Social behavior was observed in 2 groups of 17 cows during 13 wk to determine dominance rank. On the basis of dominance ranking during 2 biweekly intervals (wk 3 and 4, 7 and 8), 6 cows were exchanged between groups (2 during the 1st and 4 during the 2nd exchange). The effect on milk yield, milk leukocyte concentration and milk prolactin of dominance rank and introduction of "strange" cows into a group was studied. Dominance rank was greater for older, heavier, taller cows in a later stage of lactation. Fat-corrected-milk yield was greater for dominant cows but milk prolactin was lower. Dominance rank for cows moved to a new group did not change appreciably. Persistency of milk yield was associated negatively with dominance rank for 5 of 6 exchanged cows. Since the average persistency of milk yield for all cows dropped 5% from wk 7-8, it appears exchange of cows between groups affected both exchanged and nonexchanged cows in a random manner. Neither dominance rank nor exchanging cows between groups had a significant effect on leukocyte concentrations in milk. Milk-prolactin concentrations were most highly correlated with dominance rank, stage of lactation, body weight and height at withers.
