Digestibility, ruminal fermentation and duodenal flux of amino acids in steers fed grass forage plus concentrate containing increasing levels of Acacia mearnsii tannin extract

• The nutritional impact of the Acacia mearnsii tannin offered to steers was evaluated.
• The levels of tannin extract tested were 9, 18 or 27 g/kg of total dietary DM.
• The amino acid supply improves by adding up to 18 g/kg DM of tannin extract.

Four Holstein steers (156 ± 33 kg of body weight (BW)) fitted with duodenal cannula and rumen catheter, were housed in metabolism pens and used in a 4 × 4 Latin square trial to evaluate the effect of dietary inclusion of levels of Acacia mearnsii tannin extract on rumen fermentation, digestion and duodenal flux of amino acids. The diet was offered at a restricted amount of 20 g dry matter (DM)/kg BW and consisted of oat (Avena strigosa) plus concentrate, in a proportion of 0.55:0.45 (DM basis) respectively. Treatments were no tannin (Control) or inclusion of tannin extract in the concentrate at a rate of 20, 40 or 60 g/kg DM (i.e. 9, 18 or 27 g/kg of total dietary DM). The apparent total-tract organic matter (OM) digestibility was not affected whereas the neutral detergent fiber (aNDF) digestibility and the OM true total-tract digestibility tended to linearly decrease (P < 0.10) at increased levels of tannin extract inclusion. Tannin extract linearly reduced (P < 0.05) the total-tract digestibility of N compounds, as well as the urinary N excretion, and linearly increased (P < 0.05) the fecal N excretion, N retention and the efficiency of N utilization by steers. The ruminal pH was similar for all treatments whereas the concentration of ammonia N and reducing sugars in ruminal fluid linearly decreased (P < 0.05) with tannin extract inclusion. Linear positive responses (P < 0.05) to tannin treatments were observed for duodenal flux of total N, α-amino N and non-ammonia non-microbial N. The microbial N supply tended to be negatively affected (P < 0.10) whereas both the ruminal OM digestibility and ruminal degradability of feed N compounds linearly decreased (P < 0.05) with increasing tannin extract levels. The efficiency of ruminal microbial protein synthesis tended to increase quadratically (P < 0.10) at increased levels of tannin extract. The dietary inclusion of tannin extract positively impacted (P < 0.05) or tended to impact (P < 0.10) the duodenal flux of all amino acid groups, as well as of most individual amino acids. The amino acid profile in duodenal digesta was more closely related to the profile in feed at increased tannin extract inclusion. In conclusion, dietary inclusion of Acacia mearnsii tannin extract up to a level of 18 g/kg DM decreased the urinary N excretion and improved the amino acid supply in steers fed fresh-frozen oat forage plus concentrate without significantly affecting the total OM digestibility.