Different periods of feed restriction before compensatory growth in Belgian Blue bulls: II. Plasma metabolites and hormones.

Plasma metabolites and hormones were studied in 16 double-muscled Belgian Blue bulls maintained at low growth (.5 kg/d) for 114 (G2), 243 (G3), or 419 (G4) d (low growth period, LGP) before fattening (rapid growth period, RGP). Animals from the control group (CG) were fed a diet high in energy and protein. The animals from G2, G3, and G4 were fed a restricted amount of a diet low in energy and protein during LGP and the same diet as CG during RGP. Plasma glucose, alpha-amino nitrogen (AAN), NEFA, urea, creatinine, thyroxine (T4), 3,3',5'-triiodothyronine (T3), and IGF-I were measured in blood samples taken fortnightly. Plasma GH and insulin (I) profiles were measured in serial blood samples obtained at three times during growth. The RGP was characterized by an initial compensatory growth, by higher plasma glucose, AAN, and urea levels, and by lower plasma NEFA and creatinine levels. Plasma GH concentration decreased after refeeding. Plasma T4 increased linearly during refeeding, as opposed to T3, which showed a different profile in each group. Plasma IGF-I showed a curvilinear increase during RGP and reached a plateau after 3 mo in each compensating group. In G4, changes of plasma metabolites and hormones differed often distinctly from G2 or G3. During refeeding, higher nutrient supply improved the functionality of the somatotropic axis and increased the concentration of anabolic hormones, allowing rapid muscle deposition. However, animals underfed the longest period behaved differently from the other groups, possibly because they reached a more complete sexual maturity.