Abstract
Sixteen crossbred (Sahiwal $\times$ Holstein) male rumen fistulated calves of 18 to 24 months of age were randomly divided into four groups of four animal, each. Animals in all the groups were fed wheat straw ad lib as basal roughage. However, the animals in group I were fed concentrate mixture at maintenance level, whereas, the animals in groups II, III and IV had free access to existing, modified (A) and modified (B) urea molasses mineral block licks respectively. Daily wheat straw intake (kg) was significantly (p<0.01) higher in groups II ($4.20{\pm}0.13$), III ($4.07{\pm}0.16$) and IV ($4.22{\pm}0.20$) as compared to group I ($3.21{\pm}0.14$). Total N and TCA precistrained rumen liquor) was significantly higher in groups II ($22.36{\pm}0.25$), III ($21.63{\pm}0.25$) and IV ($21.77{\pm}0.55$) as compared to group I ($18.31{\pm}0.41$). Bacterial production rate (g/day and g/kg digestible organic matter intake) were non-significantly different amongst groups I ($214.4{\pm}13.28;\;85.38{\pm}3.69$); II ($198.7{\pm}5.70;\;86.17{\pm}3.53$); III ($214.4{\pm}8.19;\;96.15{\pm}2.16$) and IV ($218.2{\pm}10.62;\;94.44{\pm}5.52$). Similarly, percent efficiency of N incorporation into bacterial protein was not found significantly different amongst groups I, II, III and IV. These studies indicate that when concentrate mixture (upto maintenance level) in the diet of ruminants was replaced with UMMB licks, various N fraction in SRL and efficiency of bacterial production rates in the rumen were not affected.