BW)] were randomly assigned for 30 days to one of the three experimental diets. Feed intake, digestibility, and nitrogen balance were assessed. At the end of the trial, the lambs were slaughtered, and samples of rumen (dorsal and ventral sacs), proximal intestine, and liver were collected and subjected to histomorphometric and histopathological evaluations. Data were analyzed using a general linear model and mixed models for the in vitro and in vivo trials, respectively; histopathological data were analyzed by a Kruskal-Wallis test. Results of Exp. 1 showed that total gas production was lower ( P = 0.021; η 2 = 0.359), while DM disappearance ( P = 0.021; η 2 = 0.233) and microbial crude protein production ( P = 0.015; η 2 = 0.347) were higher, for the TMM diet when compared to the FM diet, while the SBM diet showed intermediate values. Results of Exp. 2 revealed that the DM intake of TMM-fed lambs was comparable to that of FM-fed lambs, but lower than that of SBM-fed lambs ( P = 0.035; η 2 = 0.407). The average daily gain was the lowest in the TMM-fed lambs ( P = 0.033; η 2 = 0.373), while the final BW remained unaffected by diet. Higher total tract apparent digestibility of DM and organic matter was obtained with the TMM and FM diets than with the SBM diet ( P < 0.001; η 2 = 0.411 and 0.408, respectively). Nitrogen balance ranked in the following order: FM > TMM > SBM ( P < 0.001; η 2 = 0.385). Ruminal morphometric indexes (i.e., papillae length and width, absorption area, and thickness of mucosa, submucosa and muscular layers) were not affected by diet. When compared to SBM-fed lambs, the TMM- and FM-fed ones exhibited significantly lower severity of epithelial keratinization in the ruminal dorsal sac ( P = 0.034; ε 2 = 0.299). Intestinal inflammation was predominantly characterized by mononuclear cells and eosinophils in the TMM and SBM groups, respectively. No differences among the experimental groups were observed for other histopathological features at rumen, intestine, and liver levels. Overall, our results suggest that TMM can be used as a protein source for fattening lambs, but further studies should explore its use in combination with conventional protein sources to optimize the performance of lambs. • Tenebrio molitor larvae meal (TMM) was evaluated as protein source for fattening lambs • The TMM diet showed comparable total gas production than the soybean meal (SBM) diet • The TMM and fishmeal (FM) diets showed higher organic matter digestibility than the SBM diet • Nitrogen balance ranked in the following order: FM > TMM > SBM • Dietary inclusion of TMM did not affect ruminal morphometric indexes