Submitted to: Domestic Animal Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 29, 1998
Publication Date: N/A
Interpretive Summary: Milk urea nitrogen is currently a popular measurement used by dairy farmers to assess the protein status of cows. Metabolism of nutrients is a complicated process and use of one metabolite sampled from a cow on one occasion may not represent nutritional status. Circadian (24h) and ultradian (less than 12 h) rhythms in body temperature and of blood concentrations of insulin and urea were identified in six lactating cows. The existence and magnitude of the rhythms found in this study strongly suggest that to correctly measure these variables, measurements must be made at least every 30 min or must be integrated over 24-h. For example, it has been proposed that milk urea nitrogen (MUN) be used as a measure of the state of an animals' metabolism. The finding of this study indicates that a 24-h average is needed to account for the circadian variation in urea.
To investigate possible circadian and ultradian periodicities for peripheral insulin and urea in lactating dairy cows, integrated 15-min blood samples were taken sequentially over 48 h from six cows. In addition, radiotelemetry measurements of body temperature were averaged over the same 15-min periods. Cows were housed in an environmental chamber at 19 with lights on 0700 to 2300 h; fed daily at 0900 h; and milked at 0800 and 2000 h. For 5 of the 6 cows, body temperature showed a circadian rhythm peaking at 2323 h with an amplitude of .34 C. For the sixth cow, body temperature was 180 degrees out-of-phase peaking at 1230 h with an amplitude of .12 C. Circadian rhythms for insulin and urea were consistent for all six cows peaking at 1743 h with an amplitude of .73 ng/ml for insulin and at 10.56 h with an amplitude of 3.83 mM for urea. Body temperature and insulin also displayed episodic increases which often exceeded the amplitudes of circadian rhythms. For body temperature, a broad increase in spectral power was seen for periods between 100 and 175 min; time intervals between peaks averaged around 100 min. For insulin, power spectra for individual cows universally indicated rhythms with periods around 45 and 80 min; time intervals between peaks averaged around 65 min. For urea, almost all spectral energy was confined to the 24-h rhythm although there was evidence of a low amplitude 60 min rhythm. In conclusion, when animals are acclimated to a rigidly controlled environment and frequent blood sampling is accomplished with minimal intervention, it is possible to detect rhythms inherent in the regulation of metabolic variables.