Yak is a vital component of animal husbandry in China’s high-altitude grazing regions, providing herders with high-quality milk, meat, and fur, thus holding significant economic value. Grazing on native pasture is the traditional way of raising yaks. Yet, these pastures experience substantial seasonal fluctuations in forage availability, often leading to considerable stress on the pastures, particularly in winter (1). Typically, due to the intake of nutrients falling below the necessary maintenance levels, the weight of grazing yaks will be reduced by about 25% after a cold season, which extends the feeding cycle of yaks, reduces the slaughter yield, and seriously hinders the sustainable development of the yak industry (2, 3). Off-site fattening has increasingly become the primary feeding method for yaks during the cold season, mitigating the traditional contradiction between the forage availability and livestock demand in grazing areas, while enhancing the growth efficiency and meat quality of yaks, resulting in higher economic returns.

Some herders relocate yaks from high-altitude grazing areas to low-altitude farms for breeding. However, live transportation means yaks are exposed to multiple stressors, including noise, crowding, starvation and high temperature, which can exacerbate the adaptive and defensive responses, causing transport stress and compromising health and welfare in animals (4, 5). It has been reported that feed and water deprivation during transport caused dehydration in the body, weight loss and negative energy balance, thereby damaging organismal metabolism and reducing slaughter value (6). Furthermore, prolonged crowding and standing can increase muscle tension and fatigue, and may also promote the development of respiratory diseases (7, 8). Currently, nutritional regulation is a commonly used measure to alleviate transport stress in animals.

Rumen-protected glucose (RPG) can be released directly from the small intestine into the blood circulation, because its surface is covered with special materials like fatty acids, preventing it from being fermented and degraded in the rumen. Unlike monogastric animals, ruminants have a strong dependency on the endogenous glucose produced by gluconeogenesis, so exogenous glucose intake is relatively low. The RPG supplement can change energy supply path, improve energy utilization efficiency, and alleviate negative energy balance (9–11). Zhang et al. (12) also reported that RPG enhanced the large intestine fermentation function and mucosal immunity of transition dairy cows. Taurine (TAU) is a sulfur-containing amino acid that plays an important role in muscle growth, immune response and energy metabolism (13, 14). However, the TAU synthesized in the liver of most mammals has a limited ability to remove free radicals, while supplementation with exogenous TAU improved the antioxidant enzyme activity of animals and their liver tissues, thereby enhancing antioxidant capacity (15, 16).

Our previous study uncovered that supplementing with RPG and TAU reduced weight loss in yaks during transportation. However, the optimal addition of both supplements to effectively alleviate transport stress in yaks remains unclear. Our study aimed to investigate: (1) the effects of four supplemental levels on the weight of yaks before and after transportation; (2) the changes of plasma antioxidant, immune and stress response indicators before and after transportation; and (3) the impacts of continuous supplementation of RPG and TAU on the above indicators in yaks. We hypothesized that one of the four additive combinations of RPG and TAU could minimize the adverse effects of transportation on growth performance in yaks, and that sustained treatment could improve their antioxidant capacity, immunity and oxidative stress status. This study provides evidence for the use of nutritional intervention to alleviate the stress of yaks during transportation.

Harsh conditions during transportation, like noise and overcrowding, can induce oxidative stress in animals, resulting in weight loss. Lendrawati et al. (17) reported that transport reduces the weight of sheep, and the weight loss increases with the longer the transport time. González et al. (18) reported that while rearing patterns, age, transportation time, and truck facilities all affect the weight of cattle transported long distances, these tests all show a decrease in weight. In our study, all yaks experienced weight loss after 9 h of long-distance transport, and even supplementation with high levels of RPG and TAU could not reverse this outcome. However, yaks fed high supplemental TAU had lower weight loss (8.42 vs. 11.9 kg) and lower weight loss percent (4.83 vs. 6.87%), which was closely related to the biological function of TAU. Mice with inadequate TAU intake reportedly exhibited low body weight, slower responses to stimuli, and exercise intolerance, indicating that TAU was crucial for maintaining body weight and muscle endurance (19). Kim et al. (20) demonstrated the fatigue resistance of TAU following acute exercise, which were attributed to increased oxygen uptake and enhanced lipid oxidation in a mouse model. Taurine is also a promoter of energy metabolic balance (13) and may help increase energy supply of yak during transport. In addition, feeding TAU can improve the performance of animals (16, 21). However, we found that the growth performance of yaks within 30 days after transport did not change due to the increase in TAU and RPG. One possible reason is that this study focused more on the dosage of RPG and TAU rather than whether they were added, which may limit the comparison of this work with other studies and will be refined in future work.

Plasma TP is clinically significant for assessing liver synthesis function, nutritional status, and water balance. Mufarrej et al. (22) observed that transport increased the levels of TP and albumin in plasma of lambs. In our study, long-term transport may promote the fluid loss in yaks, causing blood to become concentrated and plasma TP levels to rise. Plasma TC concentration is commonly used to evaluate lipid metabolism and diagnose certain diseases (23, 24). We observed an increase in plasma TC concentration post-transport, likely due to the damage and dysfunction of cell membranes caused by lipid peroxidation, which affects the metabolism of lipoproteins and thus indirectly affects the TC in plasma. Plasma levels of LDH and CK are considered biomarkers of muscle injury (25, 26). Fang et al. (27) reported that transport induced an increase in plasma LDH concentration and changes in other blood indicators, suggesting the occurrence of oxidative stress. Alcalde et al. (28) also reported that road transportation, irrespective of the duration, consistently increased plasma CK levels in suckling goat kids, suggesting significant stress and muscle damage. Similarly, we observed elevated levels of LDH and CK in yak plasma post-transport, indicating potential health risks for yaks.

