Evaluation of chronic heat stress by hair cortisol and its relationship with coat color in Holstein cows

• Grant number: 24/14842-8
• Support Opportunities: Scholarships in Brazil - Master
• Start date: January 01, 2025
• End date: December 31, 2026
• Field of knowledge: Agronomical Sciences - Animal Husbandry - Ecology of Domestic Animals and Ethology
• Principal Investigator: Cristiane Gonçalves Titto
• Grantee: Mariana Matheus Evangelisti dos Santos
• Host Institution: Faculdade de Zootecnia e Engenharia de Alimentos (FZEA). Universidade de São Paulo (USP). Pirassununga , SP, Brazil

Abstract

The thermal environment can be a chronic stress factor for dairy cows, and selection of more thermotolerant animals is the main tool for mitigating the negative effects of stress. Coat color and cortisol accumulation in the coat can be useful in determining whether Holstein cows with predominantly dark coats are better adapted to high temperatures because they have greater pigmentation and, consequently, greater protection against ultraviolet rays. The aim of this study was to evaluate chronic heat stress in Holstein cows through the accumulation of glucocorticoids in the coat, tegumentary, physiological, and behavioral changes, to identify changes in the expression of coat genes, and to determine whether there are variations in relation to the predominance of coat color. Twenty-six lactating Holstein cows, homogeneous in terms of body score, were divided into two groups: thermal comfort (free-stall system with adiabatic evaporative cooling) and thermal stress (unshaded paddock). Collections will take place at the end of winter and summer. Physiological parameters, such as rectal and body surface temperature, sweating rate, and respiratory rate, were analyzed. Blood samples to measure prolactin and melanotrophic hormone (MSH), hair length and thickness, and skin histology (characterization of sweat glands) will be collected once each season. Hair samples will be collected to measure cortisol and melanin concentrations in the hair. Observations of daily feeding behavior and body posture were made every 15 minutes on three non-consecutive days. Milk production was measured daily throughout the experimental period. Finally, the skin samples were used for RNA sequencing to search for differentially expressed genes. The animals will be characterized in relation to the percentage of white and black coloration by computer analysis of the RGB images. The environmental variables of air temperature and speed, relative humidity, rainfall, black globe temperature, and solar radiation will be monitored in each facility, and the temperature and humidity index (ITU) will be calculated. The design used will be entirely randomized, considering two treatments (thermal comfort and thermal stress) and two treatments (late winter and late summer), and analysis of variance will be carried out with time and animal as random effects and comparison of means by Tukey-Kramer at 5%. It is expected that different heat tolerance profiles will be found in each group and that the methodology for assessing cortisol in the coat will be efficient in assessing the condition of chronic heat stress, with a correlation for the percentage of predominance of dark color in the coat and identifying differentially expressed genes in relation to thermotolerance and variations in relation to predominance of color in the coat.