Interactions of implant strategies and ractopamine on carcass characteristics, muscle growth, and tenderness

To determine the interactions of implants and ractopamine on carcass characteristics, muscle growth, and tenderness, 120 Angus steers implanted with 36 mg zeranol were allocated to a five-group serial slaughter (n = 24 per group). Within group during the finishing phase, steers were randomly assigned to a 2 x 4 factorial treatment arrangement: 0 and 200 mg/hd/day ractopamine feeding and four levels of steroid reimplants: (0) = no reimplant, (ES) = 20 mg estradiol benzoate + 200 mg progesterone, (TES) = 24 mg estradiol + 120 mg trenbolone acetate, (TS) = 140 mg trenbolone acetate. Reimplants and ractopamine feeding were administered to steers at 100 d and 28 d respectively before their scheduled harvest. Carcass traits and meat tenderness were determined. There was no significant interaction of reimplants and ractopamine feeding on most traits studied. Relative to control steers, reimplanted steers produced heavier weights of biceps femoris (P = 0.028) and infraspinatus (P = 0.033) while steers fed ractopamine produced heavier weights of biceps femoris (P = 0.031) and semitendinosus (P = 0.001). There were no differences (P > 0.05) in activities of mu- or m-calpain or calpastatin at death or calpastatin at 2 d postmortem in the longissimus in either reimplanted or ractopamine-fed steers compared to control steers, except that calpastatin activity at death was higher in TES-reimplanted steers than control (P = 0.002) or TS-reimplanted (P = 0.003) steers. However, ractopamine-fed steers had lower desmin degradation in longissimus (P = 0.002), semitendinosus (P = 0.004), biceps femoris (P = 0.003) and gluteus medius (P = 0.008), and higher Warner-Bratzler shear force in longissimus (P = 0.0002) and semitendinosus (P = 0.0004) compared to controls. Desmin proteolysis and shear force values were not different (P > 0.05) between reimplanted and control steers. Neither reimplants nor ractopamine had an effect on collagen concentration of muscles relative to controls ( P > 0.05). Ractopamine may increase muscle mass by reduced proteolytic capacity and consequently reduced meat tenderness.