الفهرس 
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Abstract
This study was carried out at two stages, the first stage [laboratory trials (in vitro)] and the second stage [Farm trials (in vivo digestibilities, growth performanse and carcass characteristics)] to examine the possibility of improving the utilization of some low quality roughages by using exogenous fibrolytic enzyme (fibrozyme) in sheep diets. In the first stage (laboratory experiment), dry matter and organic matter disappearance (IVDMD and IVOMD) were determined for corn stalks (CS), wheat straw (WS), rice straw (RS) and Sugarcane bagasse (SCB). Those four low quality roughages were used at three roughage : concentrate ratios (100: 0, 40:60, 30:70%). Enzyme was supplemented at 4 levels (0, 2, 2.5 and 3 g/kg DM). In the second stage (farm experiments), the rations contained 30% roughage: 70% concentrate. The control ration consisted of berseem hay (Control) and concentrate feed mixture (CFM) while the two other rations contained corn stalks (T1) and wheat straw (T2) as a roughage source. Fibrozyme was supplemented with 2g/ animal/ day for T1 and T2 rations. In growth trial, thirty male Barki lambs were randomly divided into three equal groups. The digestion trial was carried out by the end of growth trial on four lambs of each treatment. Three animals from each group were slaughtered at the end of the experiment for some carcass and meat quality measures. Fibrozyme supplementation significantly (P<0.05) increased IVDMD and IVOMD for CS and WS compared with the control. Corn stalks and wheat straw showed the highest values of IVDMD and IVOMD especially with 30% roughage at the level of 2g fibrozyme. The highest average daily gain and daily feed intake had been recorded with lambs fed control and T1 rations. Digestibilities of dry matter (DM), organic matter(OM) and crude fiber(CF) of T2 ration was significantly (P<0.05) higher than the control ration. Crude protein (CP) and nitrogen free extract (NFE) digestibilities were significantly (P<0.05) higher for T2 ration than the control and T1 rations. Crude fiber and hemicellulose digestibilities of lambs fed both T1 and T2 were significantly (P<0.05) higher than the control. A significant (p<0.05) increase of TDN values was recorded for T1 and T2 rations compared to the control one. No significant differences (P<0.05) were found among the experimental rations in ruminal pH, but higher significant (P<0.05) values of ammonia nitrogen (NH3-N) concentration was occurred of lambs fed T1 and T2 rations. Total volatile fatty acids (TVFA’s) concentrations of rumen liquor showed significant (P<0.05) increase by lambs fed T1 ration for all sampling times compared with the other treatments. There were no significant effects (P>0.05) among the dietary treatments on carcass characteristics and meat quality. The economic evaluation indicated that the corn stalks diet supplemented with enzyme indicated the lowest feeding cost, highest gain price over cost, and consequently the highest revenue per Kg gain over feeding cost, neversless the better response that recorded with wheat straw. It could be concluded that using 2g/h/d of fibrolytic enzyme supplementation increased nutrients digestibilities and growth performance of lambs fed on ration contained 30% rough