Long-term effects of dairy calf social housing on first lactation survival and milk yield.

Given advantages of preweaning social housing for dairy calf performance and behavioral development, the objective of this study was to characterize long-term effects of social housing on survival and milk yield during first lactation. Holstein heifers were previously raised in individual housing (IH; n = 90), pair housing (PH; n = 60 pens, 120 heifers), or in groups of 10 (GH; n = 12 pens, 80 heifers) for the first 2 mo of life, then commingled among housing treatments and raised in groups on pasture where they underwent standard rearing and breeding practices. At 30 d before expected calving, heifers were moved to a free stall barn. Following calving, heifers entered the lactating herd where they were milked twice daily and followed until the completion of their first lactation. We found no effect of social housing on pre-calving culling risk, but socially housed heifers were less likely to be culled during lactation (PH: hazard ratio = 0.35, 95% CI: 0.16-0.77, and GH: hazard ratio = 0.48, 95% CI: 0.25, 0.92). In cows that completed a 305 d lactation, social housing also resulted in greater first lactation milk yield, with greatest effects in the first 100 d post-calving. Effects of treatment on milk yield were predominantly direct, with pre-weaning ADG having a partial mediating effect explaining 15% of the total treatment effect. Early lactation milk yield, estimated in wk 2 to 8 for all cows regardless of culling events in lactation, predicted lower culling risk in PH and GH, compared with IH, suggesting that reduced milk yield over the full lactation in IH animals was exacerbated due to involuntary culling. These results support long-term production advantages attributed to social housing and provide some evidence to suggest that long-term effects of social housing may be attributed to behavioral mechanisms resulting in increased resilience rather than differences in biological potential arising from pre-weaning growth.