The effect of diet enriched with rapeseed meal on endogenous thiouracil contents in urine and milk of cattle
Thyreostatic compounds, such as thiouracil, are orally active drugs that can be used to increase the weight of cattle before slaughter. Due to potentially teratogenic and carcinogenic effects of their residues on public health, the use of thyreostats in animal production has been banned in the European Union since 1981. Systematic detection of low concentrations of thiouracil in the urine of livestock in many countries is believed to be of endogenous origin due to the use of Brassicaceae plants in the animal diet. Therefore, the purpose of the study was to determine the effects of diets rich in rapeseed meal on formation of thiouracil in urine and milk of dairy cows. For two weeks three cows were subjected to a diet supplemented with rapeseed at 30%, compared to the control cattle diet which contained up to 11% rapeseed. During the experiment, samples of urine and milk were collected and analysed by LC-MS/MS. The increase and decrease of thiouracil concentration in urine samples in different animals was individual and cyclic. The highest concentration of natural thiouracil determined in urine was 3.61 μg l−1. It has been found that endogenous thiouracil exists in two tautomeric forms. A few days of storage of frozen urine samples affected the stability of natural thiouracil, whereas an acidic medium improved the stability of the compound and its isomer, which remained stable even after two months of storage at temperatures below −18°C. Due to the instability of thiouracil, urine samples upon sampling should be delivered to the laboratory as soon as possible or properly preserved. In milk samples, thiouracil was not found above the decision limit of the applied method of 0.63 μg l−1. Preliminary studies have shown that faecal examination for banned thiouracil can be a complementary test for urine samples, and may be helpful in determining the origin of the compound present in urine.
