Seasonal and lactational effects on response to thyrotropin releasing hormone injection in Holstein cows

Fifty-four lactating Holstein cows were used to define relationships between stage of lactation and environmental measures on thyrotropin releasing hormone (TRH)-induced secretion of triiodothyronine (T₃, thyroxine (T₄) and prolactin. Mean black globe temperatures were -.3, 20.6, and 25.5C for winter, spring, and summer months, respectively. The circulating hormone responses of T₃, T₄ and prolactin were significantly affected by season, sampling time and there was a season by sampling time interaction. T₄ response was affected by stage of lactation, while prolactin responses differed by stage of lactation across the three seasons. Both T₃ and T₄ responses were lowest in winter and may indicate greater utilization rates to meet metabolic demands. T₃ responses peaked near 3 hr post TRH, while T₄ increased to the end of the 7 hr sampling period. Prolactin responses to TRH increased from winter through summer and were responsive to changes in ambient temperature and photoperiod. T₄ response increased with advancing stage of lactation and could be due to changes in metabolic activity and clearance. Spring TRH-induced prolactin response decreased with advancing lactation whereas, the response increased as lactation progressed during the summer. Monthly milk yield was affected by stage of lactation and lactation number. Early lactation cows produced most milk and milk yield peaked in third lactation. Differential responses to TRH may reflect alterations in milk yield during different stages of lactation.