Early-life social environment and weaning strategy play a critical role in shaping the development, health, and welfare of dairy calves. This study investigated the interactive effects of housing system (individual vs. pair) and weaning age (50 vs. 65 days) on feed intake, growth, health, behavior, and metabolic indicators in pre-weaned Holstein calves. A total of 66 four-day-old calves (33 males, 33 females) were randomly assigned to four treatments: individual housing with weaning at 50 days (IN-WA50, n = 11), individual housing with weaning at 65 days (IN-WA65, n = 11), pair housing with weaning at 50 days (PA-WA50, n = 11 pairs), and pair housing with weaning at 65 days (PA-WA65, n = 11 pairs), resulting in 22 individually and 44 pair-housed calves. Calves remained in their assigned housing until 80 days of age. Total dry matter intake (TDMI) tended to be higher in pair-housed calves (PA; P = 0.09), while final body weight (BW) and average daily gain (ADG) tended to be greater in calves weaned at 65 days (WA; P = 0.09 and P = 0.06, respectively). Starter intake also tended to be higher in WA50 (P = 0.06) and PA (P = 0.09) groups. Feed efficiency (FE) improved significantly in WA65 calves (P = 0.03). At day 80, WA65 calves had greater withers and hip heights (P = 0.03), and a tendency for longer body length (P = 0.09). While average fecal and respiratory scores were unaffected, PA-WA50 calves had significantly fewer days and episodes of diarrhea (P < 0.01). Pair housing also tended to reduce both the number (P = 0.07) and duration (P = 0.09) of respiratory episodes. PA calves spent more time standing (P < 0.01), whereas IN calves spent more time lying (P = 0.01). Calves weaned at 50 days exhibited more feeding activity (P = 0.02) and a tendency for increased rumination time (P = 0.09) than those weaned at 65 days. WA65 calves showed higher serum glucose concentrations throughout the trial (P = 0.046), while WA50 calves had higher overall BHB levels (P < 0.01). PA calves exhibited the highest BHB at day 80 (P = 0.04), with a significant interaction observed at day 50 (P = 0.045). Cortisol levels showed only a trend toward increase in WA65 calves at weaning (P = 0.09). In conclusion, delayed weaning improved growth and metabolic adaptation, whereas early social housing enhanced behavioral activity and contributed to a lower incidence and shorter duration of specific health symptoms. Stress indicators showed only mild and non-significant trends.
