Social rank and physiological traits of dairy cows as influenced by changing group membership

Social behavior was observed in two groups of 17 cows during 13 wk to determine dominance rank. On the basis of dominance ranking during two bi-weekly intervals (wk 3 and 4, 7 and 8), a total of six cows was exchanged between groups (two during the first and four during the second exchange). The effect on milk yield, milk leucocyte 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. Dominace rank for cows moved to a new group did not change appreciably. Persistency of milk yield was associated negatively with dominance rank for five of six exchanged cows. Since the average persistency of milk yield for all cows dropped 5% from wk 7 to wk 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 leucocyte concentrations in milk. Milk-prolactin concentrations were most highly correlated with dominance rank, stage of lactation, body weight, and height at withers.