Jm. Dawson et al., INFLUENCE OF DIET AND BETA-AGONIST ADMINISTRATION ON PLASMA-CONCENTRATIONS OF GROWTH-HORMONE AND INSULIN-LIKE GROWTH FACTOR-I IN YOUNG STEERS, British Journal of Nutrition, 70(1), 1993, pp. 93-102
The effect of feeding grass silage or a forage-concentrate (dried gras
s-barley) diet ad lib. to young cattle on growth rate, plasma growth h
ormone (GH) and insulin-like growth factor-1 (IGF-1) concentrations wa
s examined. The effect of including the beta-adrenergic agonist cimate
rol in the forage-concentrate diet was also investigated. Significantl
y higher growth rates were observed in animals fed on the forage-conce
ntrate diet than in those fed on the silage diet (P < 0-001), and thes
e 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 po
sitively with growth rate across all three treatments (r 0.84, P < 0.0
01, n 17). Spectral analysis of the GH data obtained for each animal w
as used to determine whether regular periodicities could be detected i
n the 24 h profile and whether they differed between the three treatme
nt groups. Regular periodicities of four-five cycles/d were detected i
n the averaged GH profiles of the silage-fed and the control forage-co
ncentrate-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 al
tering the overall mean concentrations. The data also suggest that due
to an inadequate nutrient supply, the GH-IGF-1 regulatory mechanism w
as uncoupled in the cattle fed on silage, which may have contributed t
o the poor growth response of these animals.