The aim of this work was to determine whether developmental changes in growth hormone (GH) secretory patterns and carcass composition were influenced by nutrition and genotype in sheep. Four-month-old wether lambs from lean (low backfat), fat (high backfat) and control selection lines were nutritionally restricted to maintain a 28 kg live weight or given food ad libitum for 24 weeks. Plasma concentrations of GH and insulin-like growth factor 1 (IGF-1) were measured at predetermined times and carcass composition of the animals determined at the end of the trial.
From week 3 on, restrictions in dry matter (DM) intake were observed as the ad libitum treatment group had a significantly greater intake than the restricted treatment group (7·70 v. 5·80 kg DM per week, s.e.d. = 0·81). Differences in live weight between the feeding treatments were significant (P < 0·05) at week 9. The restricted feeding regime was associated with significant reductions in plasma levels of IGF-1 but had no effect (P > 0·05) on carcass weight-adjusted carcass fat proportion at the close of the trial. The effect of food restriction on GH secretory patterns was variable. Although there was initially a suppression in mean plasma GH, there was subsequently significantly higher mean plasma GH in the restricted feeding treatment. Periodogram analysis indicated that both the absolute levels of GH and the GH secretory pattern were altered by restricted feeding. In all animals, mean and basal GH concentrations, as well as the frequency and amplitude of pulses, declined from February to March and then increased from May to July (P < 0·001).
DM intake and live weight did not differ (P > 0·05) between genotypes, however the fat genotype had greater carcass fatness than lean or control genotypes (P < 0·01). There were no consistent differences between genotypes in plasma IGF-1 concentrations. In the ad libitum treatment, the lean and control genotypes had higher plasma GH levels than the f at genotype but the pattern of GH release did not vary. Under restricted feeding, both the pattern and the level of plasma GH varied between genotypes.
It is concluded that the developmental change in GH secretory patterns is affected by nutrition but not in a consistent manner. Although restricted feeding resulted in higher mean plasma GH concentrations later in the trial, this did not result in a change in carcass composition. The biological cues which lead to increased fat deposition in older lambs need further study but plasma GH levels may not he an important mechanism in this process.