Herd reproductive performance related to urea concentration in bulk milk.

Bulk milk samples were collected from 256 dairy herds from 4 districts in Southern Norway: Gudbrandsdal (n = 35), Bergen (n = 15), Hedmark (n = 117) and Hardanger (n = 89). A total of 6–8 samples were collected in each herd during the indoor season. Data concerning reproductive performance, milk production and nutrition were registered for a period of 15 months starting 7 months before the first milk sample was collected. The urea levels obtained during the sampling period were averaged generating a mean urea level of each herd. The mean urea levels were related to reproductive performance. The urea levels within districts showed only minor variations during the sampling period. The mean urea level in Hardanger was significantly higher (p < 0.05) than in the other distrikts. This district also had the lowest fertility. Significant correlations between mean urea level and fertility were observed only in Hedmark. Fertility status (FS), an integrated index, and number of inseminations per animal inseminated were significantly affected by the interaction between mean urea level and district. Herds in Hardanger with cows treated for ovarian cysts had a significantly higher mean urea level (p = 0.02) than herds with non-treated cows. The results appear to support the notion that a negative relationship may exist between bulk milk urea levels and reproductive performance. The great district variations observed indicate, however, that the practical value of such measurements is limited.