Balancing the dietary ratio of nitrogen to sulfur by adding inorganic sulfur improves nitrogen retention and consequently decreases urine nitrous oxide emissions in steers

Abstract

Two consecutive trials were carried out to study the effects of dietary supplementation with inorganic sulfur (S) on nitrogen (N) metabolism and nitrous oxide (N2O) emissions of urine from steers. In Trial 1, six castrated Simmental steers [449 ± 19 kg liveweight (LW)] were used as the experimental animals. Three levels of Na2SO4, i.e., 0 (control), 20 and 40 g/d were added to a basal ration [S content 1.15 g/kg dry matter (DM)] as experimental treatments. The animals and the treatments were randomly assigned to a replicated 3 × 3 Latin square design. Each experimental period included 15 days for adaptation and 5 consecutive days for sampling. The results of Trial 1 indicated that dietary supplementation with Na2SO4 linearly decreased the urinary excretions of N (P = 0.05) and urea (P = 0.03), and linearly increased rumen microbial N flow (P = 0.04), and tended to increase N retention rate (P = 0.06) in a linear manner. Supplementing Na2SO4 also linearly increased the excretions of fecal S (P = 0.02), urinary S (P < 0.01), urinary sulfate (P < 0.01) and S retention (P = 0.01). S retention rate was not influenced by Na2SO4 supplementation. Sulfate was undetectable in feces at a limit of 20 mg/kg DM. In Trial 2, the N2O emissions of the urine samples from Trial 1 were measured using the static incubation technique. The N2O-N emissions of steer urine applied to soil linearly decreased with increasing addition of Na2SO4 in the diet. Urinary sulfate did not affect the N2O-N emissions of steer urine. In conclusion, dietary supplementation with Na2SO4 at 20 g/d was effective for balancing the dietary N/S ratio, obtaining an ideal N retention rate in steers and decreasing the N2O emissions from steer urine.