The diurnal variation of urea in cow's milk and how milk fat content, storage and preservation affects analysis by a flow injection technique
Carlsson, J.; Bergström, J.
Acta Veterinaria Scandinavica 35(1): 67-77
1994
ISSN/ISBN: 0044-605X PMID: 8209822 Document Number: 429298
Six Swedish Red and White dairy cows, producing 20-39 kg 4%-FCM, were given a ration balanced in energy and protein. They had access to feed from 05.15 to 09.00 h and from 13.00 to 16.30 h, and were milked at 06.15 and 15.30 h. The milk was analysed for urea with a flow injection analysis technique. There was a significant diurnal variation in milk urea. The highest values were found 3-5 h after the beginning of a.m. feeding and the lowest values (>=60% of the maximum values) during late night. Within 1 h after the start of a.m. feeding the urea values had increased significantly, but they had decreased within the same time after the start of p.m. feeding. Since there was a pronounced diurnal variation in the milk fat content, the urea concentration was also recalculated to concentration in the water phase of the milk. It was higher in that phase, but the pattern of the diurnal variation was not significantly altered. However, analyses of milk with a very high fat content may give misleading results. There were no significant differences in the milk urea concentration of different udder quarters. When calculated as concentration in the water phase of the milk, no differences in urea concentration were found between the beginning and the end of milking. The analytical method had a good precision (CV maximum 3%). The milk urea concentration was not significantly different after storage for 10 days at 4 degrees C when no preservative was added; but after 17 days the milk had turned sour and the urea value had increased. When a preservative (bronopol) was added the urea concentration remained unchanged for 17 days. Deep-freezing did not influence the urea concentration.