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

Abstract 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 × 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 × 4 Latin square design. The treatments were in a 2 × 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