Stress during transport negatively affects the antioxidant system in animals, resulting in reduced antioxidant enzyme activity, which hinders the effective removal of free radicals and peroxide products from the body (29, 30). MDA is the end product of lipid peroxidation, shows a positive correlation between its plasma concentration and the severity of oxidative stress (31). Transport leads to increased plasma MDA concentration, indicating the onset and progression of oxidative stress (32, 33), which is also supported by our study findings. We observed that increasing TAU levels reduced plasma MDA concentration in yaks after transport, mitigating transport stress. Han et al. (16) found that TAU alleviates oxidative damage by increasing gene expression of NRF2, GPX and HO-1. Seidel et al. (34) also found that TAU reduces cellular stress levels, possibly by upregulating iron-related proteins like iron storage protein, thereby inhibiting lipid peroxidation. Therefore, we speculated that increasing TAU levels alleviated transport stress in yaks by inhibiting lipid peroxidation, and the specific mechanism needs to be further explored. GSH-PX is crucial for cellular antioxidant defense as it eliminates ROS from the body. Han et al. (35) indicated that TAU supplementation increased the glutathione contents and the GSH-PX activity, alleviating the oxidative stress induced by LPS. In our study, at 0 and 10 days after transport, increased TAU supplementation elevated plasma GSH-PX activity, implying an enhanced antioxidant capacity in yaks. It is to be noted that the plasma antioxidant capacity differences among treatments diminished over time after transport, possibly because the antioxidant function of TAU was activated in majority cases when exposed to oxidative stress risk.

During oxidative stress, the body generates a surplus of ROS, which activate the inflammasome and trigger the release of numerous pro-inflammatory cytokines, leading to cellular damage (36). Transport increased the concentration of proinflammatory cytokines like TNF-α and IL-1β in animal plasma (37, 38). However, in our study, transport did not change the concentration of IL-1β, IL-6 and TNF-α, while decreased the concentration of anti-inflammatory factors like IL-10, which also means that the health status of yaks may be potentially damaged. In addition, we found that increasing TAU supplementation decreased plasma IL-1β concentrations at 30 day after transport and TNF-α concentrations at 0 day after transport. TAU has been demonstrated to mitigates the adverse effects of LPS by dampening pro-inflammatory responses and oxidative stress (39). Zheng et al. (40) reported that TAU deficiency aggravated the DSS induced inflammatory model in mice, while TAU supplementation reduced proinflammatory factors levels like TNF-α and IL-6 by inhibiting the TLR4/NF-κB pathway. Han et al. (35) also indicated that TAU supplementation decreased the expression of IL-1β, TNF-α, and IL-6 genes, thereby resisting the LPS-induced inflammatory response in broilers. The release of pro-inflammatory cytokines accompanies the onset of inflammation, and over time, the pathogen is cleared by the body, and anti-inflammatory cytokines then come into play to repair the damaged tissue (41). TAU has a positive effect on the formation of the immune system of body (42). Zhou et al. (43) reported that TAU increases IL-10 concentration in weaned piglets and enhances immunity against weaning stress and inflammation. Elevated plasma concentrations of IL-4 and IL-10 in yaks were observed following transport when RPG and TAU were combined at high levels, indicating enhanced immunity, yet these levels stabilized after 30 days of feeding. It is likely that TAU exerts an anti-inflammatory effect during transport stress occurs, but the influence wanes when transport stress diminishes or even disappears.

The COR is an important glucocorticoid, and its concentrations serve as a crucial index for assessing stress responses, with their dynamic changes being closely tied to stress progression (44). Lee et al. (45) reported that road transport increased the plasma COR concentration of dairy cows. In our study, the concentrations of COR and LPS in yak plasma increased after transport, indicating the occurrence of transport stress. We also observed that the increase of TAU supplementation decreased the COR concentrations in yaks throughout the experiment, which suggested that TAU could mitigate transport stress in yaks, with a lasting beneficial effect (30 d). LPS stimulates the immune system, thereby triggering inflammation. He et al. (46) reported that transport impaired morphology and tissue in jejunum, induced stress responses, and elevated LPS concentrations. In our study, the LPS concentration of yaks decreased before and after transport with the increase of TAU addition, which was beneficial to the health of yaks after transport.

Transport induces oxidative stress in yaks, which can lead to reduced body weight and health impairment. TAU mitigates the adverse effects of transport on yak weight. Increasing TAU levels enhanced antioxidant capacity and immunity of yaks after transport, alleviating their oxidative stress, but this beneficial effect was weakened with time after transport. RPG had less effect on transport stress than TAU. Considering both cost and efficacy, the recommended dietary inclusion of RPG for yaks is 1%, with a TAU supplement of 15 g/d to mitigate transport stress. This work presents an effective strategy for mitigating transport stress, which is of great significance for off-site fattening of grazing yaks in the cold season.