Access to water is one of the primary requirements for animal well-being and productivity. The results from the present study showed that short-term restriction in water supply did not affect the weight of animals. This could be related to the short duration of the imposed water stress in the current work (two weeks) as similarly reported by Vosooghi-Postindoz et al. (2018). In longer-term trials it is common for a reduction in body weight to be observed as feed intake is closely related to water supply which both subsequently influence weight gained or lost (Chedid et al. 2014). Observations of rectal (core) body temperature and respiration rate provide an indication of the thermoregulatory response of animals (Chedid et al. 2014). The rectal temperature of animals was not affected by the short-term imposed water stress. Similar results were reported by others with different breeds of sheep, for example, Jaber et al. (2004) and Casamassima et al. (2016). However, as highlighted by Chedid et al. (2014), others have reported contrasting results, i.e. notable increases in rectal temperature in response to water stress. This could be related not only to prevailing environmental conditions (temperature and relative humidity) but also specific breeds, exposure to radiation, etc.
The ability to maintain normal respiratory rates following exposure to an applied stress is indicative of tolerance to the imposed stress (Hales 1973). Sheep that display symptoms of water stress reportedly reduce respiratory/panting rates in order to prevent further water loss (McKinley et al. 2009; Al-Ramamneh et al. 2012; Ghassemi and Sung 2017; Vosooghi-Postindoz et al. 2018). The restriction of water in the diet of animals, could translate into reduce blood circulation, impede the exchange of nutrients and restrict basal metabolism, with a consequent reduction in energy production. These results are supported by Al-Ramamneh et al. (2012), Ghassemi and Sung (2017) and Vosooghi-Postindoz et al. (2018) in studies on water restriction, salinity, and ruminal digestibility in sheep. In the current trial, it was observed that the respiratory rate of sheep declined with the imposition of increasing levels of water stress (Fig. 1d from 23.3 to 13.3 respirations per min in control and water stressed animals, respectively). This could suggest a thermoregulatory adaptation to cope with water stress, as discussed above.
The blood chemistry of animals can also be influenced by environmental variables and therefore is routinely measured to follow the progression of the physiological responses of animals to stresses (Okoruwa 2014). It is noted that the average hemoglobin levels of sheep exposed to limited water supply and the control group, from our study were within the normal reference ranges of 9 to 16 g L− 1 (Wang et al. 2015; Ahmadi-hamedani et al. 2016). Sejian et al. (2017) highlighted that determinations of hematocrit (packed cell volume) and hemoglobin levels provide good indications of thermo tolerance in sheep as these factors correlate with the body water balance. In the current study, hemoglobin and hematocrit levels displayed a similar trend – they were stable in the control and 3 d water restricted treatments but were elevated with further water stress (Fig. 2a and b). Hemoconcentration (i.e. increased hemoglobin and packed cell volume) has been identified as a consequence of water stress in sheep (Abdelatif et al. 1994; Ghanem et al. 2008). However, others have reported conflicting results, e.g. Igbokwe (1993) and Jaber et al. (2004). It has been suggested that the observation of increased hemoglobin levels might be as a result of reduced plasma volume as a direct consequence of water loss (Kaliber et al. 2016; Kumar et al. 2016; Ay and Ulutas 2020). The maintenance of plasma volume levels might be indicative of an adaptive response by animals to tolerate stress (Sneddon 1993) and the inability to maintain levels could suggest some degree of stress. Interestingly, the phenomenons of hemoconcentration and increase in hemoglobin and hematocrit levels have also been noted under conditions of heat stress. It has been suggested that the increase in blood hemoglobin levels may be a consequence of the higher oxygen demand (Okoruwa 2014; Al-Dawood 2017; Ribeiro et al. 2018; Vicente-Pérez et al. 2018). One hypothesis is that the mobilization of fluids into the circulatory system causes hemoconcentration due to the loss of water through evaporative means (Kumar et al. 2016), similar to the hemoconcentration that occurs when there is restriction of drinking water.
