Bovine Somatotropin Alters Myosin Heavy Chains and Beta Receptors in Skeletal Muscle of Feedlot Heifers with Little Impact on Live or Carcass Performance

The objective was to determine whether recombinant bovine somatotropin (rbST) enhanced live performance,skeletal muscle biological activity, and beta-adrenergic receptor expression of feedlot heifers during the finishing phase. Heifers (n = 16; initial body weight 457 ± 3 kg) were randomly assigned pens (4 pens/treatment; 2 heads/pen) treatment: (1) no rbST (Control); (2) 500 mg/hd sometribove zinc at day 0 14 (rbST; Posilac®; Elanco AnimalHealth, Greenfield, IN). Longissimus biopsies for chemistry collected on 0, 14, 28, 42, 56. had increased AMP-activated protein kinase alpha beta adrenergic (P < 0.05). Day study affected myosin heavy chain-IIA (MHC-IIA), MHC-IIX, receptor, peroxisome proliferator-activated gamma, stearoyl-CoA desaturase a significant effect fiber cross-sectional area proportion As days feed increased, MHC-I fibers decreased whereas MHC-IIA IIX proportions MHC-IIX greatest density Paired Box 7-positive cells 42 0.05), Myogenic factor 5-positive 56 Also, positive 7:Myogenic 5 measured 28 These data indicate that, as increase, effects skeletal activity are not dependent administration but may be more due physiological changes occurring animal reaches physio-logical maturity.