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Effects of ractopamine supplementation on growth performance and carcass characteristics of feedlot steers differing in biological type

S. L. Gruber^*, J. D. Tatum^*,1, T. E. Engle^*, M. A. Mitchell^*, S. B. Laudert, A. L. Schroeder and W. J. Platter

^* Department of Animal Sciences, Colorado State University, Fort Collins 80523-1171; and Elanco Animal Health, Greenfield, IN 46140

	Abstract

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION LITERATURE CITED

 
Effects of ractopamine hydrochloride (RAC) supplementation on growth performance and carcass characteristics of feedlot steers differing in biological type were investigated using British, Continental crossbred, and Brahman crossbred calf-fed steers (n = 420). Steers of each type were weighed at reimplantation [British, mean BW = 375 kg (SD = 38 kg); Continental crossbred, mean BW = 379 kg (SD = 42 kg); Brahman crossbred, mean BW = 340 (SD = 32 kg)] and sorted into 7 BW blocks, each block consisting of 2 pens (10 steers per pen) per type. Pens within a block x type subclass were randomly assigned to RAC treatments (0 or 200 mg · steer^–1 · d^–1 fed during the final 28 d of the finishing period). The type x RAC interaction did not affect (P > 0.05) any of the traits evaluated in this study. Feeding RAC improved (P = 0.001) ADG (1.50 vs. 1.73 ± 0.09 kg) and G:F (0.145 vs. 0.170 ± 0.005), but did not affect (P = 0.48) DMI of steers. Dressing percentage, adjusted fat thickness, KPH percentage, and yield grade were not affected by RAC supplementation. Carcasses of steers fed RAC had heavier (P = 0.01) HCW (359 vs. 365 ± 4.9 kg), larger (P = 0.046) LM areas (81.7 vs. 84.0 ± 1.1 cm²), and tended (P = 0.07) to have lower mean marbling scores (487 vs. 477 ± 5.2; Slight = 400, Small = 500) than did carcasses of control steers. Among the 3 biological types, Brahman crossbred steers had the lowest DMI and produced the lightest-weight carcasses that had the lowest mean marbling score (P < 0.05). Compared with Continental crossbred and Brahman crossbred steers, British steers produced carcasses with the greatest (P = 0.001) mean marbling scores. Continental crossbred steers had the heaviest BW and greatest dressing percentages and produced the heaviest carcasses with the largest LM areas (P < 0.05) compared with British and Brahman crossbred steers. In the present study, 28 d of supplementation with RAC at a dosage rate of 200 mg · steer^–1 · d^–1 elicited consistent responses in growth performance and carcass traits among 3 diverse biological cattle types.

Key Words: beef • biological type • breed • carcass • growth • ractopamine

	INTRODUCTION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION LITERATURE CITED

 
Ractopamine hydrochloride (RAC; Optaflexx, Elanco Animal Health, Greenfield, IN), a phenethanolamine ß-adrenergic agonist approved in 2003 by the US Food and Drug Administration for use in cattle diets during the last 28 to 42 d of the finishing period, has been shown to improve growth performance of finishing steers by increasing ADG and improving the efficiency of gain (Laudert et al., 2005a; Schroeder et al., 2005a; Van Koevering et al., 2006b). Including RAC in cattle diets also has been reported to increase HCW (Crawford et al., 2006), LM area (Laudert et al., 2005b), and carcass conformation (Schroeder et al., 2005b).

Numerous studies have documented that including RAC (Paylean, Elanco Animal Health; Greenfield, IN) in swine finishing diets improves ADG, feed:gain, and carcass leanness (Watkins et al., 1990; Uttaro et al., 1993; Crome et al. 1996); however, several researchers have reported that effects of RAC on carcass characteristics differ among genetic lines of swine (Yen et al., 1991; Bark et al., 1992; Stoller et al., 2003).

Cattle that differ in biological type have been observed to respond differently when administered growth-promoting implants. Williams et al. (1991) reported that zeranol implants improved ADG and feed:gain of Angus and Brangus steers, but the magnitude of this response was greater for Angus steers. In addition, Botts (1992) reported data suggesting that implants containing estradiol and trenbolone acetate produced a greater growth response in Continental steers than in English or Brahman steers. To date, no studies have examined the responses of different biological cattle types to RAC.

Therefore, this study was conducted to determine the effects of RAC on growth performance and carcass characteristics of steers differing in biological type.

