Dietary Rumen-Protected Gamma-Aminobutyric Acid Supplementation to Alleviate Stress in Beef Cattle Caused by Long-Distance Transport by Improving Antioxidant Capacity, Immune Function, and Hormonal Balance: A Related Study

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.