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Department of Dairy Husbandry, University of Missouri, Columbia 65201
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 61 ±5 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 44.5±4.3 ng/ml.
The mean average duration of estrus of 16.8 h at 18.2 C differed from 11.9 h at 33.5 C. The mean length of estrous cycle was 19.5 days at 18 C and 21.6 days at 33.5 C with some cycles being as long as 23 days under the hot conditions.
1 Contribution from the Missouri Agricultural Experiment Station. Journal Series No. 6579. Approved by the Director.
2 Present address: Animal Production Physiology, College of Animal Sciences, Haryana Agriculture University, Hissar (Haryana), India.
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