SCREENING OF PROTEIN METABOLIC UNBALANCE BY BULK MILK UREA DETERMINATION IN DAIRY HERDS

Bulk milk urea concentration, similar to mean serum urea, has been use d as an indicator of the protein-energy balance intake in dairy cows. Accordingly, to determine the association between blood and milk urea values the herd blood urea concentration (mean of 7 lactating cows) an d bulk milk urea concentration (tank or churn milk samples) were deter mined in 21 dairy herds. Besides, to determine reference values, seaso nal and herd variance, and the frequency of herds with abnormal values , bulk milk samples were obtained monthly during 1 year from 82 suppli ers at two dairy plants in the South of Chile. Mean bulk urea concentr ations (6.2 +/- 2.0 mmol/l) was similar to mean herd blood urea value (6.2 +/- 2.0 mmol/l) and a significant association between these param eters was found (r = 0.9477; p < 0.01). Mean urea concentration in the milk samples obtained during a year from 82 herds was 4.9 +/- 1.2 mmo l/l, with a reference range of 2.5 to. 7.4 mmol/l. No differences were found between samples obtained from tanks or churns (p > 0.05). The h ighest values were found during autum and spring, and the lowest durin g summer. The annual coefficient of variation (CV) ranged from 13% to 47%, showing a high seasonal variation. The between herds CV was also high, ranging from 20% to 31%. Of the 82 herds, 27% showed abnormal va lues in two or more opportunities. 5.4% of the samples had urea values above the reference values specially during spring, and 3.8% were bel ow the reference values mainly during summer. According to these resul ts, bulk milk urea concentrations render information similar to herd b lood urea values, and present high seasonal and herd variations. Also, there was a high frequency of herds with nutritional unbalance of pro tein-energy.