Urinary benzylated compounds as potential markers of forage intake and metabolism of their precursors in ruminants
The quantitative relationship between the urinary excretion of TBA and the uptake of PPA and CHCA was examined by continuous intraruminal infusion of PPA (8, 16 or 24 mmol/d) either alone or with CHCA (8 or 16 mmol/d) in sheep nourished by intragastric infusions of all nutrients. The daily excretion of TBA was linearly correlated (r 0.99, P<0.001) with the amounts of PPA and CHCA infused. The urinary recovery of infused PPA and CHCA as TBA was 0.79 (s.e. 0.01). Recovery of infused SA (8 mmol/d) as urinary total salicylic acid (TSA) was 0.89 (SD 0.08). Both TBA and TSA excretions were found to be exclusively of exogenous origin, with negligible faecal excretion. It is concluded that urinary TBA is a potential estimator of PPA + CHCA absorption from the digestive tract. SA may have potential as a marker of urine volume. The relationship between urinary excretion of TBA and forage intake was assessed with steers fed different amounts of fresh herbage from pastures comprising oat, rye, vetch and lucerne. Herbage samples collected daily were incubated in vitro with buffered rumen fluid for 48 h. The PPA production per unit of herbage DM incubated was fitted to the exponential function of time Y = a + [b. (1 - e-c.l)]. Regression analysis showed that the TBA:creatine concentration ratio was a good explanatory variable (r2 0.92-095; P<0.001) of herbage intake. TBA:creatinine concentration ratio divided by the product b.c, both constants from the fitted curves of PPA production of herbage cultures, was another good index (r2 up to 0.98). It is concluded that a method for the prediction of herbage intake for grazing animals based on TBA excretion in urine seems promising.