his study aimed to evaluate the effect of feeding supplemented diets by two levels of thiamin or niacin on the productive performance of growingfattening calves. A feeding trial was conducted using twenty five yearling Friesian calves of an initial weight ranged from 200 -250 kg/head. Animals were divided into five similar groups (5 each) using the randomized complete block design. They were fed individually about 180d of an experimental period on a basal concentrateroughage ration that consisted of approximately 56% concentrate feed mixture (CFM), 28% rice straw (RS) and 16% corn silage (CS), on DM basis, wherein T 1 (control) had no supplement, T 2 and T 3 had 200 or 500 mg thiamin/h/d and T 4 and T 5 had 500 and 1000 mg niacin/h/d, respectively. Animals were fed according to NRC,(1984) standard for beef cattle. At the same time, five digestibility trial were conducted to determine the digestibility and feeding value of the experimental rations. Results indicated that digestion coefficients of DM and OM were insignificantly increased with all supplemented rations except that of high levelniacin ration (T 5 ) which significantly increased, compared with control. Most nutrients digestibility were markedly improved by both supplements, the highest value (p<0.05) was occurred with the high level of niacin supplement (T 5 ). Similar trend for the feeding values (TDN, DCP and DE) was found among dietary treatments. Showing the highest feeding value with (T 5 ) total live body gain and daily gain were significantly increased due to both supplements (thiamin & niacin) and its levels, being 158.8, 187.6, 164.6, 167.6 and 188.8 kg total gain versus 882, 1042, 914, 931 and 1049/gm as daily gain for T 1 , T 2, T 3, T 4 and T 5 , respectively. Feed conversion as DM: gain was slightly improved for calves fed rations whose supplemented by low level thiamin and both levels of niacin. Blood total protein and albumin concentrations were significant higher for all supplemental rations than those of unsupplemented one. Economically, the best net revenue was occurred with low thiamin and high niacin supplemented rations, being 12.10 and 12.70 LE /head/ day, respectively. The corresponding economical efficiency values were 112 and 114% based on control ration (100%). It cloud be concluded that using either thiamin or niacin as feed additive in ration formulation of growing and fattening Friesian calves tended to improve productive performance and economical efficiency for fattening calves, with the best results are associated with niacin supplement.
