Genotype × environment interactions for beef cow performance traits expressed during lactation were evaluated for 10 F1 cow crosses. The crosses (genotypes) were represented by the Hereford-Angus (HA) and nine crosses produced by mating Charolais (C), Simmental (S), and Limousin (L) sires with cows of Hereford (H), Angus (A) and Shorthorn (N) breeding. Environmental differences were provided by two climatically distinct locations (Brandon, Manitoba; Manyberries, Alberta) with two treatments (drylot confinement vs. pasture) imposed at each location in each of 3 consecutive years. The total data set comprised 1274 cow-calf pairs with milk production data recorded for 730 lactations. Substantial (P < 0.01) year, treatment and breed cross effects were present at each location for virtually all of the traits evaluated (cow weights and average subcutaneous fat, weight and fat changes during lactation, milk yield, calf growth rate, conception rates). Substantial (P < 0.0001) year × treatment interactions were also present. No genotype × environment interactions (year × breed cross, breed cross × treatment) were identified at either location for preweaning growth rate of calves, average daily milk yield of the cows or date of conception. Location differences were large for all of these traits, but rankings of the breed crosses at the two locations were in close agreement, indicating the absence of any location × genotype interactions. The ranking of breed crosses for conception rate was unaffected by treatment or year within each location, but the rankings did differ between locations. Under the environment provided at Manyberries conception rates tended to be inversely related to average milk yield of breed crosses, while no such relationship was evident under die conditions at Brandon. Key words: Genotype × environment interaction, beef cattle, cow productivity, calf performance