	MATERIALS AND METHODS

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION LITERATURE CITED

 
Animals and Management
Care, handling, and sampling of animals described herein were approved by the Colorado State University Animal Care and Use Committee. Four hundred twenty weanling steer calves (6 to 9 mo old) were obtained from 5 sources to represent 3 distinct biological cattle types (British, Continental crossbred, and Brahman cross-bred). British steers included pedigreed Angus from Wyoming (n = 60); British composites from Colorado (hybrids with Red Angus, Angus, and Hereford breed influence, n = 40); and Angus x Hereford crossbreds from Colorado (n = 40). Continental crossbreds were 50% Charolais x 50% British-composite (n = 40) or 50% Limousin x 50% Angus (n = 100), originating from Colorado and Wyoming, respectively. Beefmaster steers (37.5 to 50% Brahman x 50 to 62.5% British) procured from 2 herds (single owner) in Texas were used to represent the Brahman crossbred biological type (n = 140). Information documenting breeds, typical ADG during finishing, and average finished BW for steer calves from each herd was obtained from all 5 cooperators.

Steer calves selected for use in this study were transported from their points of origin to the Colorado State University Beef Research Feedlot at the Agricultural Research, Development, and Education Center (Fort Collins, CO). Upon arrival at the feedlot, a 5-step feeding program that began with a receiving diet (1.07 Mcal/kg NE_g and 14% CP) fed for 21 d. Steers were then transitioned from the receiving diet through a series of incremental increases in NE_g (from 1.07 to 1.42 Mcal/kg NE_g) and decreases in CP (from 14 to 12.85% CP) until the final finishing diet (Table 1) was achieved. All diets were provided once daily (±2.3 kg) to allow ad libitum access to feed throughout the day. Shortly after arrival at the feedlot, the steer calves were implanted (20 mg of estradiol benzoate and 200 mg of progesterone; Component ES with Tylan, VetLife, West Des Moines, IA) and vaccinated (Pyramid 4, Fort Dodge Animal Health, Fort Dodge, IA).

Experimental Design and Treatments
Steers were stratified by BW at reimplantation and sorted into 7 BW blocks, each block consisting of 2 pens (10 steers per pen) per type. Steers within a block x type subclass were obtained from the same source. Each pair of pen-replicates within a block x type subclass was randomly assigned to RAC supplementation treatments: 1) control, 0 mg · steer^–1 · d^–1 of RAC; 2) treatment, 200 mg · steer^–1 · d^–1 of RAC. The experimental design used in this study resulted in 7 BW blocks, 21 pen-replicates per RAC treatment, 14 pen-replicates per type, and 7 replicates of each RAC x type subclass.

The study design involved feeding RAC for the final 28 d of the finishing period and required that each pen of steers be assigned a predetermined slaughter date. A target slaughter date was projected for each pair of pen-replicates (control and treatment) using pen means for BW at reimplantation, together with estimates of ADG (based on historical herd records), and predicted finished BW (computed using individual scores for frame size; Grona et al., 2002). Mean predicted finished BW for British, Continental crossbred, and Brahman crossbred steers were 538 kg (SD = 19.9 kg), 591 kg (SD = 17.9 kg), and 545 kg (SD = 19.6 kg), respectively. Paired pens were targeted for slaughter on 4 slaughter dates (103, 124, 138, or 159 d after reimplantation). All types were represented on the first 3 slaughter dates; however, due to differences in growth and developmental patterns among the 3 biological cattle types, the final slaughter group included only Continental crossbreds and Brahman crossbreds. The experimental protocol specified that steers would be slaughtered no sooner than 100 d from the date of reimplantation. This constraint caused British steers to be fed to an actual mean final BW that exceeded their predicted finished BW.

Experimental treatments were initiated when paired pens of steers were 28 d from their respective predetermined slaughter dates. On each designated treatment initiation date (74 to 130 d after randomization of steers to treatments), steers were weighed individually before feed was issued and returned to their pen as a group (initial BW was reduced by 4% to represent a standard industry shrink).

Steers in the treatment group were administered RAC via a Type-B, medicated, ground corn supplement formulated to contain 400 g of RAC/907 kg. The supplement was thoroughly hand-mixed into each pen’s daily feed issue at a rate of 0.45 kg/steer. A similar nonmedicated premix (identical formulation to that of medicated supplement, except that RAC was excluded) was hand-mixed into each control pen’s daily feed issue at a rate of 0.45 kg/steer.

