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This study aimed to determine the effect of rosemary (Rosmarinus officinalis) essential oil (REO) in lamb fattening diets on blood, rumen parameters, fattening performance and meat quality. Thirty Norduz male lambs weaned at 4 months of age with average body weight 22.0 ± 4.41 kg were used. Lambs were divided into three groups: no rosemary oil (control, R0), with 250 mg/kg DM (R250) and with 500 mg/kg DM (R500) rosemary oil added to the basal ration, and fed for 70 d. The daily feed intake of the R500 lambs (1.63 kg) was lower than other groups (1.70 kg) (p ¼ .01). Rosemary oil did not change the density of methane-producing bacteria in the rumen fluid. Rumen pH was 6.31 in R250 lambs, while it was 6.16 in control (p < .04). The proportion of propionic acid (PA) increased in R250 (26.5 molar%) and R500 (26.0 molar%) lambs compared to control lambs (22.7 molar%) (p < .001). Serum glucose levels increased with REO dose (p < .01) and serum IGF-1 levels were significantly higher in R250 lambs (p < .001). The dose rates of REO used in fattening lambs had limited effect on fattening performance, carcase and meat quality. The results showed that although it does not affect the final live weight, the negative effect of REO on feed intake at 500 mg/kg was considered as a limiting factor and 250 mg/kg dose of REO may have positive effects on ruminal fermentation. Hence, it may be beneficial to try doses lower than 500 mg in further studies. HIGHLIGHTSRosemary oil may improve rumen pH and the propionic acid (PA) concentration at 250 mg of REO/kg DM of the diet. Serum IGF-1 levels were significantly increased with 250 mg/kg dose of REO. Feed intake and feed conversion ratio were negatively affected by REO at 500 mg/kg DM. Lambs supplemented with 500 mg REO/kg DM in the diet had the lowest CLA. The effect of rosemary oil on slaughter-carcase characteristics and meat quality was limited.

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