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The Effects of Days on Feed and Ractopamine HCL Administration on Growth and Carcass Traits of Yearling Heifers




Hittle, Mindy

Type of Degree



Animal Sciences


Heifers comprise 30% of the total beef animals slaughtered in the United States yearly. However, heifers are less efficient and have lighter HCW than steers which affects the profitability of heifers. Beta adrenergic agonists increase production efficiency in steers but little is known about the effects of administering them to heifers. By increasing days on feed producers can increase marbling scores but decrease yield grades and increase inputs. Thus, the goals of this study were to examine the effect of RAC and days on feed on growth and carcass traits in growing heifers, to determine if either could reduce production costs. This study examined crossbred commercial yearling heifers (n=71 for carcass evaluation and n=67 for growth analyses, age = 397 ± 34 d, initial BW = 433 ± 34.9 kg) that were placed on ad libitum feed (DM = 89%, CP = 13.5%) for either 79 (n=15), 100 (n=15), 121 (n=16), 142 (n=16) or 163 (n=5) days for growth analyses or 79 (n=16), 100 (n=16), 121 (n=16), 142 (n=16) or 163 (n=7) days for carcass analyses. Days on feed (DOF) group assignments were stratified across initial weight and height. Individual birth dates and breed composition were known. A Calan System® (American Calan, Northwood, NH) was utilized to record daily feed intake. Body weights were recorded weekly. Thirty five days prior to harvest, one half of each DOF group were placed on Ractopamine-HCL (RAC) at a rate of 300 mg/hd/d (treatment phase). Performance traits analyzed were overall gain, ADG, DMI, and DM feed efficiency (DMFE). Carcass traits included hot carcass weight (HCW), longissimus dorsi muscle area (LMA), adjusted 12th rib fat thickness (BF), kidney pelvic and heart fat % (KPH), marbling score (MS), and USDA Yield Grade (YG). During the treatment phase, performance traits were recorded as treatment (TRT) gain, TRT ADG, TRT DMI, and TRT DMFE. Data were analyzed using general linear procedures of SAS and significance levels accepted at P<0.05. Independent variables included DOF, treatment, and breed composition. Covariates were initial weight or age at slaughter for growth analyses. Covariates for carcass characteristics included HCW, age at slaughter, and BF. DOF affected post weaning growth and carcass characteristics more than feeding a beta-agonist for 35 days. Adjusting data to a common initial weight, DOF increased TRT gain and TRT DMI. Adjusting to a common age, DOF increased gain and ADG. Addition of RAC to the diet affected improved DMFE by 25% over control heifers during the treatment phase of the experiment (7.38 vs 9.82; P<0.05). RAC did not affect carcass characteristics, tenderness, or sensory evaluation. These data are consistent with published results of the effects of RAC on feedlot heifers. Breed was not a source of variation during the treatment phase for any performance trait. Breed had an effect on LMA, KPH, USDA YG, and MS. Data from this study suggest heifers on feed longer than 100 or 121 d will not provide more salable product. Heifers fed longer than 121 days have improved MS, but significantly larger USDA yield grades. From this dataset, feeding yearling heifers 100 days is optimum. The administration of RAC to heifers at 300 mg/ hd /d be beneficial to producers by increasing the efficiency level of feedlot heifers and thus reducing feed costs.