Slaughter and Carcass Data Collection
The day before each shipping date (28 d after the respective treatment initiation date), steers designated for slaughter were weighed individually before the daily feed issue (final BW was reduced by 4% to represent a standard industry shrink). The next day (approximately 30 h after obtaining the final BW), steers scheduled for slaughter were transported approximately 64 km to a commercial packing facility, where they were slaughtered using conventional, humane procedures.

Carcasses were chilled in a cooler with an air temperature of 2°C for 36 h. For the first 8 h of the chill period, carcasses were sprayed intermittently (2 min on, 8 min off) with a mist of 2°C water. After the carcass-chilling period, a USDA grading supervisor assigned scores to each carcass for marbling and lean maturity. In addition, a panel of 2 experienced evaluators (Colorado State University personnel) independently evaluated each carcass and recorded measurements/assessments of fat thickness, adjusted fat thickness, KPH, and skeletal maturity. Values for each trait recorded by the 2 evaluators were averaged, resulting in a single value for each grade factor for each carcass. Measurements of LM area were obtained for each carcass using a video image analysis system (Computer Vision System, Research Management Systems Inc., Fort Collins, CO).

Statistical Analysis
No animals died during the treatment period; however, 3 steers were excluded from the dataset for illnesses unrelated to the treatment. One British control animal was removed during the study period; following euthanasia and necropsy, it was determined that the animal suffered from heart failure. A second British control steer completed the study period in its respective pen, but lost BW during the study period and was excluded from the dataset. A third British steer, supplemented with RAC, was treated for a respiratory condition during the 28-d study period and could not be returned to his pen to complete the study.

Analyses of growth traits and carcass characteristics were conducted for a split-plot design using the MIXED procedure (SAS Inst. Inc., Cary, NC); pen served as the experimental unit. The statistical model included RAC and type as independent fixed effects. Block and block x type were included as random effects. The 2-way interaction of RAC x type initially was included in the statistical model but subsequently was removed if not significant (P > 0.05). The Kenward-Roger approximation was used to calculate denominator degrees of freedom, and means were separated using the PDIFF option at a significance level of P < 0.05.

Frequency distributions of calculated USDA yield grades (YG) and quality grade marketing categories (e.g., USDA Prime; upper USDA Choice; lower USDA Choice; USDA Select; USDA Standard) were computed for main effects of RAC and type. Grade frequencies for treatment groups were compared (i.e., control vs. RAC; British vs. Continental vs. Brahman) using ² test at a significance level of 0.05.

	RESULTS AND DISCUSSION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION LITERATURE CITED

 
RAC x Type Effects
The primary objective of this study was to determine whether cattle of diverse biological types would respond differently to dietary supplementation with RAC. Previous research has shown certain genetic lines of pigs differ in their response to RAC supplementation (Gu et al., 1991; Bark et al., 1992; Stoller et al., 2003). Bark et al. (1992) compared 2 lines of pigs differing in genetic capacity for lean tissue growth (low vs. high) and reported significant RAC x genotype effects for weights of dissected muscle and fat, rates of muscle and fat accretion, and amount of muscle gained per unit of feed consumed. The authors concluded that responsiveness to RAC was greater in pigs that have a high genetic capacity for lean tissue growth.

In the present study, the RAC x type interaction did not affect (P > 0.05) any of the traits evaluated, indicating that, despite their genetic diversity, steers differing in type responded similarly to supplementation with RAC. Therefore, only the main effects of RAC and type will be presented and discussed. No other studies have examined the interaction between RAC supplementation and cattle type.

Ractopamine Effects
Least squares means for growth performance traits, corresponding to the main effect of RAC, are presented in Table 2. Supplementation with RAC during the final 28 d of finishing improved (P = 0.001) ADG and G:F but did not affect (P = 0.48) DMI (Table 2). Laudert et al. (2005a) and Schroeder et al. (2005a) observed similar improvements in growth performance of steers supplemented with RAC. Positive effects on ADG and the efficiency of gain have been documented in swine, when diets were supplemented with RAC (5 to 20 ppm) during the final stages of finishing (Crome et al., 1996; See et al., 2004; Carr et al., 2005). Moreover, ADFI of swine has been reported to decrease (Watkins et al., 1990; Crome et al., 1996; Mimbs et al., 2005) or remain unchanged (Stites et al., 1991; Armstrong et al., 2004; Carr et al., 2005) following RAC supplementation.

