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Two experiments were conducted to evaluate the effects of zinc propionate and vitamin A status on feedlot steer growth performance, carcass characteristics, and muscle fiber characteristics. First, 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 dietary Zn (KemTRACE® Zinc Propionate 27; Kemin Industries, Inc. Des Moines, IA) supplementation on 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 detected. Steers fed ZnP had greater ADG (P = 0.02) and G:F (P = 0.03) during the RH feeding period compared to CON. Fiber type I and IIA had no differences for cross-sectional area; however, IIX area was greater (P < 0.04) for CON compared to ZnP, and increased (P < 0.02) over time. 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 over time (P < 0.01). These data indicated that 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. The second study utilized a randomized complete block design with 30 yearling crossbred steers (average BW = 436.3 ± 39.8 kg) fed a steam-flaked corn-based diet was used to evaluate the effects of dietary vitamin A (Rovimix® A 1000; DSM Nutritional Products Ltd., Sisseln, SUI) depletion (phase 1) and repletion (phase 2) on cattle growth performance during the finishing phase. Steers were blocked by BW (n = 5 blocks; 6 steers/block), assigned to pens (n = 2 steers/pen), and randomly assigned to one of the following treatments: no added dietary vitamin A (0IU; 0.0 IU/kg [DM basis] of vitamin A), vitamin A supplemented at the estimated dietary requirement (2,200IU; 2,200 IU/kg of dietary DM of additional vitamin A), and vitamin A supplemented at 5× the estimated dietary requirement (11,000IU; 11,000 IU/kg of dietary DM of additional vitamin A). The basal diet included minimal vitamin A dietary activity (< 200 IU of vitamin A activity/kg of dietary DM) via the provitamin A, beta-carotene. After all treatments underwent a 91-d vitamin A depletion period, additional vitamin A was top-dressed at feeding via a ground corn carrier. Liver biopsy samples, BW, and blood were obtained on d -91, -35, 0, 28, 56, 84, and 112. Final BW was collected prior to shipping on d 112. Carcass data was collected by trained personnel upon harvest. Sera and liver samples were used to monitor circulating vitamin A and evaluate true vitamin A status of the cattle. Vitamin A status did not effect, interim ADG or G:F (P > 0.05). Throughout the duration of the study, DMI for the 0IU cattle was depressed (P < 0.05). Overall G:F was lower in the 0IU cattle compared to 2,200IU and 11,000IU. A treatment × day interaction occurred for both sera retinol and liver retinol (P < 0.01) during phase 2 of the trial. The treatments and sera retinol levels were incorporated into a repletion model, resulting in an estimation of liver retinol changes (P < 0.01; R2 = 0.682). However, models used to evaluate depleted animals were less effective. Expression for MHC-I diminished and rebounded (P = 0.04) from the beginning to the end of the trial. The intermediate fiber type, MHC-IIA, had a similar pattern of expression (P = 0.01) to that of MHC-I. On d 84, C/EBPβ expression was also the greatest (P = 0.03). The pattern of PPARγ (P < 0.01) and PPARδ (P < 0.01) expression seemed to more closely mimic that of MHC-I expression, increasing from d 84 to d 112. Distribution of MHC-IIA demonstrated a treatment × day interaction (P = 0.02). Although the proportion of MHC-IIA fibers equalized across treatments by d 112, the distribution of this muscle fiber type varied greatly across treatments and time. The distribution for MHC-I differed among treatments (P = 0.02): 2,200IU cattle maintained the greatest proportion of MHC-I, 11,000IU had the least MHC-I fibers, and 0IU was intermediate. Muscle fiber cross-sectional area increased for each MHC (P < 0.01) with the notable increase between d 28 and d 56. Among treatments, MHC-I had a tendency to have the greatest cross-sectional area in the 2,200IU steers and the smallest cross-sectional area in the 0IU cattle, with the 11,000IU cross-sectional area being intermediate. For both MHC-IIA and MHC-IIX, myofiber cross-sectional area was not different for the 11,000IU and 2,200IU treatments, but both were greater than the 0IU treatment (P = 0.03 and P < 0.01, respectively). Total nuclei density decreased (P < 0.01) gradually over time. Cells positive for only Myf5 increased (P < 0.01) in density early in the feeding period, then declined. There was a treatment × day interaction (P = 0.04) for PAX7-positive cells. Initially the 0IU treatment had the greatest PAX7 density; however, by d 56, the PAX7 density had declined, and there was no longer a difference among treatments. The dual positive (PAX7+Myf5) nuclei also increased through d 56 prior to a decline to d 112. As Myf5 positive cells, these nuclei are no longer pluripotent and will be readily collected into the muscle. The wax and wane of these transcription factors seemed to be unaffected by the vitamin A status of the animal. Rather, each cell type appeared to follow an expected pattern as animals mature. These data indicate that vitamin A should be supplemented in the diet at a concentration no less than 2,200 IU vitamin A/kg diet DM to prevent a loss in growth performance, and that oxidative gene expression may be independent of vitamin A status. Moreover, animals with an inadequate vitamin A status do not deposit protein as well as those animals that have adequate vitamin A.



beef, zinc propionate, vitamin A, fiber type, gene expression, satellite cells