Thirty lambs (Avishaan genotype, 10 weeks old, 14.0 ± 0.2 kg live weight) were randomly assigned into three equal treatment groups to study the effect of complete feed blocks (CFB; concentrate and roughage at 70:30 ratio) with three different tanniniferous tree leaves on intake and utilization of nutrients, rumen fermentation, microbial protein synthesis and growth performance. The treatments were T1 (Control), CFB1 with Vigna sinensis hay; T2, CFB2 with Acacia nilotica leaves rich in hydrolysable tannins (HT); and T3, CFB3 with Ziziphus nummularia leaves rich in condensed tannins (CT). The three CFBs were fed ad libitum to the respective groups of lambs for a period of 12 weeks. There was lower (p < .05) intake of dry matter (DM), total carbohydrates (TCHO) and fibre components in T2 compared with T1 and T3. However, the digestibility of nutrients except crude protein (CP) was higher in T2. Diet had no effect (p > .05) on the LW gain in lambs. Amongst the three groups, T3 showed enhanced N utilization with a comparable microbial protein synthesis, the lowest being in T2. The T2 group of lambs had higher propionate and lower non-glucogenic: glucogenic short-chain fatty acids ratio. It may be concluded that tanniniferous tree leaves at 30% of total mixed ration can meet the requirement of nutrients for desired post-weaning growth. © 2019 Blackwell Verlag GmbH.