White hair coat color does not influence heat tolerance of sheep grazing under a hot arid environment

Highlights

• Heat stress is critical factor hampering small ruminant’s productivity.

• Black coat colored sheep showed better heat tolerance than white coat colored ones.

• White coat color is not a crucial trait for heat tolerance in grazing sheep.

Abstract

High climatic temperature is the most important factor hampering livestock productivity in tropical and arid regions. Therefore, the current study aimed to investigate the interaction between coat color and heat tolerance in two principal Saudi sheep breeds; Najdi (black colored coat) and Naemi (white colored coat). Najdi and Naemi adult sheep of different age groups and both sexes, raised under direct solar radiation in different parts of the Riyadh region were used. The thermo-physiological measurements were taken from sheep flocks raised under a similar production system in different parts of Riyadh region during the summer season. The recorded rectal temperature (T_re) was significantly (P < 0.05) higher in the white coat colored breed compared to the black coat colored breed. Consequently, the calculated heat tolerance coefficient (HTC) was significantly (P < 0.05) higher in the black coat colored breed compared to the white coat colored one. No significant (P> 0.05) differences were observed concerning skin temperature (T_sk), packed cell volume (PCV) and plasma albumin levels between the two breeds. The obtained results indicate that white coat color of sheep grazing under a hot arid climate is not a crucial trait of heat tolerance and adaptation. Other hair coat characteristics and evaporative cooling traits might play important role in the adaptation of the black coat colored sheep breed grazing under hot arid conditions.

Introduction

Saudi Arabia has an arid climate characterized by high temperature and solar radiation. The average summer temperature in the central region is about 45 °C, but readings of up to 54 °C are common (Metz, 1993). This high temperature is the most crucial factor hampering livestock productivity in the tropical belt and arid regions. Sheep constitute a large part of the livestock production in Saudi Arabia, and the total number of sheep is estimated at 17.5 million (GaStat, 2015). Almost half of the sheep population is raised in the pastures under direct solar radiation. Najdi breed, which has a black coat color, and Naemi breed, which has a white coat color, represent the principal breeds of sheep that are reared in Saudi Arabia (GaStat, 2015). In fact, both breeds are fat-tailed sheep, and known for their hardiness and adaptability to adverse environments. Najdi is characterized by black long hairy coat with white head, as well as long legs and large pendulous ears. While Naemi is characterized by white, short and stiff hair coat with brown head, as well as long legs and pendulous ears. Generally, Najdi is heaver in weight, larger in size and more docile than Naemi breed.

Most studies on heat tolerance use the temperature-humidity index (THI), which considers only temperature and humidity for calculating the heat stress index (Abdoun et al., 2012; Alhidary et al., 2018). However, in a natural grazing system, other factors like solar radiation and wind speed also contribute to the severity of the climatic conditions (Gaughan et al., 2008; Mengistu et al., 2017). Thus, heat load index (HLI), which considers other environmental variables (solar radiation and wind speed), was also used in the current study. McManus et al. (2011) reported that sheep phenotype affects heat tolerance-associated traits. For example, haired sheep usually tolerate heat better than woolly sheep. The color of the coat can influence the magnitude of absorbed and reflected radiation, and consequently the amount of heat transfer between the surrounding environment and the animal body (Gerken, 2010). It has been reported that the dark coat color could absorb more energy from solar radiation than does the light coat color (McManus et al., 2011). Meanwhile, blood biochemical profile has been reported to provide a reliable tool for assessing the level of heat stress in sheep (Fadare et al., 2012). The same authors revealed that physiological and hemato-biochemical attributes are influenced by the coat color, and are associated with the adaptation to hot environment in four color categories of West African dwarf sheep.

Accordingly, we hypothesized that differences in the coat color of sheep breeds could reflect on the efficiency of their heat tolerance and adaptation to the hot environment. Therefore, this study was designed to investigate the thermo-physiological responses and heat tolerance of sheep breeds with different coat color grazing under a hot arid environment.