Dietary Rumen-Protected Gamma-Aminobutyric Acid Supplementation to Alleviate Stress in Beef Cattle Caused by...

γ-aminobutyric acid (GABA), an inhibitory neurotransmitter, plays an important role in regulating stress responses through modulation of the hypothalamic-pituitary-adrenal axis, neuroendocrine activity, antioxidant defense, and immune function. Therefore, this study evaluated the effects of dietary rumen-protected γ-aminobutyric acid (RP-GABA) supplementation on alleviating long-distance transport-induced stress in beef cattle. Forty-eight healthy Simmental bulls were randomly assigned to four treatment groups: CON, G-L, G-M, and G-H, receiving RP-GABA at 0, 300, 500, and 700 mg/kg of concentrate dry matter, respectively. The experiment lasted 53 days, including a 21-day pre-transport feeding period, a 2-day transportation period, and a 30-day post-transport feeding period. Samples were collected on experimental Days 21, 23, and 54. The results showed that long-distance transport impaired antioxidant status, immune function, and stress hormone secretion in beef cattle. The responses to different dietary RP-GABA supplementation levels varied among serum parameters. Long-distance transport reduced triiodothyronine and thyroxine (T4) concentrations, while the G-L group alleviated these reductions and maintained higher levels than the CON group after transport (Day 23). The G-M group enhanced glutathione peroxidase (GSH-Px) activity after transport (Day 23) (p < 0.001) and promoted the elevation of interleukin (IL)-4 levels both before and after long-distance transport (Days 21 and 23). The G-H group significantly decreased serum cortisol concentration after transport (Day 23), while reducing epinephrine levels and increasing glutathione peroxidase activity (all p < 0.001). Moreover, the G-H group further enhanced the increases in IL-4 and total antioxidant capacity before and after transport (Days 21 and 23), and increased IL-6 concentrations before and after transport (Days 21 and 54). Regarding dose-specific effects, the G-L group showed limited effects on stress-related hormones, immune function, and antioxidant capacity, whereas the G-H group enhanced some antioxidant and immune-related responses but had higher pre-transport stress hormone concentrations. In contrast, the G-M group exhibited a relatively balanced response across stress hormone levels, antioxidant status, and immune regulation, characterized by higher T4 (p = 0.003), GSH-Px (p = 0.001), and IL-4 (p = 0.006) concentrations and lower alkaline phosphatase (p = 0.005) and tumor necrosis factor-alpha concentrations. Therefore, under the conditions of this study, dietary supplementation with 500 mg/kg RP-GABA in concentrate may represent a potential level for maintaining balanced physiological responses in beef cattle.