Effect of programmed diurnal temperature cycles on plasma thyroxine level,body temperature,and feed intake of holstein dairy cows

Holstein cows exposed to simulated summer diurnal ambient temperature cycles of Phoenix, Arizona and Atlanta, Georgia and diurnal modifications of these climates displayed daily cycles fluctuations in plasma thyroxine (T₄) and rectal temperatures (T_re). There were daily diurnal changes in T₄ and T_re under all simulated climate conditions. Maximal values generally occurred in the evening hours and minimum values in the morning. Although the diurnal rhythm was influenced by the various simulated climates (diurnal modifications) a diurnal rhythm was very evident even under constant conditions at thermoneutral (Tnc) and at cyclic thermoneutral conditions (TN). The major significance of the study is that the initiation of night cooling of the animals at a time when their T_re was highest was most beneficial to maintenance of a TN plasma T₄ level. There was a highly significant negative relationship of average T₄ and average T_re. There was also a significant negative relationship of feed consumption and average temperature-humidity index (THI).These data suggest that night cooling may be a most effective method to alleviate thermoregulatory limitations of a hot climate on optimal animal performance. Decreasing the night time air temperature (T_a) or THI or increasing the diurnal range allows the cows to more easily dissipate excess body heat accumulated during the day and minimize the thermal inhibition on feed intake, and alterations in plasma T₄ and T_re.Contribution from the Missouri Agriculture Experimental Station.Reference to a company or product name is for specific information only and does not imply approval or recommendation of product by the University of Missouri or the U.S. Department of Agriculture to the exclusion of others that may be suitable.