Interaction between tannin and urea on nitrogen utilization by beef cattle grazing during the dry season

Tannins are secondary plant products that may reduce ammonia in the rumen and increase amino acid flow to intestine, but a reduction in ruminal N availability can limits its use for beef cattle fed low-quality tropical forage. Two studies, in vivo and in vitro, were carried out with the objective of evaluating the effects of the inclusion of a blend of condensed and hydrolysable tannins on protein-energy supplements (PES) with or without urea on intake, total nutrient digestibility and protein dietary efficiency of grazing beef cattle during the dry season. In the in vitro study, the effects of the interaction of urea and tannin in a 2 x 3 factorial arrangement of treatments were evaluated. Urea was included at 2% of the dry matter (DM) of the PES. Tannins were supplied at 0, 1 and 2 g/kg DM of the PES. The substrate was a mixture of forage and PES (50:50, w/w). In vitro DM and neutral detergent fiber (NDF) digestibility as well as ruminal pH and ammonia were measured. In the in vivo study, four rumen-cannulated Nellore bulls (470 +/- 8 kg) were randomly distributed in a 4 x 4 Latin square design. The treatments were in a 2 x 2 factorial arrangement: PES with or without the inclusion of urea x PES with or without the addition of tannin. Urea was included at 2% of the DM of the PES. Tannins were supplied at 1 g/kg DM intake estimated, totalizing 10 g/animal/day. In the in vitro study, gas production decreased with the addition of urea (P < 0.001). It was observed an interaction between urea x tannins (P < 0.01) for in vitro DM and NDF digestibility at 24 h. Increasing tannin concentration, linearly decreased (P < 0.05) in vitro DM digestibility, and quadratically decreased (P < 0.05) in vitro NDF digestibility in the PES with urea. In the in vivo study, lower digestibility was observed (P < 0.01) for DM and organic matter with inclusion of urea in PES. The addition of tannins increased (P < 0.05) the digestibility of crude protein when urea was provided. Lower fecal nitrogen excretion was observed in the PES containing urea and tannin (P < 0.05). The results indicate that the combination of the tannin blend with urea modified the route of N utilization, lowering its excretion in the feces.