Effects of growth rate on carcass composition and lipid partitioning at puberty and growth hormone, insulin-like growth factor I, insulin, and metabolites before puberty in beef heifers.

The effect of three rates of gain on carcass composition, lipid partitioning, age and BW at puberty, and concentrations of growth hormone (GH), IGF-I, insulin, glucose, and NEFA in plasma were evaluated in 38 Angus x Hereford heifers. Heifers were allotted by BW and age to three treatments with a replication in each of 2 yr: full-fed (n = 13; FF) to gain 1.36 kg/d; limit-fed (n = 12; LF) to gain .68 kg/d; maintenance-full-fed (n = 13; MFF) to gain .23 kg/d for 16 wk, then full-fed to gain 1.36 kg/d. Heifers were slaughtered within 10 d after the onset of puberty. At slaughter, kidney, pelvic, and heart fat (KPH) and udder (UDDER) were separated from carcass, as was fat surrounding viscera (OM). After 48 h at 4 degrees C a carcass side was dissected into subcutaneous fat (SC), intermuscular fat (SEAM), soft tissue (SFT = inseparable lean and fat), LEAN, and BONE. In yr 1, LF heifers (431 d) were older (P < .05) than MFF heifers (371 d) at puberty, but age of FF heifers (389 d) did not differ (P > .10) from that of LF and MFF heifers. In yr 2, FF heifers (351 d) were younger (P < .05) than LF and MFF heifers (398 and 434 d, respectively). The FF heifers had greater (P < .05) BW and a greater (P < .01) percentage of lipid in the carcass at puberty than LF and MFF heifers. During the first 16 wk of treatment, concentrations of NEFA were greater in heifers with slower daily gains (MFF > LF > FF; P < .01). Concentrations of NEFA were lesser and concentrations of IGF-I and insulin were greater in plasma of FF than in that of MFF heifers during the 10 wk before puberty. Treatment significantly altered age, BW, carcass composition, and lipid partitioning at puberty in beef heifers. We conclude that the percentage of body fat is not the sole regulator of puberty, and age may be an important modulator in determining the onset of puberty in beef heifers.