A total 20 male Ghezel lambs were fed diets containing different concentration of monensin (0, 10, 20 and 30 mg/kg DM) in a completely randomized design. Diets were offered Ad libitum in the form of total mixed ration. Feed conversion ratio (FCR) improved signifiCantly by supplementation of diets containing high amount of concentrate with monensin (p< 0.01). Rumen liquor pH was significantly (p < 0.01) affected by monensin and the level Of concentrate in the ration (6.38 for 30 mg monensin/kg DM vs. 5.72 for 0 mg monensin/kg DM).Treatments had significant effect on the rumen reduction potential (RP) and total Acidity (p < 0.01) with higher rate (342.50 Sec., and 16.39) for 0 mg monensin/kg DM and Lower (271.25 Sec., and 15.26) for 10 mg monensin/kg DM and control diet, respectively. Monensin did not have a remarkable effect on sedimentation and flotation (SAF) and blood metabolites. The results showed that diets containing monensin may reduce the incidence of laminitis in lambs fed high concentrate: forage ratio due to the increased rumen pH and acidosis management.

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