Circadian and ultradian rhythms of peripheral cortisol concentrations in lactating dairy cows.

To investigate possible circadian and ultradian periodicities for plasma cortisol in lactating dairy cows, integrated 15-min blood samples taken sequentially over 48 h from six cows were analyzed by radioimmunoassay. The cows were housed in an environmental chamber at about 19 degrees C, 50% relative humidity, and 16 h of light and 8 h of darkness (lights on at 0700 h); fed daily at 0900 h; and milked at 0830 and 2000 h. Peripheral concentrations of cortisol for all six cows exhibited weak circadian rhythms--average minima were 3.

Body temperature of dry cows on pasture: environmental and behavioral effects.

To investigate interrelations among factors potentially affecting body temperature, we observed body temperature, activity, and relative aggregation of 3 to 5 dry cows from a herd of 12 to 15 every 20 min. Ambient conditions were noted every 1 h and fly counts every 3 h. Observations during daylight for 5 d with and without shade indicated effects of shade were minimal and were extended to 24 h without shade at six weekly intervals.