Thirty-six Naeimi ram lambs were equally and randomly allotted to four treatment groups with three replications per treatment to determine the simple and additive effects of monensin and zeranol on growth performance, carcass characteristics and nutrient digestibility. The treatment groups were: basal diet-fed lambs (C), monensin-fed lambs (M) where the basal diet was supplemented with 33 mg monensin per kilogram DM, lambs implanted with 12 mg zeranol (Z), and monensin-fed lambs implanted with

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... eranol (MZ). Lambs fed monensin-containing diet consumed 10.5% less (p<0.05) DM/100 kg weight and were 8.3% more (p<0.05) efficient in converting feed than lambs fed control diet. Zeranol implanted lambs tended to grow 35.2% (p<0.05) faster, consumed 5.1% more (p<0.05) feed and were (p<0.05) 21.9% more efficient in their feed conversion than control lambs. Responses of lambs to monensin and zeranol implants were not additive. Except for Z treatment, there were no marked differences in all carcass characteristics among the various treatment groups. Z-lambs produced 12.7% heavier (p<0.05) carcasses compared with those from C treatment. Also fat parameters, namely, kidney and pelvic fat (KP), body wall thickness and fat thickness, indicated trends for higher finish in Z treatment lambs (p<0.05) than for those lambs from other treatments. Except for CF and ADF, no significant differences in nutrients digestibility were noticed between various treatments; feeding monensin resulted in 24.5% and 8.5% depressions (p<0.05) in CF and ADF digestibility, respectively in comparison to C treatment. Nitrogen retention as percentage of total N-intakes was averaging 7.5 and 20.2% higher (p<0.05) in lambs implanted with zeranol than those fed the M and C diets, respectively. (Asian-Aust.