Long-term effects of preweaning social housing on growth and reproductive development of dairy heifers

Social housing provides early life behavioral and performance benefits for dairy calves, yet potential longer-term effects on heifer growth and development remain less studied. We assessed the effects of preweaning social housing on growth until 1 yr of age, mounting activity, and age at onset of cyclicity of heifers. Dairy heifer calves were assigned to (1) individual housing (IH; n = 55 heifers), (2) pair housing (PH; n = 55 focal heifers; 1 focal heifer/pair), or (3) group housing (GH; n = 53 focal heifers; selected from 9 groups of 10 calves/group). Calves assigned to GH were managed identically to PH calves until 15 ± 3 d of age, at which point they were introduced to a group of 10 calves (<10 d age range) and fed via an automated milk feeder. All calves received 8 L/d of milk replacer and were weaned over 10 d beginning at 7 wk of age. At 9 wk of age, calves were mingled between previous housing treatments and housed on pasture. Growth outcomes were recorded weekly from 6 to 12 mo of age. Mounting activity was assessed using Estrotect breeding indicators fixed to the tailhead and examined weekly, and age at onset of cyclicity was assessed by ultrasound confirmation of a corpus luteum and retrospective analysis of blood progesterone. We found that PH heifers had greater hip height than both IH and GH heifers (at 12 mo of age: 130.7, 128.5, and 129.0 cm, SE = 0.6, respectively) and final BW was greater for PH than IH heifers, with GH intermediate (at 12 mo of age: 334.3, 318.8, and 325.6 kg, SE = 5.3, respectively). Mounting activity was expressed approximately 3 wk earlier in PH and GH than IH heifers, with no effect of preweaning social housing on age at onset of cyclicity. Season of birth affected some growth and reproductive outcomes, with lower final BW and later age at first estrus in heifers that experienced heat stress concurrent with data collection. Overall, these findings support long-term growth advantages particularly attributed to housing calves in pairs, as well as increased indication of social interaction in heifers in either preweaning social housing treatment compared with individual housing.