Thirty-six Finn-Ossimi crossbred male lambs with an average live body weight of 22 kg and 4 months age were randomly assigned to 6 nutritional groups each of 6 animals. Six mixed rations based on bean straw were formulated to be isonitrogenoues and isocaloric so as to provide 14% CP and 64% TDN according to NRC (1985) recommendations. Soybean meal (SBM), corn gluten meal (CGM) and cotton seed meal (CSM) were used as the sole dietary protein source or it was partially substituted by urea (U) at 1% level. Experimental animals were allotted to one of the following rations in a fattening trial for 106 days; Tr1 SBM, Tr2 CGM, Tr3 CSM, Tr4 SBM + 1%U, Tr5 CGM + 1%U and Tr6 CSM + 1%U. A digestibility trial and nitrogen balance were conducted to evaluate the nutritive value of the experimental rations.
Results obtained showed that:
1- Dietary protein source (SBM, CGM and CSM) affected partially (p> 0.05) digestibility coefficient of nutrients, however urea supplementation caused an increase in EE,DM and OM digestibility but it decreased CF digestibility.
2- Higher (P<0.05) protein digestibility was obtained with diets contained SBM either with or without urea.
3- Urea supplementation resulted in a significant reduction of DMI for CGM (1353vs1159gm) and CSM groups (1319vs 1151gm) but it was not for SBM (1349 and 1341gm/h/d).
4- SBM diet had higher(P<0.05)TDN% value either it was supplemented with U or not.
5- DCP% value was increased (p < 0.05) for CGM supplemented with U, but it was decreased with the other two protein sources (SBM and CSM).
6- Similar nitrogen intake was observed for all diets irrespective of source of protein or supplementation with U.
7- SBM group with or without U had higher (P<0.05) nitrogen retention (NR), however urea supplementation resulted in less NR for CGM and CSM groups.
8- Dietary nitrogen source had insignificant effect on live body weight gain of the experimental animals, however animals fed CGM with or without U had insignificant higher daily gain compared with the other treatments.
9- Incorporation of U in the diet improved insignificantly daily weight gain of SBM group (235 vs. 255 g/h/d), however it decreased daily weight gain of CGM and CSM groups.
10- Animal' s feed consumption was decreased insignificantly due to U supplem-entation, however it improved lamb's feed efficiency (as DM and DCP/kg gain).
11- The best feed utilization and the lowest feed cost/kg live body weight gain was achieved by (CGM+1%U) group; being 4.56kg DMI, 0.427 kg CPI/kg gain and 1.73LE.
It was concluded that to maintain maximum feed efficiency of a resistant dietary nitrogen source; inclusion of rapidly degradable N i.e. urea should be considered.