C and N stable isotope variation in urine and milk of cattle depending on the diet

The stable carbon and nitrogen isotopic composition of urine and milk samples from cattle under different feeding regimes were analysed over a period of six months. The isotope ratios were measured with isotope ratio mass spectrometry (IRMS). The delta C-13 values of milk and urine were dependent on different feeding regimes based on C-3 or C-4 plants. The delta C-13 values are more negative under grass feeding than under maize feeding. The delta C-13 values of milk are more negative compared to urine and independent of the feeding regime. Under grass feeding the analysed milk and urine samples are enriched in C-13 relative to the feed, whereas under maize feeding the C-13/C-12 ratio of urine is in the same range and milk is depleted in C-13 relative to the diet. The difference between the N-15/N-14 ratios for the two feeding regimes is less pronounced than the C-13/C-12 ratios. The delta N-15 values in urine require more time to reach the new equilibrium, whereas the milk samples show no significant differences between the two feeding regimes.