Abstract
We compared the thermotolerance of Dorper (D) and second cross (SC) (Poll Dorset x Merino/Border Leicester) lambs by assessing physiological and biochemical responses. After acclimatization, 4–5 month old lambs of each breed were exposed to either thermo-neutral (18 °C–21 °C, 40%–50% RH, n = 12/group) or cyclic heat stress (HS) (28 °C–40 °C; 40%–60% RH, n = 12/group) for 2 weeks in climatic chambers. The HS involved exposure to temperatures of 38 °C–40 °C between 0800 and 17.00 h daily; otherwise the temperature was maintained at 28 °C. Elevated temperature increased rectal temperature (p < 0.01), respiration rate (p < 0.01) and skin temperature (p < 0.01) in both breeds, (data for 12.00 and 16.00 h pooled), but to a lesser extent in D than in SC lambs (p < 0.01). The HS increased (p < 0.01) water intake to a greater extent in SC than in D lambs and HS reduced (p < 0.05) food intake in SC lambs but not in D lambs. There were no treatment effects on blood glucose and lactate levels in either breed. Significant effects of breed (p < 0.01) and treatment (p < 0.01) were observed in blood creatinine levels, being higher in SC lambs. Higher pH (p < 0.01) and lower pCO2 (p < 0.01) were recorded under HS in both breeds. Among blood electrolytes, Cl−, Na+ and base excess were significantly (all p < 0.01) reduced under HS, with no breed differences. In conclusion, the attenuated physiological responses to HS in Dorper lambs indicates better adaptation of this breed to high environmental temperature.
Author Contributions
Conceptualization: A.J., S.S.C. and F.D.; Research execution and Methodology: A.J., S.S.C., A.J., A.A., M.Z. and R.O.-A.; Data Analysis: A.J. and S.S.C., Writing—Original Draft Preparation: A.J. and S.S.C.; Writing—Review & Editing: A.J., S.S.C., B.J.L., K.D., I.J.C. and F.R.D.; Supervision: S.S.C., K.D., B.J.L. and F.R.D.; Project Administration: S.S.C.; Funding Acquisition: S.S.C.
Funding
This research was funded by Faculty Research Initiative Fund, Faculty of Veterinary and Agricultural Sciences, The University of Melbourne.
Acknowledgments
The authors acknowledge the facilities provided by The University of Melbourne and Faculty of Veterinary and Agricultural Sciences to conduct this research at the Animal Facility, University of Melbourne Dookie Campus, Victoria 3647, Australia.
Conflicts of Interest
The authors declare no conflict of interest.
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