The use of rangelands for domestic livestock implies having resilient and efficient animals, able to adapt to environmental constraints of these generally low-producing environments. We studied the roles of early rearing mode and genetic lineage on this adaptation in lambs. Eighty Romane ewe lambs of divergent lines for feed efficiency (EFF-, EFF+) were reared from birth to 3 months, either indoors with artificial milking (AR) or in the rangeland with their mothers (MR). Then, they were grouped by 20 (separated for experience and mixed for genetic line) and placed in 4 experimental rangeland plots for 3.5 months. For the first 1.5 days after entry into the plots, the behavioural activity of 12 lambs per group (6/genetic line) was recorded by scan sampling on 3 sessions of 2.5h. The AR lambs spent 34% of the scans exploring the environment on morning 1 (vs 1% for MR lambs), but only 5% on morning 2. Rumination increased through sessions in AR and MR lambs and more strongly in EFF- than EFF+ lambs. Feeding represented 36% of scans in AR lambs vs 73% in MR ones on morning 1, similarly to morning 2. After a similar growth for AR and MR lambs from D0 to D90 (average daily gain: 240 g/d), the entry into the plots made ADG strongly decrease, particularly for AR lambs (D90-D120: 46 vs 124 g/d for MR ones). Beyond this age, lambs’ growth was similar, maintaining the weight difference between AR and MR lambs. The genetic lineage had no effect excepted a trend for a greater ADG in EFF+ lambs between D90 and D200 (96 vs 86 g/d).