Diurnal Variations of Serum and Milk Urea Levels in Dairy Cows

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Abstract
Blood and milk urea levels were determined to disclose differences in nitrogen metabolism of cows fed with home-produced feed in accordance with the Finnish feeding standards. The cows were divided according to the type of feed into hay-urea and silage groups. The cows were fed twice a day; in the morning from 6 to 10 a.m. and in the afternoon from 2 to 6 p.m. Four daily blood samples were obtained at 5.30 and 10 a.m. and at 1.30 and 6 p.m. and two daily milk samples between 7–9 a.m. and 4–6 p.m. The stability of urea in milk was determined at different conditions. In azide-preserved milk or in milk stored at +4° the milk urea levels remained unchanged for one week, while urea levels gradually declined in milk kept at room temperature. This shows that milk samples must be preserved with sodium azide or be kept at +4°C for urea determination. An increase of serum urea level was found after the morning feeding but during the afternoon feeding the serum levels remained unchanged. The increase of serum urea level was significant from 6 to 10 a.m., while from 10 a.m. to 2 p.m. only a slight increase was observed. The milk urea levels also increased significantly from the morning to the afternoon sampling. In the hay-urea group the milk and serum urea levels were significantly higher than in the silage group. The milk urea concentration closely correlated to that of the serum at any particular time point. This correlation varied from 0.86 to 0.96 in the morning and afternoon samplings, respectively. Therefore milk urea assays provide a convenient method to follow the nitrogen balance of dairy cattle.