ENDOCRINE RESPONSES AND BODY-COMPOSITION CHANGES DURING FEED RESTRICTION AND REALIMENTATION IN YOUNG BULLS

Changes in serum metabolic hormones, carcass composition, and body wei ght gains were examined in 21 Angus bulls (9 mo old) subjected to feed restriction and realimentation. They were allotted to three feeding r egimens: 1) control (CON); 2) restricted (REST); and 3) realimented (R EAL). The CON group was fed 3.2% of their body weight; the REST group was fed 1.5%. The REAL group was fed the restricted diet (1.5% BW) for 84 d then fed the control diet (3.2% BW) until slaughter. The CON and REAL groups were slaughtered at approximately 400 kg and the REST gro up at 346 kg For the experiment, average daily gains (kg/d) were diffe rent (P < .05) (CON = 1.60; REAL = 1.35; REST = .64). Bulls were bled every 20 min for 6 h on d 14, 70, 98, and 127 of the experiment. Overa ll carcass characteristics (yield grade, muscle area, marbling) and ch emical analysis of 9-10-11 rib sections indicated changes in quantity and percentages of protein and fat commensurate with the dietary intak e treatments. The REST group had the least lean and fat (P < .05); the REAL group had less fat than (P < .05) but the same amount of lean (P > .05) as the CON group. Serum GH was higher in the REST than in the CON group (P < .05). In the REAL group, serum GH values rose to a leve l similar to that of the REST group; realimentation lowered serum GH ( P < .05). The REAL group had low IGF-I concentrations initially, like the REST group, but access to the control diet increased serum IGF-I t o concentrations similar to those in the CON group. Serum insulin and cortisol were affected, but less so, by feed restriction. The REAL reg imen resulted in a serum GH:IGF-I ratio that was inverted as the bulls underwent realimentation, followed by a change in the lean to fat rat io of the carcass.