N-nitrosoproline excretion in urine, faeces and milk from cows in relation to feed composition

The technique of Ohshima and Bartsch for measuring the excretion of N-nitrosoproline (NPRO) was applied in a balance trial with cows to estimate the in-vivo formation of N-nitrosamines. In the first trial, a cow with a high milk production received rations of concentrates and hay with low or high contents of nitrate and free proline. Samples were taken from the urine, faeces and milk. In the second trial, two non-lactating cows were put on hay rations with different nitrate and free proline contents, and samples were taken from the rumenal fluid every 15 min to measure the formation of nitrite from the ingested nitrate. Samples of urine and faeces were also taken. It was found that dried roughage may contain considerable amounts of free proline, and the NPRO formed is related to the nitrate content. No NPRO was found in the urine and faeces when the NPRO content of the hay was low, not even when the ingested nitrate was reduced drastically to nitrite or when the concentration of free proline was high. When NPRO was present in the feed, it was recovered from the urine and faeces but was not transmitted to the milk.