Effects of letrozole supplementation on growth performance, blood indexes, ruminal fermentation parameters, and microbiome composition of hu lambs

This study aimed to explore the effects of dietary letrozole (LE) supplementation on growth performance, rumen microbiota, fermentation profiles, and blood metabolites in <i>Hu</i> lambs, providing insights into its potential for enhancing animal production. Twenty-eight male <i>Hu</i> lambs (20.21 kg ± 0.56 kg, 70 days old) were randomly assigned to four groups, with seven replicates per group: a control group (CON), and three test groups (T1, T2, T3). Lambs in the CON group were fed a basal diet, while T1, T2, and T3 groups received 0.05, 0.1, and 0.2 mg/kg BW of LE, respectively, in addition to the basal diet. The experiment lasted for 46 days. The findings were as follows: (1) There were no significant differences among groups in Initial Body Weight (IBW), Final Body Weight (FBW), Average Daily Feed Intake (ADFI), Average Daily Gain (ADG), and feed conversion ratio throughout the entire trial (<i>P</i> > 0.05). (2) Compared with the CON group, plasma testosterone (T) concentrations in Groups T2 and T3 were significantly elevated at 0 h post-supplementation (<i>P</i> < 0.05). Compared with the control group, nitric oxide (NO) levels in Groups T1 and T2 were significantly reduced 4 h after LE supplementation (<i>P</i> < 0.05). (3)Nitric oxide (NO) levels in experimental groups exhibited a secondary change 4 h after supplementation (<i>P</i> < 0.05). There were no significant differences in plasma Total Antioxidant Capacity (T-AOC), Catalase (CAT), Superoxide Dismutase (SOD), Glutathione Peroxidase (GSH-Px), or Malondialdehyde (MDA) levels between 0 h pre-supplementation and 4 h post-supplementation across all experimental groups (<i>P</i> > 0.05). At 0 h before and 4 h after supplementation, Total Protein (TP), Albumin (ALB), and Globulin (GLB) levels in all experimental groups showed no significant differences compared to the CON group (<i>P</i> > 0.05). (4) Ammonia nitrogen (NH₃-N) levels were extremely significantly higher in all test groups compared to the CON group (<i>P</i> < 0.05). Propionic acid and isovaleric acid concentrations in Group T3 were significantly higher than in the CON group (<i>P</i> < 0.01), while the ethyl-to-propyl ratio was significantly lower (<i>P</i> < 0.01). (5) At the phylum level, LE-treated groups showed a higher relative abundance of <i>Firmicutes</i> than the CON group (21.04%), with increases proportional to the LE dose: Group T3 (37.88%), Group T2 (32.74%), and Group T1 (30.66%). At the family level, the relative abundance of <i>Prevotellaceae</i> was significantly lower in all test groups compared to the CON group (<i>P</i> < 0.05), while <i>Lachnospiraceae</i> abundance was significantly higher in the test groups (<i>P</i> < 0.01). Under the experimental conditions, supplemental feeding of LE did not significantly affect the overall growth performance of lambs. but it did increase plasma testosterone concentration, elevated the relative abundance of Firmicutes in the rumen, reduced the relative abundance of Bacteroidetes, and altered the rumen fermentation pattern. This shift occurred by decreasing the acetate-to-propionate ratio, increasing isovaleric acid concentration, and promoting a propionic acid fermentation pattern, thereby improving feed utilization. Among all groups, the optimal supplemental feeding rate was determined to be 0.2 mg/kg BW.