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.

Comparison of hindquarter metabolite uptakes in Belgian Blue double-muscled bulls at maintenance or during fattening

Metabolism of muscle growth in the hindquarter was investigated by the arterio-venous difference (AVD) technique in Belgian Blue double-muscled type bulls at maintenance or at fattening. The bulls were fitted with an aortic ultrasonic blood flow probe and with catheters in the aorta and vena cava. They were offered a diet allowing for maintenance (MP) during a period of 15 d, at the end of which measurements were made over 3 d. Bulls were then given a fattening diet (FP) and the measurements were repeated. Arterial blood flow was approximately 1 L/min greater when the bulls were standing than when lying. Blood flow was 2 L/min higher during FP than during MP. The AVD and uptake of glucose were maximal at 1400 and 1600. Uptake of alpha-amino nitrogen decreased immediately after a meal. The increase in glucose from MP to FP fitted very well with the calculated energy needs for muscle growth. The AVD and uptake of alpha-amino nitrogen, total amino acids, and total nonessential amino acids were negative during MP and positive and significantly higher during FP. There was also a significant increase in AVD and uptake of essential and branched-chain amino acids when the bulls were changed from MP to FP. When changing from maintenance to fattening, the incremental glucose and amino acid hindquarter uptake provided energy and supply for muscle protein accretion, respectively. The level of alanine transamination was also sharply reduced.