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Heat Production of Cattle Acclimated to Cold, Thermoneutrality and Heat When Exposed to Thermoneutrality and Heat Stress^1,2,

Kansas State University,S Manhattan 66506

Abstract

Four Hereford x Red Angus yearling steers were acclimated to each of the following environments: cold (3 C), thermoneutrality (TNZ; 20 C) and heat (35 C). Intake was equalized for all treatments at 4.9 · kg · head^–1 · d^–1 (2.9 Mcal metabolizable energy/kg). Heat production, respiration rate and rectal temperature were determined after 3- and (21-h later) 24-h exposures to thermoneutral and heat stress test-temperatures: 25, 30, 32.5, 35, 37.5 and 40 C. Thermoneutral heat production (kcal · kg^–.75 · d^–1), after 3- and 24-h exposures, was greater (P<.05) for the cold-acclimated cattle (139.6 ± 5.0 and 153.0 ± 5.8) as compared with the TNZ-acclimated cattle (117.7 ± 5.0 and 121.6 ± 5.8). Heat production of the heat-acclimated cattle after 3- and 24-h exposures to thermoneutrality was 121.0 ± 5.1 and 123.5 ± 3.2 and was not different from the TNZ-acclimated cattle. Heat production of steers acclimated to different temperatures was variable during the 3- and 24-h exposures to test-temperatures ranging from 25 to 40 C. Heat production increased linearly in the TNZ-acclimated cattle (24-h exposure) and in the heat-acclimated cattle (3-h exposure) at the rate of 1.3 and 2.3 kcal · kg^–75 · d^–1 · C^–1 increase in test-temperature, respectively. In the other four comparisons, analysis by regression indicated no significant change in heat production. Rectal temperature and respiration rate increased significantly in either a linear or quadratic manner in all treatment groups exposed to test-temperatures from 25 to 40 C. During exposure to environmental temperatures that were high enough to stimulate an increase in respiration rate and rectal temperature, heat production of cattle on equalized intake did not change in a predictable or consistent manner.

¹ Appreciation is expressed to Drs. Fedde, Kuhlmann and Hodgeson for assistance with the calorimetry system.

² Contribution No. 84-201-J, Dept. of Anim. Sci. and Ind., Kansas Agr. Exp. Sta., Manhattan 66506

³ Present address: Dept. of Anim. Sci., Univ. of Alberta, Edmonton. Alberta. Canada T6G 2P5

⁴ Present address: Dept. of Anim. Sci., Colorado State Univ., Fort Collins 80523

⁵ Dept. of Statist., Kansas State Univ., Manhattan 66506.