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Two consecutive trials were carried out to study the effects of dietary crude protein (CP) and tannic acid (TA) on nitrogen (N) metabolism of beef cattle and consequently, the N2O emissions from the urine of cattle. In Trial I, eight growing castrated cattle were used as the experimental animals. Two levels of dietary CP (110.6 and 135.7 g/kg dry matter [DM]) and two levels of TA (0 and 16.9 g/kg DM) were allocated in a replicated 2 × 2 crossover design. In Trial II, the N2O emissions from the urine of cattle collected from Trial I were determined using the static incubation technique. An interaction between dietary CP and TA on the urinary N excretion (p < .05) was found but not on the N2O‐N emission of cattle urine. Increasing dietary CP level from 110.6 g/kg DM to 135.7 g/kg DM increased the total N excretion (p < .001), the N retention (p < .05) and the ratio of urinary urea‐N/urinary N (p < .01), did not affect the N use efficiency (NUE; p > .05) and shifted the N excretion from faeces to urine. Increasing the dietary CP level increased the N2O‐N emission of cattle urine. Dietary addition of TA decreased the urinary excretions of urea (p < .001) and shifted the N excretion from urine to faeces, did not affect the NUE of beef cattle (p > .10), and decreased the N2O‐N emission of cattle urine. Pyrogallol and resorcinol of the TA metabolites were detected in urine with dietary addition of TA. Feeding beef cattle with relatively low CP level and adding TA in rations are effective approaches to mitigate the N2O‐N emissions from cattle urine.

Two consecutive trials were carried out to study the effects of dietary CP and adding gallic acid (GA) in basal rations on nitrogen (N) metabolism and nitrous oxide (N 2 O) emissions from the urine of beef cattle. In Trial I, eight Simmental castrated male cattle with initial liveweight of 310.5 ± 21.5 kg were used as experimental animals. Two levels of dietary CP (113.5 and 150.8 g/kg DM) and two levels of GA (0.0 and 15.2 g/kg DM) were used as experimental treatments in a 2 × 2 reversal design. Two cattle received each treatment in each of two experimental periods. Each experimental period lasted 19 d, of which the first 14 d were for adaptation and the last 5 d were for sampling. In Trial II, the urine samples collected from Trial I were used for measuring N 2 O-N emissions using static incubation technique. Glass jars containing soil were used as the incubation vessels. Three jars were used for each of the urine samples as replicates and two jars without urine samples were used as blanks. The incubation lasted 15 d, and the daily N 2 O-N emission from each jar was determined using gas chromatography. The results showed that no effects of interactions were found between dietary CP and GA on the N metabolism of beef cattle and the estimated cattle N 2 O-N emissions (P > 0.05). Increasing dietary CP from 113.5 to 150.8 g/kg DM increased the excretions of total N, urinary N, and urea (P < 0.001), whereas adding GA at 15.2 g/kg DM in ration did not affect these parameters (P > 0.05). Increasing dietary CP from 113.5 to 150.8 g/kg DM increased the estimated cattle urine N 2 O-N emissions by 36.8% (without adding GA) and 32.3% (adding GA at 15.2 g/ kg DM) (P < 0.01), whereas adding GA at 15.2 g/kg DM in ration decreased the estimated cattle urine N 2 O-N emissions by 28.5% (dietary CP 113.5 g/kg DM) and 30.9% (dietary CP 150.8 g/kg DM) (P < 0.01). The inhibiting effects of GA on decreasing the N 2 O-N emissions of urine could have been resulted from the effects of GA metabolites including pyrogallol and resorcinol excreted in urine. Feeding cattle with relatively low dietary CP or adding GA in ration is effective to decrease the N 2 O-N emissions from the urine patches of beef cattle applied to soil.

This trial was conducted to study the effects of dietary rapeseed cake (RSC) containing high glucosinolates (GLS) on rumen fermentation, nutrient digestion and the rumen microbial community in steers. Eight growing steers and four rations containing RSC [GLS 226.1 μmol/g dry matter (DM)] at 0.00, 2.65, 5.35 and 8.00% DM were assigned in a replicate 4 × 4 Latin square design. The results indicated that increasing RSC levels increased the ruminal concentration of thiocyanate (SCN) (P < 0.01), decreased the ruminal concentration of ammonia nitrogen (NH3-N) and the molar proportion of isovalerate (P < 0.05), did not affect the ruminal concentration of total volatile fatty acids (VFA) (P > 0.05), decreased the crude protein (CP) digestibility (P < 0.05) and increased the ether extract (EE) digestibility (P < 0.01). Increasing RSC levels tended to decrease the abundances of ruminal Ruminobacter amylophilus (P = 0.055) and Ruminococcus albus (P = 0.086) but did not affect methanogens, protozoa, fungi and other bacteria (P > 0.05). Increasing RSC levels in the ration did not affect the ruminal bacterial diversity (P > 0.05), but it increased the operational taxonomic units (OTUs) and the bacterial richness (P < 0.05) and affected the relative abundances of some bacteria at the phylum level and genus level (P < 0.05). In conclusion, RSC decreased the ruminal concentration of NH3-N and the CP digestibility, increased the EE digestibility and partly affected the ruminal bacterial community. SCN, as the metabolite of GLS, could be a major factor affecting these indices.

BACKGROUND: Two consecutive trials were carried out to study the effects of dietary supplementation of rumen-protected methionine (RPM) on nutrient digestibility, nitrogen (N) metabolism (Trial 1), and consequently the nitrous oxide (N 2 O) emissions from urine in beef cattle (Trial 2). Eight 24-month-old castrated Simmental bulls with liveweights of 494 ± 28 kg, and four levels of dietary supplementation of RPM at 0, 10, 20, and 30 g head −1 d −1 , were allocated in a replicated 4 × 4 Latin square for Trial 1 and the N 2 O emissions from the urine samples collected in Trial 1 were measured using a static incubation technique in Trial 2.RESULTS: Supplementation of RPM at 0, 10, 20, and 30 g head −1 d −1 to a basal ration deficient in methionine (Met) did not affect the apparent digestibility of dry matter, organic matter, neutral detergent fiber, or acid detergent fiber (P > 0.05), but decreased the urinary excretions of total N (P < 0.05) and urea (P < 0.001), increased the ratio of N retention / digested N (P < 0.05) in beef cattle, and decreased the estimated cattle urine N 2 O-N emissions by 19.5%, 23.4%, and 32.6%, respectively (P < 0.001).CONCLUSION: Supplementation of RPM to Met-deficient rations was effective in improving the utilization rate of dietary N and decreasing the N 2 O emissions from urine in beef cattle.