Nitrogen balance and myofibrillar protein turnover in double muscled Belgian Blue bulls in relation to compensatory growth after different periods ofrestricted feeding

Nitrogen balance and myofibrillar protein breakdown were studied in 16 doub le-muscled Belgian Blue bulls during a low growth period (0.5 kg d(-1)) (LG P) of 4 mo (L4), 8 mo (L8), or 14 mo (L14) and the subsequent fattening per iod (rapid growth period, RGP). The control group (CG) was given a conventi onal fattening diet; the others received a low-energy, low-protein diet dur ing LGP, and the same diet as the CG during RGP. Measurements were made hal fway through the LGP, I mo after the beginning of the fattening period, and 1 mo before slaughter. Nitrogen balance was about half of CG (P < 0.001) d uring LGP, e.g., 50.8, 21.3, 25.8, and 23.8 g d(-1), for CG, L4, L8, and L1 4, respectively. Between LGP and RGP, N balance increased by about 18 g N d (-1) above the control in the compensating groups L4, L8 and L14. This was due to the higher digestibility and the higher metabolizability of the nitr ogen in the fattening diet. Lower muscle protein accretion during the LGP r esulted from decreased synthesis (P < 0.001) and degradation (P < 0.05) com pared with the GC. When changing to RGP different evolution patterns were o bserved in the three formerly restricted groups, e.g. after a short restric tion (L4) both synthesis and degradation rose during the RGP but declined t owards the end. After a longer restriction (L8 and L14), synthesis and degr adation increased and remained high. The magnitude of these increases was i nversely proportional to the length of the restriction period.