This study was conducted to investigate the effects of dietary supplementation with rumen-protected arginine (RP-Arg) on growth performance, rumen fermentation parameters, microbial diversity, and blood physiological and biochemical indices in fattening Altay sheep. A total of 24 healthy, 6-month-old Altay male lambs were randomly assigned to three groups, with eight replicates per group. The control group received a basal diet, while the experimental groups were supplemented with either 0.50% or 1.00% RP-Arg on a dry matter basis, respectively. The results indicated that RP-Arg supplementation had no significant effect on feed intake, growth performance, or slaughter performance of the lambs (p > 0.05), whereas backfat thickness decreased linearly (p < 0.05). With increasing RP-Arg levels, serum low-density lipoprotein cholesterol, leptin, and catalase activity increased linearly (p < 0.05), while blood ammonia, alanine aminotransferase, and Ig M exhibited a significant quadratic increase (p < 0.05). RP-Arg supplementation led to a linear decrease in ruminal propionate and valerate concentrations (p < 0.05). Analysis of bacterial diversity revealed that the class Vampirivibrionia and the order Gastranaerophilales were biomarkers for the 0.50% RP-Arg group, while several taxa within the phyla Proteobacteria and Thermoplasmatota served as biomarkers for the 1.00% RP-Arg group. In summary, although supplementation of a forage-based diet with RP-Arg partially modified rumen microbial composition and fermentation profile, and regulated several blood biochemical parameters, it did not translate into any beneficial effects on growth performance. Larger-scale studies are therefore warranted to further elucidate the role of RP-Arg in fattening lambs.

This study aimed to evaluate the effects of dietary supplementation with a defined blend of phytochemicals (DBP) composed of carvacrol, thymol, and cinnamaldehyde on the growth performance, slaughter performance, rumen fermentation, microbial diversity, and blood physiological and biochemical parameters of sheep. Twenty-four healthy male Altay lambs, aged six months, were randomly assigned to three groups: (1) fed a basal diet (CON), (2) basal diet with 400 mg/kg DM of DBP (DBP1), and (3) basal diet with 800 mg/kg DM of DBP (DBP2). Results show that DBP supplementation had no significant effect on growth or slaughter performance (p > 0.05). The molar proportion of acetate and the acetate-to-propionate ratio increased linearly, and the molar proportions of propionate and valerate decreased linearly (p < 0.05). DBP supplementation had no significant effect on rumen bacterial α-diversity; however, in the DBP1 group, the relative abundances of Succinivibrionaceae UCG-002, Prevotellaceae UCG-004, Sphaerochaeta, Monoglobus, and Moryella were significantly increased, whereas in the DBP2 group, the relative abundances of Coprococcus and U29-B03 were significantly increased (p < 0.05). DBP exhibited a significant quadratic effect on interleukin-2 and superoxide dismutase activity (p < 0.05). In conclusion, although the DBP altered the rumen microbial community structure and rumen fermentation pattern in sheep to some extent, it showed minimal efficacy in improving growth performance, slaughter performance, immune function, and antioxidant status. Further large-scale studies are warranted to determine the optimal inclusion level and timing of this phytochemical blend in sheep diets.