The aim of the study was to assess the effect of feed restriction levels followed by realimentation on long bone growth and concentrations of growth hormone, osteocalcin, bone-specific alkaline phosphatase (ALP), calcium (Ca) and phosphorus (P) in plasma. Thirty-six Najdi ram lambs weighing 30.1 ± 0.3 kg were randomly allotted to one of three feeding regimens. The first group was fed ad libitum throughout the trial (AD group). Other groups were subjected to 5-week feed restriction at either 0.75 or 0.60 of ad libitum intake (0.75AD and 0.60AD groups, respectively). Following the restriction period, lambs were returned to ad libitum feeding (realimentation) for 7 weeks. Feed restriction suppressed (P < 0.01) the growth of bone lengths in proportion to the restriction level. After the realimentation, bone lengths of animals from either 0.75AD or 0.60AD groups did not recover to the lengths of AD group. Plasma concentrations of inorganic P and osteocalcin decreased (P < 0.01) as the level of feed restriction increased. Concentration of plasma bone-specific ALP increased (P < 0.01) by 39.6 and 72.7% in 0.75AD and 0.60AD groups, respectively. After 4 weeks of realimentation, plasma concentrations of osteocalcin, ALP and P did not differ between all groups. The ulna, humerus and femur linear lengths were directly correlated with the level of osteocalcin and P; however, they showed a negative correlation with the level of ALP. Therefore, plasma concentration of P, osteocalcin and ALP appeared to be suitable bone markers for detecting changes in bone length due to variation in feed restriction levels
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