Abstract
Summary
This study indicates that at 15° higher producing cattle (milk yield) have higher plasma glucocorticoid concentrations compared to lower producing cattle with glucocorticoid levels appearing to be positively correlated with lactational intensity. Short term thermal (30°) exposure for 18 hr resulted in glucocorticoid levels being markedly lower in high producing cattle compared to low producers. This shift at 30° (after 18 hr) is possibly due to different time sequence of glucocorticoid response to thermal exposure between high and low producing cows. These data support the concept that gluco-corticoids assist the animal in efficiently meeting the greater energy demand of lactation and further studies should be undertaken to denote free plasma levels and their utilization by dairy cattle at various levels of milk production.
The authors gratefully acknowledge Richard Morgan and Lewis Croy for cooperation on animal management, L. Hahn and Dale Clark for engineering support on this experiment, Carolyn Hebert for statistical analysis, Katy Douglass for typing the manuscript, and Chris Lippincott for preparing tables and figures.
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