Inclusion of sorghum grain rich in condensed tannins in the diet of steers did not affect the nitrogen utilization efficiency but increased the urine nitrous oxide emissions

The objectives of this study which included two experiments were to investigate the effects of dietary inclusion of sorghum grain rich in condensed tannins (CT) (18.9 g kg^–1 dry matter (DM)) on nitrogen (N) metabolism and urine nitrous oxide (N₂O) emissions of beef steers. In experiment 1, six Limousin×Luxi crossbreed steers with an initial liveweight of (245.0±18.7) kg were used as experimental animals. Three levels of sorghum grain, i.e., 0, 167 and 338 g kg^–1 DM were included in diets as experimental treatments. The animals and the treatments were randomly assigned to a replicated 3×3 Latin square design. In experiment 2, static incubation technique was used to determine the N₂O emissions of the urine samples collected in experiment 1. The results of experiment 1 showed that dietary inclusion of sorghum grain linearly increased the faecal N excretion (P=0.001), the total N excretion (P=0.010) and the faecal N to N intake ratio (P=0.021), but it did not affect the N retention and the N utilization efficiency (P>0.10). The plasma metabolomic data showed that dietary inclusion of sorghum grain increased the relative concentrations of phenolic acid (N1, N5, N10-tris-trans-p-coumaroylspermine and prenyl cis-caffeate) and carnitine (3-hydroxyisovalerylcarnitine and linoelaidyl carnitine). The results also showed that dietary inclusion of sorghum grain linearly increased the urinary urea excretion (P=0.010) and decreased the urinary excretion of purine derivatives (P=0.041) as well as the estimated rumen microbial N supply (P=0.012) based on urinary purine derivatives. The results of experiment 2 showed that including sorghum grain in the diet linearly increased the average concentrations of NH₄⁺-N (P=0.012), NO₂^–-N (P=0.009), NO₃^–-N (P=0.001) and the total inorganic N (P<0.001) in the soil enriched with urine samples. The urine sample N₂O-N flux (P=0.001), the estimated steer urine N₂O-N flux (P=0.021) and the N₂O-N to urinary N ratio (P=0.038) linearly increased with increasing inclusion of sorghum grain in the diet. In conclusion, dietary inclusion of sorghum grain containing high CT at 167 and 338 g kg^–1 DM did not affect the N utilization efficiency of steers but increased the urine N₂O-N emissions by 5.7 and 31.4%, respectively. For reducing the N₂O emissions to the environment, high levels of sorghum grain should not be included in the diet of steers.