In the last half-century, a significant increase in meat yield has been mainly (85–90%) achieved by genetic selection for rapid growth traits, nutrition, and management practices (Havenstein et al., 2003; Erensoy et al., 2020b). However, the thermoregulation ability of broilers could not develop simultaneously with their growth traits. This has made metabolic heat dissipation, which has become more difficult as a result of increased body weight and feed intake, especially in the last 2–3 weeks of the production period, even more, difficult in hot environmental conditions (Deeb et al., 2002; Zaboli et al., 2019). The presence of feathers and the absence of sweat glands in birds increase the sensitivity to high environmental temperatures (Lara Rostagno et al., 2013; Loyau et al., 2013) and negatively affect performance, welfare status, and meat quality traits (Akşit et al., 2006; Wang et al., 2013). In our study, IF and CW were given between 22–42 days of age to reduce the harmful effects of heat stress on some welfare and meat quality traits in broilers.
Approximately 4.5-5 oC higher temperature and 9–10% lower relative humidity levels were determined in the period when heat stress was applied (11–17 h) compared to the hours that were not applied. In our study, although 6-h of IF treatment under heat stress conditions provided a numerical decrease in rectal temperature, this was not at a significant level (as seen in Table 1). These results were found consistent with Farghly et al. (2018a) and inconsistent with the results of Özkan et al. (2003) and Farghly et al. (2018b). Marai et al. (1999), Abioja et al. (2011), and Park et al. (2015) reported that giving cold water suppressed the increase in body temperature of broiler chickens. In addition, it has been reported that drinking water at a lower temperature than the bird's body temperature will help to dissipate the body temperature more easily (Fairchild and Ritz, 2012). However, in the current study, CW treatment did not decrease the body temperature of broiler chickens at six weeks of age.
Panting is known to thermoregulate in many bird species (Steenfeldt et al., 2019). Syafwan et al. (2012) reported that rearing male broilers at 32°C during the day and at 25°C at night between 21–42 days did not change the panting rate. Abioja et al. (2011) and Farghly et al. (2018a) reported that CW intake reduces the panting rate in maintaining homeostasis in hot conditions in poultry. However, our study results showed that broilers in the AF×CW group under hot conditions showed the significantly highest panting rate at 4, 5, and 6 weeks of age in parallel (see Fig. 1). In previous studies, the effect of such an interaction on indicators of heat stress has not been tested before. While the panting rate is expected to decrease in broilers consuming CW (Farghly et al., 2018a), an unexpected effect occurred with AF×CW interaction. The fact that the body temperature of the AF chickens during heat stress hours (11–17 h) was 0.3 oC higher, although not at a significant level, may have caused more panting to remove metabolic heat from feed intake, in accordance with Farghly (2011). In addition, we speculated that consuming CW probably contributes to eliminating the harmful effects of heat stress on broilers and is consistent with Fairchild and Ritz (2012).
Broilers consuming CW had worse litter quality than those consuming NW (as seen in Table 3, P < 0.05). In the previous study we conducted on the same chicken materials (Erensoy et al., 2020a), it is known that IF and CW does not affect water consumption, so we speculated that the difference in water consumption does not cause the deterioration in the litter quality. However, chickens consuming CW showed more panting behavior at 4, 5, and 6 weeks of age (see Fig. 1). It is known that the duration of sitting or lying down (resting) behaviors increase with advancing age in broilers (Bessei, 2006). In addition, body weight increase as the growth period progress, so the floor area is almost completely covered by birds, making it difficult to ventilate the litter surface (Bessei, 2006). Although behaviors were not examined in our study, it is speculated that activity decreases with advancing age, possibly due to panting behavior, and that chickens consuming CW have more contact with relatively high moisture litter. Conversely, chickens in the AF×NW group showed less HB and BB (see Fig. 2). Water temperature treatment rather than feeding type seems more effective in the interaction effects for HB and BB, as shown in Table 3. In our study, more prolonged contact of the hock and breast with the litter increased the severity of HB and BB, consistent with Mench (2002) and Allain et al. (2009). However, the fact that FPD and LP were not significantly affected by this situation was not compatible with the general literature (Shepherd and Fairchild, 2010; Cengiz et al., 2011; Mello et al., 2015; Dunlop et al., 2016). In our study, both FPD and LP varied within minimal limits, and FPD was seen in only 3 chickens and LP in 5 chickens among all broilers (data not shown).
Pre-slaughter stress factors such as heat stress, fasting, handling, and water deprivation may affect meat quality (Mota-Rojas et al., 2006). One of our hypotheses for this study was that the negative effects of heat stress on meat quality could be reduced by management practices such as IF and CW treatments. However, it was seen that the results obtained partially supported our hypothesis. IF treatment decreased the panting rate compared to AF, but unexpectedly, the highest panting was observed in AF×CW broilers for all age periods (see Fig. 1). Panting behavior is a physiological response of birds to maintain thermal homeostasis under heat stress conditions, provide heat dissipation through respiration, and regulate body temperature (Yahav et al., 2005). The increase in the panting rate decreases the H + ions concentration in the blood plasma, causing a decrease in pH (respiratory alkalosis), and it becomes difficult to maintain body temperature balance (Sandercock et al., 2006; Zaboli et al., 2019). This mechanism results in a faster pH drop and lower ultimate pH in breast meat of broiler. PSE (pale, soft, and exudative) syndrome develops due to lower pH in breast meat, higher lightness (L*), lower redness (a*), and yellowness (b*) (Petracci et al., 2004; Akşit et al., 2006). However, in our study, IF and CW treatments were insufficient to significantly change the ultimate pH and color traits of breast meat compared to consuming AF and NW chickens (as seen in Table 4). These results were consistent with the report of Lippens et al. (2000) that IF does not affect the pH and color traits of breast meat. However, IF and CW treatments significantly decreased the ultimate pH of the thigh meat in broiler chickens. It is speculated that CW intake increases the panting rate and contributes to the decrease in ultimate pH of the thigh meat. In addition, the yellowness (b*) of thigh meat was also found the highest in IF×CW broilers (P = 0.038, see Fig. 3). Consistent with Lu et al. (2007), a further decrease in thigh meat pH of broilers in the CW group probably caused more yellowness (b*).
We concluded that management practices such as IF and CW in fast-growing broilers could not completely reduce the harmful effects of heat stress on some welfare and meat quality traits, and in some cases, even caused more negativity. When AF broilers consumed CW, the panting rate unexpectedly increased, and the severity of HB and BB increased. While IF and CW treatments did not affect the quality traits of breast meat, they decreased the ultimate pH value and increased the yellowness (b*) in thigh meat.