Effects of dietary inclusion with rapeseed cake containing high glucosinolates on nitrogen metabolism and urine nitrous oxide emissions in steers

Two consecutive trials were conducted to investigate the effects of glucosinolates (GLS) in rapeseed cake (RSC) on nitrogen (N) metabolism and urine nitrous oxide (N(2)O) emissions in steers. In trial 1, 8 steers and 4 levels of RSC, i.e. 0%, 2.7%, 5.4% and 8.0% dry matter (DM) (0, 6.0, 12.1, 18.1 μmol GLS/g DM) were allocated in a replicated 4 × 4 Latin square. In trial 2, the static incubation technique was used for measuring the N(2)O emissions of the urine samples collected from trial 1. The results of trial 1 indicated that dietary inclusion of RSC decreased the digested N and increased the fecal N excretion (P < 0.01), whereas it did not affect the urinary N excretion, total N excretion and N retention (P > 0.10). Dietary inclusion of RSC decreased the urinary excretion of urea while it increased allantoin, total purine derivatives, the predicted rumen microbial N flow and thiocyanate (SCN) (P < 0.05). Dietary inclusion of RSC did not affect the plasma triiodothyronine and thyroxine while it down-regulated the plasma relative concentrations of 4-aminohippuric acid, 3α,7α-dihydroxycoprostanic acid, phosphatidylserine (14:0/16:0), 6β-hydroxyprogesterone, pyrrhoxanthinol, tatridin B, mandelonitrile rutinoside, taraxacoside (P < 0.05), and up-regulated hypoglycin B, neuromedin N (1-4), dhurrin, 5-deoxykievitone (P < 0.01). The results of trial 2 indicated that dietary RSC increased the steer urine N(2)O–N fluxes, the ratio of N(2)O–N to N application and the estimated steer urine N(2)O–N emissions (P < 0.01). A close correlation was found between the estimated steer urine N(2)O–N emissions and the output of urinary SCN (P < 0.001). In conclusion, dietary RSC increased the fecal N excretion, whereas it did not affect the urinary N excretion and the N retention rate in steers. Dietary RSC increased rather than decreased the urine N(2)O–N emissions even though it decreased the urinary excretion of urea. The SCN excreted in urine could be the major factor in increasing the urine N(2)O–N emissions. Whether other metabolites excreted into urine from RSC have an impact on the urine N(2)O–N emissions in steers needs to be investigated in the future.