Pair housing benefits performance and activity of dairy calves with influences of season.

The objective of this study was to evaluate effects of social housing on calf performance and activity within different seasons. Within 24 h of birth, Holstein heifer calves were assigned to either individual (IH; n = 50 pens) or pair housing (PH; n = 50 pens), balanced across seasons in a humid subtropical climate (cool season, mid-October to mid-April, 17 ± 5°C, mean ± SD, and warm season, mid-April to mid-October, 26 ± 3°C, temperature-humidity index consistently >65). Calves received 8 L/d of milk replacer, until gradual weaning over 10 d beginning at 43 ± 3 d of age, and calf starter and water ad libitum. The presence of fever (≥39.4°C) and scours (fecal consistency assessed visually at the pen level) were assessed 5 d/wk. Calves were weighed at birth, before and after weaning, and 1 wk postweaning. Pen-level starter intake was recorded 4 d/wk. Standing time was determined using leg-based accelerometers. Calves born in the cool season scoured later and for fewer days, with no effect of social housing. Fever presentation was similarly reduced in the cool season, in interaction with housing treatment; fewer PH than IH calves had a fever in the cool season. Calves in PH pens consumed more starter by wk 5 of life, had greater ADG pre- and postweaning, and had greater BW by the end of weaning, compared to IH calves. Some performance advantages attributed to PH were most apparent in the cool season, including starter intake during weaning (+0.17 kg/d; SE = 0.021) and postweaning ADG (+0.35 kg/d; SE = 0.1). Calves in PH pens maintained greater BW 1 wk postweaning, with no effect of season. Calves in PH pens spent more time standing, particularly in the evening, with daily peaks in activity falling later in the day during the warm season. These results support clear benefits of housing dairy calves in pairs from birth, alongside evidence that performance advantages attributed to social housing may be enhanced in cool weather or dampened by heat stress.