Impact of Cold Stress on Physiological, Endocrinological, Immunological, Metabolic, and Behavioral Changes of Beef Cattle at Different Stages of Growth

**Background:** Climate change is expected to increase extreme weather events, including colder winters. In South Korea, mean minimum winter temperatures over the last 30 years range from −3.2 to −5.5°C. Cold stress (CS) in beef cattle reduces growth performance, alters metabolic status, impairs immune function, and causes economic losses. While physiological indicators like heart rate (HR) and rectal temperature (RT) are known to change under CS, the internal metabolic, hormonal, and behavioral responses—and how these differ by growth stage—remain poorly characterized. This study aimed to evaluate physiological, blood, and behavioral responses to CS in Korean native beef calves (growing stage) and steers (early fattening stage).

**Methods:** Twelve calves (220.4 ± 12.33 kg, 209.9 ± 10.90 days old) and twelve steers (314.2 ± 18.44 kg, 324.3 ± 15.86 days old) were randomly assigned to three groups (four per group per stage): threshold, mild–moderate cold stress (MCS), and extreme cold stress (ECS), based on ambient temperature (AT). The experiment lasted 14 days per group with a 7-day acclimatization period. Animals were housed in individual pens indoors (roofed, protected from wind/rain). AT and relative humidity (RH) were recorded at 1-second intervals. Diets consisted of 40% roughage (Timothy grass) and 60% concentrate. Feed and water intake were recorded daily. HR and RT were measured weekly at 1400 h. Blood samples were collected weekly at 1100 h for serum cortisol, HSP70, glucose, NEFA, WBC, lymphocyte, and platelet analysis. Behavioral patterns (standing and lying time) were recorded weekly from 0900 to 1900 h (600 min). Data were analyzed using repeated-measures analysis with Tukey's HSD test (SAS 9.4). Post hoc power analysis showed power of 0.81 for calves and 0.85 for steers.

**Key Results:** Mean AT differed significantly across groups: threshold (4.66°C), MCS (−1.05°C), and ECS (−4.33°C) (p < 0.001). RH was lower in ECS (44.74%) vs. threshold (62.24%) and MCS (64.32%) (p < 0.001). Calves in ECS showed a tendency toward decreased dry matter intake (4.02 vs. 4.44 kg/day, p = 0.064), while steer intake did not differ (p = 0.147). Water intake was unchanged in both stages. HR increased significantly in ECS for calves (71.38 vs. 61.63 bpm, p < 0.001) and steers (74.88 vs. 62.75 bpm, p < 0.001). RT also increased in ECS for calves (39.63 vs. 38.98°C, p < 0.001) and steers (39.34 vs. 38.61°C, p < 0.001). Blood cortisol rose in ECS for calves (12.60 vs. 8.97 ng/mL, p = 0.014) and steers (11.38 vs. 7.17 ng/mL, p < 0.001). NEFA increased in ECS for calves (177.18 vs. 118.37 µEq/L, p = 0.046) and steers (153.75 vs. 112.00 µEq/L, p = 0.002). Blood glucose decreased in ECS calves (61.75 vs. 71.00 mg/dL, p = 0.018) but not in steers (p = 0.257). HSP70, WBC, lymphocytes, and platelets showed no significant differences. Standing time increased in ECS for calves (316.25 vs. 251.88 min, p < 0.001) and steers (261.88 vs. 232.50 min, p = 0.022), with corresponding decreases in lying time.

**Clinical Implications:** These findings demonstrate that extreme cold stress induces measurable physiological (HR, RT), blood (cortisol, NEFA, glucose), and behavioral (standing/lying time) changes in beef cattle, with some stage-specific differences (e.g., reduced feed intake and glucose only in calves). HR, RT, cortisol, and behavioral patterns are robust, sensitive indicators of CS across growth stages. The results can inform management strategies for winter housing, feeding adjustments, and welfare monitoring in beef cattle operations. The lack of HSP70 change suggests that this marker may not be useful for chronic CS assessment. Further research is needed to establish CS thresholds and mitigation strategies tailored to different growth stages.