Effect of heat and milk yield on bovine plasma glucocorticoid levels

This study indicates that at 15 degrees 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 degrees) exposure for 18 hr resulted in glucocorticoid levels being markedly lower in high producing cattle compared to low producers. This shift at 30 degrees (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 glucocorticoids 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.