Effect of Environmental Heat Stress on Meat Quality and Blood Biochemical Indicators in Sheep

In order to understand the impact of heat stress on the meat quality and blood biochemical indicators of sheep, this study compared the meat quality and blood biochemical indicators of Luzhong mutton sheep under heat stress generated by artificially regulating the feeding environment with those under normal feeding conditions (control). The result analysis showed that heat stress had no significant effect on the pH at 45 min and 24 h after slaughter or cooking loss of lamb meat (P > 0.05). There were significant differences in drip loss, shear force, and the color parameters brightness (L*), redness (a*), and yellowing (b*) values at 45 min and 24 h after slaughter between the two groups (P < 0.05). In terms of sensory scores for appearance, taste, juiciness, smell, flavor and off-flavor, the control group was superior to the stress group. Scanning electron microscopy showed that muscle fibers were arranged more loosely in the stress group than in the control group. The daily average levels of thyrotropin releasing hormone (TRH), thyroxin, and cortisol (Cor) in the blood of the stress group were significantly reduced compared to the control group (P < 0.01), while the levels of epinephrine (EPI) and insulin (Ins) were significantly increased (P < 0.05). There was no significant difference in the levels of triiodothyronine and adrenocorticotropic hormone. In terms of immune function, the level of immunoglobulin A (IgA) in the stress group was significantly reduced compared to the control group (P < 0.05), but there was no significant difference in the levels of IgG, IgM, and interleukin-4; blood urea nitrogen and creatinine levels in the stress group were significantly increased compared to the control group (P < 0.05), while lactate dehydrogenase levels were significantly reduced (P < 0.05). There was no significant difference in creatine kinase levels. Based on the above results, heat stress has a negative impact on the meat quality of sheep, leading to a trend toward pale, soft, exudative (PSE)-like meat. The hormones insulin, thyrotropin releasing hormone, and cortisol can be considered as biomarkers for assessing the status of heat stress in sheep.