74 President Oral Presentation Pick: Phenotypic plasticity of heat tolerance in beef cattle

Abstract

Abstract Bos taurus indicus cattle have superior ability for regulation of body temperature during heat stress due to a number of physiological and cellular level adaptive traits. The objectives of this study were to quantify the change in body temperature in heifers with various proportions of Brahman genes per unit increase in heat stress as measured by THI and to assess the impact of skin properties on the phenotypic plasticity. A total of 299 two-year old heifers from six breed groups ranging from 100% Angus to 100% Brahman were evaluated under hot and humid conditions during 2017 and 2018 summer days. The plasticity in body temperature of breed groups in response to environmental challenges was estimated using a random regression mixed model. Six mm skin biopsies were collected from the right side of sixth thoracic bone of each heifer. Cross-sections of biopsy samples with a thickness of 7 µm were stained with Hematoxilin–Eosin technique. The number and area of sweat and sebaceous glands and the thickness of dermis and epidermis were determined using the ImageJ program. The 100% Brahman group had the lowest intercept and slope (38.66 and 0.18) representing the body temperature under the low THI conditions and the increase in body temperature for 2 units of THI, respectively. Except for the 100% Angus group, which had a slightly higher intercept (38.68), all other breed groups had lower intercepts. Body temperature in the 100% Angus heifers increases by 0.42 for every 2 units of THI. There was a significant linear effect of Brahman percentage on the sweat gland size, with the 100% Brahman having the largest sweat gland size (203.57µm2) and purebred Angus the smallest (94.31µm2). Effective strategies will require the identification of the genes conferring the superior thermotolerance in Brahman cattle.