Usefulness of urea and protein concentrations in milk and blood as indicators of nitrogen utilization in the Simmental and Holstein-Friesian lactating dairy cows

The aim of this study was to determine the protein status in Simmental and Holstein- Friesian cows, as well as use possibilities of milk and blood urea values as indicator of nitrogen utilization. The digested N is mainly absorbed as free amino acids and is used by the animals for protein synthesis, or as ammonia, which needs to be detoxified by the liver into urea. The forms in which N is absorbed are critical in determining the proportion of digested N that is available to support protein synthesis. Investigation has been conducted on 17 lactating Simmental cows and 10 Holstein- Friesian lactating dairy cows divided into groups regarding to milk production (under 15 kg (S<15; H<15) and over 15 kg (S>15; H>15) and to lactating stages (under 100 days S<100d; H<100d and over 100 days). Blood pH and electrolytes (Na+, K+, Cl-) have been determined by the RapidLab 348 (Bayer, Germany) analyzer. Blood metabolites (BUN, TP and albumin) have been measured by Beckman AU400 automatic analyzer. MUN (milk urea nitrogen), protein and fat content have been determined by infrared spectrophotometric method on Combi-Foss analyzer FT 6000. Simmental cows (S<100d) had the lowest BUN and significantly lower MUN concentrations and lower MUN has been noticed also in H<15kg. Significantly higher TP concentration was noticed in H<15kg group and H>100d due to raised globulin count, probably because of inflammation. H>15kg had the lowest rumen pH, but blood pH was in referent interval in all groups. There was no correlation between BUN and MUN (r=0.32 in S>15L) which is related to the ratio of dietary CP to energy, because of excess of microbial requirements. In the condition of low milk production as a consequence of low dietary protein and energy, MUN concentration is not relevant enough as an indicator of protein/energy supply.