The effect of various mineral additives Clinoptilolite (CL) natural Bentonite (Montmorillonite) (NB) and Processed Bentonite (PB) on growth, feed intake, carcass traits, rumen fermentation parameters (pH, ammonia nitrogen) and protozoa population were studied. Holstein male calves (BW = 237± 6 Kg and 210 ± 20 days of age) were divided to 7, 8,8 and 8 calves per ration and each assigned to one of the four rations for which levels of energy and protein were equal but were different in mineral additives. Additives were PB (ration 1) B (ration 2) CL (ration 3) and Placebo (ration 4). Calves were adapted to TMR rations for 25 days followed by 165 days of individual feeding. Average daily gain in calves receiving rations 1-4 were 1310.10, 1200.62, 1294.13 and 1107.38 gr/day, respectively; the difference being significant (P<0.01). Feed conversion, rumen pH difference between 0& 3 hours after feeding, and under curve area of rumen ammonia nitrogen for 1,2,3 and 4 rations were 6.41, 6.63, 6.37, 7.10 and 0.5, 0.3, 0.1, 0.7 and 52.75, 55.51,73.83, 77.23 respectively, the differenc between rations not being significant. There were not significant differences observed among carcass traits for calves taking rations 1-4. On the basis of results in this expriment, the use of PB and CL can be recommended in rations of fattening calves.
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