Environmental Heat Effects on Bovine Luteinizing Hormone

Abstract

Six animals with synchronous estrous cycles were housed in the climatic chambers under controlled conditions of 18.2 C, 55~ relative humidity and 33.5 C, 55~; relative humidity and followed for four full estrous cycles. Two cycles were measured at 18 C and two successive cycles at 33.5 C. Plasma luteinizing hormone was measured by double antibody radioimmunoassay. At 18.2 C, luteinizing hormone peaked at 615 ng/ml near the onset of estrus; the surge persisted 8 to 16 h. Basal luteinizing hormone was approximately 2.43 ng/ml for all heifers, rose significantly during the luteal phase of the cycle on day 10 to 3.5 .27 ng/ml, and declined thereafter. Under high temperature conditions of 33.5 C where body temperature remained elevated by 1 to 1.5 C, base line as well as peak luteinizing hormone was lower at 1.25___.48 ng/ml (day 1 post-estrus) and 44.54.3 ng/ml.