Abstract
A randomized complete block design experiment with 32 yearling crossbred steers (average BW = 442 ± 17.0 kg) fed a steam-flaked corn-based diet was used to evaluate the effects of dietary Zn (KemTRACE® Zinc Propionate 27; Kemin Industries, Inc., Des Moines, IA) supplementation on live growth performance, skeletal muscle fiber, and beta-adrenergic receptor (β-AR) characteristics during the finishing phase. Steers were blocked by BW (n = 4 blocks; 8 steers/block), assigned to pens (n = 4 steers/pen), and randomly assigned to the following treatments: control (CON; 0.0 g/ (hd · d) of additional Zn) or additional dietary Zn (ZnP; 1.0 g/(hd · d) additional Zn). The basal diet contained Zn (60 ppm DM basis) from ZnSO4; additional Zn was top-dressed at feeding. Ractopamine hydrochloride (RH; Optaflexx®: Elanco Animal Health, Greenfield, IN) was included at 300 mg/ (hd · d) for the final 28 d of the 111-d feeding period. Longissimus muscle biopsy samples, BW, and blood were obtained on d 0, 42, 79, and 107. Final BW was collected prior to shipping on d 111. Biopsy samples were used for immunohistochemical (IHC), mRNA, and protein analysis. Serum urea- N (SUN) and NEFA concentrations were measured. Steers fed ZnP had greater ADG (P = 0.02) and G:F (P = 0.03) during the RH feeding period compared to CON. There were no differences (P > 0.05) in other growth performance variables, carcass traits, mRNA abundance, or relative protein concentration for fiber type and β-AR. Fiber type I and IIA had no differences for cross-sectional area; however, IIX area was greater for CON (P < 0.04) compared to ZnP, and increased (P < 0.02) over time. There were no differences between treatments for β1-adrenergic receptor density (β1-AR; P > 0.05) in skeletal muscle tissue throughout the study. A treatment × day interaction was observed in β2-adrenergic receptor density (β2-AR; P = 0.02) and β3-adrenergic receptor density (β3-AR; P = 0.02) during the RH feeding period, where the abundance of the receptors increased with ZnP but did not change in CON. Compared to CON, ZnP had greater (P < 0.01) mean NEFA concentrations. Mean SUN concentrations did increase by day (P < 0.01). Additional dietary Zn, supplied as Zn propionate, upregulates β2-AR and β3-AR, and improves growth performance in feedlot steers during the RH feeding period, likely through a shift of resource utilization from lipogenesis to muscle maintenance and hypertrophy.
