Abstract
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 steers with an initial body weight of 245 ± 18.70 kg liveweight were used as the 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, a 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 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 flow (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 retention rate of steers but increased the urine N2O-N emissions by 5.7% and 31.4%, respectively. For reducing N2O emissions to the environment, high levels of sorghum grain are not suggested to be included in the ration of steers.
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