The effects of social exchange between two groups of lactating primiparous heifers on milk production, dominance order, behavior and adrenocortical response

The effects of social disturbance, by management practice, on behavior and performance of lactating heifers in a dairy herd were investigated. Two groups of 51 primiparous heifers in mid-lactation were used. Nearly half of the heifers in each group, including the three highest-ranking (Dm), three middle-ranking (Md) and three lowest-ranking (Sb) animals, were selected by angular dominance value (ADV) and the remaining heifers selected randomly. On Day 0, heifers were exchanged between the two groups. Regrouping prolonged the duration of standing and increased the frequency of shorter bouts of lying (≤ 15 min). At the feed bunk, the feeding of Sb was frequently interrupted by an attack of Dm. The average feeding time of Sb was significantly longer than that of Dm ( P < 0.05). In the second week (Week 2) after regrouping, average milk production in regrouped heifers decreased significantly compared with the week before member exchange (Week −1) ( P < 0.05) but no difference was found with heifers who were not regrouped. Among regrouped heifers, Week 1 milk production decreased to 96.5% of that prior to the exchange ( P < 0.05) in those showing a decrease in ADV of more than 5. Week 2 milk production of subordinates also decreased significantly ( P < 0.05) but that of dominants did not. Serum cortisol response at 30 min after 200 IU of ACTH administration on Day 14 significantly increased in Dm ( P < 0.05) but did not in Md and Sb. Regrouping distressed heifers, especially among heifers of lowered dominance rank and subordinates, affected their production. Management practices that disturb social stability in a dairy herd should be avoided.