HORMONAL STATUS, METABOLIC CHANGES, AND RESTING METABOLIC-RATE IN BEEF HEIFERS UNDERGOING COMPENSATORY GROWTH

Twelve recently weaned Hereford crossbred heifers weighing 227 kg (12 kg SD) and aged 230 d (8 d SD) on d 0 were used to investigate physiol ogical responses associated with compensatory growth. Six heifers were allotted to ad libitum intake (ADLIB) and six were restricted to a ma intenance diet for 95 d followed by realimentation (REST). Plasma coll ected from all heifers during feed restriction (d 0, 20, 48) and reali mentation (d 104, 125, 153, 195) was analyzed for growth hormone (GH), insulin-like growth factor I (IGF-I), thyroid hormones (thyroxine [T- 4] and triiodothyronine [T-3]), insulin, glucose, nonesterified fatty acids (NEFA), blood urea nitrogen (BUN), and 3-methyl histidine (3-MH) . Resting metabolic rate (RMR) was measured 5 d before and 15 and 36 d after the beginning of realimentation. Feed restriction was associate d with higher (P < .05) plasma concentrations of GH and NEFA and lower ( P < .05) concentrations of IGF-I, T-4, T-3, insulin, and glucose. T he BUN concentration was lower (P = .05) in REST than in ADLIB heifers on d 48, whereas 3-MH was not affected during the first 48 d of feed restriction. The RMR was lower (P < .05) in REST than in ADLIB heifers during feed restriction. During realimentation, the ADG of REST heife rs was higher (P = .001) than that of ADLIB heifers. Concentrations of IGF-I, insulin, glucose, and NEFA in REST heifers increased and no di fferences (P > .05) between treatments were observed on d 104 (d 10 of realimentation) and thereafter. Conversely, GH concentration in REST heifers remained elevated through d 104 but dropped to ADLIB levels by d 125 (d 31 of realimentation). The T-4 and T-3 concentrations remain ed lower (P < .05) in REST than in ADLIB heifers after 10 d of realime ntation but rose to control levels by d 31 of realimentation. The RMR was lower (P < .05) in REST than in ADLIB heifers 15 d into realimenta tion; however, no difference was found between treatments by d 36 of r ealimentation. These results indicate that enhanced growth rates in th e early phase of compensatory growth are associated with the physiolog ical response of the GH-IGF-I-insulin axis coupled with reduced mainte nance requirement due to slower metabolic rate in restricted-refed hei fers.