When to pair: Effects of different pairing ages on dairy calf health, behavior, and performance.

Pair housing presents an alternative to individual or larger group housing systems, though the effects of different pairing ages on calf welfare are not well understood. In this study, we examined the effect of pairing age on the performance, health, and behavior of dairy calves. A total of 140 Holstein female calves (n = 70 pairs) were used, paired at 3 stages: early (6-7 d; 48 calves), intermediate (29-30 d; 48 calves), and late (49-50 d; 44 calves). Calves were housed individually before pairing and weaned at 78 ± 2 d of age. Weight measurements were recorded at birth, 30, 50, and 60 d, and at weaning. Calves' behavior was assessed 5 times per week through scan sampling after milk feedings from d 6 to 7 until weaning, and a food neophobia test was conducted at 50 to 51 d. Health assessments were performed 3 times per week by a veterinarian using Wisconsin clinical scores to diagnose diarrhea and bovine respiratory disease, with pulmonary ultrasonography at 55 d. The results suggest no association between pairing age and the prevalence of diarrhea or bovine respiratory disease, although the statistical power was limited. The number of calves with lung consolidation scores above 2 was also unaffected by pairing age. Early-paired calves exhibited more exploration and play behaviors and less idleness, stereotypies, and nonnutritive oral behaviors than late-paired calves. No differences were observed in cross-suckling likelihood across pairing ages. Early-paired calves also showed a shorter latency to touch a novel feed in the food neophobia test, higher calf starter intake during the first 15 d, and greater ADG up to 50 d. Although overall ADG did not differ among pairing ages, early-paired calves were more homogeneous in weaning weight than late-paired calves. In conclusion, early pairing supports calves' initial development, promoting early feeding behavior and growth and enhancing behaviors associated with positive affective states without adverse effects on health or undesirable behaviors.