Eighteen crossbred Friesian calves weighed 95.3 ± 1.74 Kg on average were used in a feeding trial lasted for 180 days. Animals were divided into three similar groups (6 calves each) to study the effects of protecting proteins of soybean meal (SBM) and un-decorticated cottonseed meal (UCSM) by zinc sulphate on nutrients digestibility, some rumen fermentation characteristics, plasma metabolite concentrations and their reflection on growth performance of calves fed corn silage based diets. The ratio of corn silage to concentrate was 67 to 33 on the basis of dry matter and fed ad libitum. Diets were supplemented with SBM and UCSM; Zn-SBM and Zn-UCSM or a 50 : 50 combination of both Zn-protected and untreated sources. The obtained results indicated that digestibility coefficients of DM, OM, CP, CF and NFE for calves fed treated proteins by zinc sulphate increased than those fed untreated materials. The improvements in digestibility of nutrients reflected better feeding values in terms of TDN and DCP for diets containing treated proteins than the control ones. Ruminal NH₃-N and NPN concentrations were lower (P<0.05) for calves fed treated proteins by zinc sulphate than those fed untreated diet, indicating efficiency of the protection and / or better synchronization between availability of energy and release of NH₃. On the other hand, ruminal true protein nitrogen (TPN) and protozoa count were higher (P<0.05) for calves fed zinc sulphate treated proteins and those received a ratio of 50 : 50 of combined treated sources than those fed the untreated ones. Ruminal acetic acid concentration decreased by 11.15% for calves fed protected proteins than those fed untreated proteins, whereas ruminal propionic acid increased by 8.29%. The concentration of plasma urea-N was decreased (P<0.05) in calves fed treated proteins by zinc sulphate than untreated materials, indicating better utilization of protected proteins. Whereas, the zinc sulphate protected protein diet resulted in the highest (P<0.05) values for plasma total protein, albumin and globulin concentrations. Average daily gain (ADG) was significantly (P<0.05) higher for calves fed treated proteins followed by calves fed diet treated and untreated proteins with a 50 : 50 combination of both Zn-protected and untreated ones, being 1.272, 1.178 and 0.962 Kg/h/d, respectively. Moreover, feed conversion efficiency (expressed as DM, TDN and DCP, Kg/Kg gain) were better for calves fed protected proteins than those fed unprotected proteins. Feed cost decreased by 29.8 and 25.7% for calves fed protected proteins and diet contained a 50 : 50 combination of both Zn-protected and untreated proteins, respectively than those fed unprotected protein. Accordingly, economical efficiency improved by 29.9 and 25.8% with diets contained protected proteins or a 50 : 50 combination of both Zn-protected and untreated sources than with diet contained untreated proteins. From the aforementioned results, this study recommends the use of zinc sulphate as a protection method for half or all SBM and UCSM protein sources from degradation in the rumen during formulating rations for crossbred Friesian calves when feeding corn silage as a basal diet.