THE EFFECTS OF SOCIAL-EXCHANGE BETWEEN 2 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 investigate d. Two groups of 51 primiparous heifers in mid-lactation were used. Ne arly half of the heifers in each group, including the three highest-ra nking (Dm), three middle-ranking (Md) and three lowest-ranking (Sb) an imals, were selected by angular dominance value (ADV) and the remainin g heifers selected randomly. On Day 0, heifers were exchanged between the two groups. Regrouping prolonged the duration of standing and incr eased the frequency of shorter bouts of lying (less than or equal to 1 5 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 r egrouping, average milk production in regrouped heifers decreased sign ificantly compared with the week before member exchange (Week - 1) (P < 0.05) but no difference was found with heifers who were not regroupe d. 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 i n ADV of more than 5, Week 2 milk production of subordinates also decr eased significantly (P < 0.05) but that of dominants did not, Serum co rtisol response at 30 min after 200 IU of ACTH administration on Day 1 4 significantly increased in Dm (P < 0.05) but did not in Md and Sb. R egrouping distressed heifers, especially among heifers of lowered domi nance rank and subordinates, affected their production. Management pra ctices that disturb social stability in a dairy herd should be avoided .