BODY-COMPOSITION AND METABOLIC PROFILES ASSOCIATED WITH PUBERTY IN BEEF HEIFERS

Rapid growth large frame (RL, n = 61) or average growth medium frame ( AM, n = 71) biotype heifers fed to achieve either moderate (MOD, .6 kg /d) or high ADG (HI, 1.0 kg/d) were used to determine whether puberty occurs at similar body composition or metabolic status. A heifer was c onsidered pubertal after being detected in estrus and then forming a f unctional corpus luteum. Live animal estimates of body composition and blood samples for assessment of metabolic status were taken at 13 +/- .2 d after estrus for all heifers. Body composition and metabolic stat us were assessed every 56 d from 7 mo of age until puberty in a subset of 80 heifers representing all biotype-diet combinations. At puberty, 32 of these 80 heifers were slaughtered and physical and chemical com position of the empty body were determined. High-gain diet heifers wer e younger, heavier, taller, and more muscular (all P <.01) at puberty than MOD heifers. Slaughter measurements paralleled Live animal estima tes; bodies of KI and RL heifers contained more (P <.01) carcass and n oncarcass components than those of MOD and AM heifers, respectively. C arcasses of RL and HI heifers were more (P <.05) muscular and fatter t han AM and MOD heifers. At puberty, HI heifers had a greater(P <.01) m ass of moisture, fat, and fat-free organic matter (FFOM) than MOD, whe reas RL heifers had more moisture, ash, and FFOM than AM. Percentage o f fat was greater (22.1 +/- 1.0 vs 19.1 +/- 1.0; P <.05) and percentag e of moisture was less (55.4 +/-.6 vs 58.1 +/-.6; P <.01) in bodies of KI than in those of NOD heifers. Concentrations of blood urea nitroge n and insulin were greater (P <.05) in HI than in MOD heifers. Diet di d not influence concentration of IGF-I or glucose, and metabolic marke rs were unaffected by biotype. No dramatic changes in body composition or metabolic signals were detected before puberty. Puberty did not oc cur at similar body composition or metabolic status in all heifers.