Feed restriction in young bulls alters the onset of puberty in relationship with plasma insulin-like growth factor-I (IGF-I) and IGF-binding proteins.

The objectives of this study were to evaluate the effect of feed restriction and re-alimentation on the onset of puberty and IGF status in peripubertal male calves and to compare the radioimmunoassay (RIA) and western ligand blotting (WLB) methods for bovine IGFBP-2. Twelve prepubertal 290 d-old Belgian Blue bulls (mean weight: +/- 290 kg) were randomly assigned in three groups: a control group (NG; n = 4) receiving a classic fattening diet to induce "normal" growth (1.48 kg/d), a feed restricted group (RG; n = 4) to obtain reduced growth (0.50 kg/d) and, a severely restricted group (SG; n = 4) to nearly stop growth (0.08 kg/d). The feed restriction period was maintained over a period of 114 d. After the period of differential feeding, all animals received the control feed regime over a period of 100 d. Blood samples were collected at fortnightly intervals. Circulating IGF-I was measured by RIA whereas plasma IGFBPs was evaluated by WLB; IGFBP-2 was additionally quantified by RIA procedure. At the beginning of the trial, IGF-I levels were low (<100 ng/ml) and similar in the three groups in accordance with prepubertal status. In the NG group, a progressive rise in IGF-I was observed from Day 42 to Day 142 whereas in the RG and SG groups, IGF-I levels did not change until the experimental restriction period ended. The delay of the rise in plasma IGF-I was longer for the SG group, IGF-I remained low until 2 wk after the end of the period of restricted feeding. Surprisingly, although differences were detected for IGF-I levels between the three groups, the IGFBP-2 and -3 data, evaluated by WLB could only discriminate between NG and SG group and not between NG and RG. However, by using a RIA method, an IGFBP-2 decrease was observed in the NG group coincident with increasing IGF-I levels. For both RG and SG groups, IGFBP-2 levels remained high throughout the feed restriction period whereas plasma IGFBP-2 levels declined upon feeding in both groups. During this feed restriction period, IGFBP-2 was significantly lower in NG than in RG or SG groups. Moreover, SG group animals had higher levels in plasma IGFBP-2 than RG animals. In conclusion, puberty is characterized by developmental changes in plasma IGF-I and IGFBPs that were altered by feed restriction. Moreover, RIA evaluation of plasma IGFBP-2 is able to better reflect group differences than WLB.