The aim of this study was to determine whether a rapid release N source can be substituted with a slow release N source without having any negative effects on intake, digestibility, rumen fermentation and microbial protein synthesis, if sheep are fed a poor quality roughage. Five rumen cannulated wethers were used in the trial in a 5x5 latin square design. Cannulated wethers were assigned to different treatments after each experimental period. The treatments studied had different proportions of urea to OptigenrII, with the same inclusion level of starch and a mineral premix between treatments. The five different treatments were: 100% urea; 75% urea:25% OptigenrII; 50% urea:50% OptigenrII; 25% urea:75% OptigenrII and 100% OptigenrII. Significant differences (P<0.05) between the 25% urea:75% OptigenrII and the other treatments in terms of intake suggested that a combination of urea and OptigenrII might be the preferred supplementation due to a significantly higher dry matter intake (DMI), organic matter intake (OMI), neutral detergent fibre intake (NDF intake) and digestible organic matter intake (DOMI). The intake variables of 100% urea and 100% OptigenrII did not differ (p>0.05). No differences (P>0.05) were recorded for dry matter digestibility (DMD), organic matter digestibility (OMD) and neutral detergent fibre (NDF) digestibility between treatments. However, the 100% OptigenrII treatment had a significant (P<0.05) lower apparent nitrogen digestibility, which might be the result of a slower rumen NH3-N release and higher nitrogen excretion than the other treatments. No differences were observed for pH and VFA between different treatments. The rumen NH3-N concentration of the 100% OptigenrII treatment was significantly (P<0.05) lower than the 100% urea treatment at 2 and 4 hours after infusion. The effective degradability of both DM and NDF did not differ (P>0.05) between treatments. Neither were there differences between treatments for total microbial crude nitrogen (MCN) production. Based on biological evaluation, it could be suggested that urea might be substituted with OptigenrII in supplements. From an economical point of view, urea might still be the preferred NPN source, as urea is cheaper than OptigenrII in terms of R/kg nitrogen. Copyright / Dissertation (MSc(Agric))--University of Pretoria, 2012. / Animal and Wildlife Sciences / unrestricted