Leukocytes are commonly measured as an indicator of stress in vertebrates (Davis et al. 2008). Leukocytosis (describes an increase in the overall number of white blood cells) has been used to infer a stress response (Ots et al. 1998). However, this must be interpreted with caution as while this parameter can indicate whether an animal has been exposed to a comparatively lesser or greater degree of stress, it is not necessarily indicative of the ability of the animal to mount an immune response (Davis et al. 2008). In terms of an immune response, a reduction in leukocyte numbers is typical (Dhabhar 2002). In the current trial, an increase in leukocytes was observed in response to water stress (Fig. 2e). It has been reported that changes associated with stress conditions include an increase in the prevalence of neutrophils (neutrophilia) and a decrease in the lymphocytes (lymphopenia or lymphocytopenia) (Davis et al. 2008). This was apparent in the current work where lymphocyte levels in water deprived animals were reduced relative to the control. The opposite trend was observed for neutrophils. This suggests that the imposition of water stress was apparent in the blood chemistry of sheep. The observation of leukocytosis might also be linked the concurrent decrease in SOD activity as this enzyme has been reported to play an important role in defense against the toxicity caused by leukocytopenia that appears in infections associated with the generation of free radicals (Flores and Medina 2013). Donia et al. (2014) also observed lower serum activity of SOD in pneumonic sheep.
Some researchers have reported an increase in protein levels in response to water stress in sheep, e.g. Jaber et al. (2004), Nejad et al. (2014), Casamassima et al. (2016), Vosooghi-Postindoz et al. (2018), etc. This was attributed to hemoconcentration due to water loss (Casamassima et al. 2016). The observed results were inconsistent in the present study since blood protein levels increased in sheep supplied with water every 3 d but declined to levels similar to the control in those supplied with water every 6 d (Fig. 3a).
It has been well established that Reactive Oxygen Species (ROS) are produced in response to stress conditions. Animals possess a suite of antioxidant systems which allow for the quenching of ROS (Halliwell and Gutteridege 2007). When ROS are produced in excess (or are not adequately quenched) oxidative damage results leading to membrane deterioration, protein, DNA and RNA denaturation, etc. (Mujahid et al. 2007). Superoxide dismutase (SOD) and catalase (CAT) are two antioxidants. Superoxide dismutase catalyzes the breakdown of the oxygen radical leading to the generation of H2O2, which is subsequently decomposed by CAT (Ghosh and Deb 2014). The current study investigated the effect of short-term water stress on the antioxidant capacity of sheep. The results indicated a decline in the levels of both antioxidants (SOD and CAT) when animals were exposed to water limitation. The effect was more pronounced in the case of catalase (Fig. 3c). There has been limited work on the effect of water stress on antioxidant systems in animals as more effort has been focused on heat stress. For example, it has been shown that thermal stress reduced the activity of SOD and CAT in broiler chickens (Zhang et al. 2015). Furthermore, Shi et al. (2020) demonstrated that heat stress resulted in reduced serum levels of CAT in lambs following 28 d of stress. However, it must be highlighted that the short duration of the water stress imposed in the present study could have been insufficient to trigger protection from antioxidant mechanisms (Rathwa et al. 2017). Furthermore, sheep have other adaptive mechanisms that largely mitigate against the negative effects of limiting factors such as lack of water. In addition, reduced SOD activity (Fig. 3b) could also be related to a deficiency of minerals that act as cofactors for the SOD enzyme (Flores et al. 2011). Even in animals under grazing conditions, serum SOD activity can be used as an indicator for the nutritional metabolic balance of certain elements such as copper, manganese and iron (Flores et al. 2011). However, the increase in body weight per day was affected (Fig. 1b), which is related to an impairment of consumption and consequently a decrease in weight gain. This was similar to that described by Akinmoladun et al. (2019) and Ay and Ulutas (2020), which refer to the influence of the type of food and the association of heat stress.
This study reports on the imposition of short-term water stress in Pelibuey sheep. This type of research is critical in order to develop meaningful strategies to protect livestock production in the face of changing climate patterns. The short- and long-term consequences of water scarcity on sheep well-being and productivity during different life cycle stages will have ultimately have impacts on the longevity of this important sector in years to come.