Rumen, plasma and urinary concentration of the undesirable phenol flavour compound 4-methylphenol was measured in two rumen-fistulated Romney wethers fed perennial ryegrass. Following a one week adaptation period, samples were collected at ca. 1 hr intervals after feeding on two sampling days one week apart. Maximal concentration of 4-methylphenol in the rumen, blood and urine was reached at ca. 2, 3, and 4.5 hr respectively, declining to a minimal concentration by 9 hr (15% of maximum), 11 hr (16%),and 18 hr (<1%) respectively. Thus phenol compounds formed in the rumen are cleared rapidly and efficiently by excretion into the urine. In a separate trial the absorption and excretion of the exogenous phenol, thymol, was measured. Four groups of 4 lambs grazed on pasture were dosed daily with either a control or 3 different levels of thyme slurry for two weeks. After one week of dosing spot urine samples were collected on two consecutive days. Upon completion of the two week dosing period the animalswere slaughtered and subcutaneous fat samples collected. The concentration of thymol (conjugates) in the urine was highly correlated with the thyme dosage (r=0.848, P<0.001), but there was no significant correlation between dose level and the concentration of (free) thymol in the fat. These results suggest that the concentration of phenol flavour compounds in ruminant products is affected by the efficiency of conjugation and excretion as much as by their formation in the rumen.
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