The effect of feeding grass silage or a forage–concentrate (dried grass–barley) diet ad lib. to young cattle on growth rate, plasma growth hormone (GH) and insulin-like growth factor-1 (IGF-1) concentrations was examined. The effect of including the β-adrenergic agonist cimaterol in the forage–concentrate diet was also investigated. Significantly higher growth rates were observed in animals fed on the forage–concentrate diet than in those fed on the silage diet (P < 0·001), and these were further enhanced by dietary inclusion of cimaterol (P < 0·05). Plasma GH levels were higher in the silage-fed animals (P < 0·001) but IGF-1 levels were significantly lower (P < 0·001) than in the forage–concentrate-fed animals. Cimaterol had no effect on mean plasma GH and IGF-1 concentrations. Nevertheless, plasma IGF-1 levels correlated positively with growth rate across all three treatments (r 0·84, P < 0·001, n 17). Spectral analysis of the GH data obtained for each animal was used to determine whether regular periodicities could be detected in the 24 h profile and whether they differed between the three treatment groups. Regular periodicities of four-five cycles/d were detected in the averaged GH profiles of the silage-fed and the control forage–concentrate-fed animals. In contrast, in animals given cimaterol regular GH cycles were not detectable at any of the frequencies tested. This suggests that cimaterol disrupts the rhythm of GH secretion without altering the overall mean concentrations. The data also suggest that due to an inadequate nutrient supply, the GH–IGF-1 regulatory mechanism was uncoupled in the cattle fed on silage, which may have contributed to the poor growth response of these animals.