Comparison between the effects of feeding copper sulphate-treated and untreated rapeseed cake containing high glucosinolates on rumen fermentation, nutrient digestion and nitrogen metabolism in steers.

Two trials were carried out to study the effects of copper sulphate (CuSO4 ) on detoxifying glucosinolates (GLS) in rapeseed cake (RSC) and compare the effects of feeding CuSO4 -treated and untreated RSC on nutrient digestion and nitrogen (N) metabolism in steers. In Trial 1, different concentrations of CuSO4 solution (1.6 vs. 3.2 g CuSO4 ·5H2 O L-1 ), soaking temperatures (25 vs. 60°C) and drying methods (air drying at 60°C vs. freeze drying) were allocated in a 2 × 2 × 2 factorial arrangement in vitro. In Trial 2, six steers and dietary inclusions of untreated RSC (control), CuSO4 -treated RSC and CuSO4 -added RSC were assigned in a replicated 3 × 3 Latin square design. CuSO4 treatment in vitro decreased the contents of GLS and thiocyanate (TC) in RSC (p < 0.001). The total amount of GLS and TC decreased by 62.7-68.5% for all treatments. The animal trial showed that CuSO4 -treated RSC inclusion decreased ruminal concentration of valerate (p < 0.01), whereas it did not affect ruminal pH, ammonia N and total volatile fatty acids. Compared with the control, feeding CuSO4 -treated or CuSO4 -added RSC had no effect on plasma concentrations of triiodothyronine and thyroxine, N excretion and N retention. CuSO4 -treated RSC tended to increase neutral detergent fibre digestibility (p = 0.072) and urinary excretion of urea (p = 0.056). Urinary excretion of purine derivatives (p = 0.076) and rumen microbial N supply (p = 0.084) tended to decrease when feeding CuSO4 -treated RSC versus control. TC was found to be the only metabolite of GLS in rumen fluid, plasma and urine. It was feasible to detoxify GLS in RSC using low CuSO4 at room temperature. However, feeding CuSO4 -treated or CuSO4 -added RSC had minor effects on rumen fermentation, nutrient digestion and N metabolism in steers. CuSO4 treatment on RSC for feeding steers seems to be unnecessary.