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 Belg ian Blue bulls maintained at low growth (.5 kg/d) for 114 (G2), 243 (G 3), or 419 (G4) d (low growth period, LGP) before fattening (rapid gro wth 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 (T-4), 3,3',5'-triiodothyronin e (T-3), and IGF-I were measured in blood samples taken fortnightly. P lasma GH and insulin (I) profiles were measured in serial blood sample s obtained at three times during growth. The RGP was characterized by an initial compensatory growth, by higher plasma glucose, AAN, and ure a levels, and by lower plasma NEFA and creatinine levels. Plasma GH co ncentration decreased after refeeding. Plasma T-4 increased linearly d uring refeeding, as opposed to T-3, which showed a different profile i n each group. Plasma IGF-I showed a curvilinear increase during RGP an d reached a plateau after 3 mo in each compensating group. In G4, chan ges of plasma metabolites and hormones differed often distinctly from G2 or G3. During refeeding, higher nutrient supply improved the functi onality of the somatotropic axis and increased the concentration of an abolic hormones, allowing rapid muscle deposition. However, animals un derfed the longest period behaved differently from the other groups, p ossibly because they reached a more complete sexual maturity.