Simple SummaryCow–calf separation immediately after birth is an issue of growing concern to global consumers of dairy products. The practice is also increasingly acknowledged by dairy scientists as coming at the detriment of the long-term behavioural, emotional and social development of replacement heifers. When dairy heifers first calve, they must successfully mix for the first time with herds of larger, more dominant, older cows, who can be aggressive and cause stress. Our study used contact with non-maternal adults and access to pasture rather than space-restricted sheds as a means to increase the complexity of the early-life environment of pre-weaned, artificially-reared dairy calves. When heifers reared under these differing conditions were then mixed with mature cows from a commercial milking herd at 23 months of age, the agonistic behaviour of those reared at pasture with adult contact suggested that these heifers were the most dominant within the wider heifer groups, and their feeding behaviour was the most similar to the cow groups into which they were mixed. Interestingly, heifers reared at pasture without adult contact seemed better able to adapt to grazing in a group of mature cows than heifers reared in sheds without adult contact. We suggest that exploring ways to increase both the physical and social complexity of the rearing environment may improve the ability of heifers to successfully integrate into a herd of older, more dominant animals.AbstractThis study aimed to determine the effects of early-life physical and social enrichment on the ability of dairy heifers to integrate into a herd of mature cows. Fifty heifer calves were reared from the ages of 2–13 weeks in one of three treatments: (1) Hand-reared and group-housed in sheds (CC); (2) Hand-reared and group-housed at pasture (−S); or (3) Hand-reared and group-housed at pasture, with 3 non-familial dry cows per group (+S). At 23 months of age, these heifers were introduced in groups to small herds of cows (Cows) at pasture. Social interactions were recorded continuously for two 1-h periods. Feeding, ruminating and resting behaviours of all animals and walking, standing and lying behaviours of 36 heifers only (+S = 14, −S = 13, CC = 9) were recorded for 48 h after mixing. Heifers that were managed as calves according to the CC treatment delivered less agonistic behaviour to other heifers after mixing than those reared in the +S or −S treatments (p= 0.002 andp= 0.041, respectively). On Day 2, +S heifers and cows spent the lowest proportion of time feeding (p= 0.961), with −S heifers spending significantly more time feeding than cows (p= 0.046), while CC heifers spent more time feeding than both +S heifers and cows (p= 0.027 andp< 0.002, respectively). Increasing the complexity of the early-life environment, particularly socially, may aid heifers in integrating into groups of multiparous cows later in life and shape their lifelong social experiences with same-age conspecifics.