The effects of pretest and genotype × environment (GE) interactions of 137 reciprocal backcross bulls produced under two contrasting environments (Brandon, Manitoba and Manyberries, Alberta) were evaluated for postweaning performance traits. Differences in weaning weight and average daily gain during the pretest periods defined as preweaning (ADGBW), weaning to on-test (ADGWT), and birth to on-test (ADGBT) associated with the fixed effects of station of origin, breed cross and station of origin by test were not significant. Station of test effects were significant (P = 0.0001) for ADGWT and ADGBT. None of the GE interactions involving the station of origin was significant (P > 0.10) for the postweaning growth traits and probabilities exceeded 0.20 for all but three of the 40 traits. The GE interactions involving station of test were nonsignificant (P > 0.20) for all but eight traits. All of these exceptions involved the cumulative average daily gain in the eight periods which excluded the first 14 d of test. Although the GE interaction for average daily gain for 140 d of test was not significant there were substantial breed cross differences in growth rates at the two test locations. Users of performance test results, however, are generally concerned with absolute performance values, not statistically significant differences. Viewed in this context, the differential responses of genotypes under different test environments, even though statistically nonsignificant, could have important implications to the industry. Key words: Cattle, postweaning growth, genotype × environment interaction
