Blood metabolic profiles: their use and relation to nutritional status of dairy cows.

Profiles of blood metabolites have been used widely to identify problem herds and to indicate dietary causes of disease or low production. Related herd survey studies and designed experiments are reviewed. An initial study of profiles and estimated nutrient intake of five "normal" and four "problem" herds was conducted between 1972 and 1974 to establish baseline values for blood metabolites. Profiles of blood metabolites for additional problem herds were obtained during 1974 through 1977. Variation due to herd of origin, production, stage of lactation, and season of year are important causes of variation in concentrations of metabolites in jugular blood of dairy cows. They need to be considered carefully in interpreting profiles of blood metabolites. Several relationships between dietary intake of nutrients and concentrations of associated components in jugular blood are significant. Concentrations of metabolites are of almost no practical use for individual cows because of extreme variations in diet required to generate abnormal concentrations of blood metabolites. Averages of glucose of blood and nitrogen in urea on seven to ten cows, adjusted for the factors mentioned above, have limited practical value in detecting inadequate energy and protein intake, respectively.