Skeletal, lean, and overall maturity scores were not affected by RAC supplementation (Table 3). There was a tendency (P = 0.07) for carcasses of steers fed RAC to have lower mean marbling scores compared with carcasses of control steers (487 vs. 477 ± 5.2; Slight = 400, Small = 500); however, the effect of RAC on marbling score was not of sufficient magnitude to influence (P > 0.05) the distribution of USDA quality grades (Table 4). Others have reported that RAC supplementation at 200 mg · steer^–1 · d^–1 (or 20 ppm) did not affect (P > 0.05) mean marbling scores of carcasses from crossbred beef steers (Laudert et al., 2005b; Schroeder et al., 2005b; Van Koevering et al., 2006a). Vogel et al. (2005) noted that marbling scores of carcasses from calf-fed Holstein steers supplemented with 200 mg · steer^–1 · d^–1 of RAC were lower (P < 0.05) than those of carcasses produced by control steers (498 vs. 515 ± 20.7; Slight = 400, Small = 500); however, the authors observed no difference (P > 0.05) in mean marbling scores between the 300 mg · steer^–1 · d^–1 and control groups.

Biological Cattle Type Effects
Results showing the effects of type on growth performance traits and carcass characteristics are summarized in Tables 2 and 3. Numerous studies have been conducted to characterize differences in growth and carcass traits among steers differing in biological type (Wheeler et al., 2005). The specific focus of the current study was to compare responses of steers of diverse types to RAC supplementation during the final 28 d of finishing. Results reported in Tables 2 and 3 are generally consistent with previous reports that have compared British, Continental crossbred, and Brahman crossbred cattle at various slaughter endpoints (Cundiff, 1992; Wheeler et al., 2001, 2005; Bidner et al., 2002).

In the current study, biological type did not affect (P > 0.05) ADG or G:F measured during the final 28 d of the finishing period (Table 2), although compared with Continental crossbred and British steers, Brahman crossbred steers had the lowest (P < 0.05) DMI.

Continental crossbred steers had the heaviest BW (initial and final), and greatest dressing percentages, resulting in the heaviest HCW (P < 0.05; Tables 2 and 3). Conversely, Brahman crossbred steers had the lightest initial and final BW, and the lightest HCW (P < 0.05). Carcasses produced by Continental-type steers had a mean LM area that was 7.2 and 4.9 cm² larger (P < 0.05) than LM areas of British and Brahman crossbred carcasses, respectively (Table 3).

British steers in this study produced carcasses with the greatest (P < 0.05) mean marbling score, Small²⁹, which would qualify for the USDA Choice quality grade, whereas Continental crossbred and Brahman crossbred steers produced carcasses with mean marbling scores (Slight⁸⁹ and Slight²⁹, respectively) that would result in quality grades of USDA high Select and USDA low Select, respectively (Table 3). Additionally, distinct differences in quality grade distributions were observed among the 3 biological cattle types (Table 4). British steers produced the greatest percentage of carcasses grading Choice and the lowest percentage of carcasses grading Select (P < 0.05), whereas Brahman crossbred steers produced the lowest (P < 0.05) percentage of carcasses grading Choice and the greatest (P < 0.05) percentage of USDA Standard carcasses (Table 4). All subclass means for skeletal, lean, and overall maturity were A-maturity, and type did not affect (P > 0.05) any of these indicators of maturity.

British steers had greater (P < 0.05) mean adjusted fat thickness and a greater mean value for calculated YG compared with Continental and Brahman crossbred steers (Table 3), due in part to the fact that British steers were slaughtered at an actual mean BW that was greater than their mean predicted finished BW. Nevertheless, all 3 type groups had means for fat thickness and YG (Table 3) that were within practical ranges. McKenna et al. (2002) surveyed carcass characteristics of the US fed cattle population and determined that fed steers and heifers had a mean fat thickness of 1.2 cm (SD = 0.5 cm) and a mean YG of 3.0 (SD = 0.9).

Preferably, steers representing the 3 biological types would have been slaughtered at more similar fat thickness endpoints; however, of greater importance, relative to the outcome of this study, is the fact that the test period spanned a growth interval during which all 3 cattle types exhibited similar ADG and G:F (Table 2), suggesting that the 3 types were evaluated across comparable segments of their respective growth curves. Feeding British steers to a slightly fatter endpoint did not appear to negatively affect their growth performance or their ability to respond to RAC supplementation (i.e., all 3 types exhibited similar increases in ADG and G:F when supplemented with RAC). Correspondingly, there is no indication that the difference in fat thickness among biological types in this study influenced RAC or RAC x type interaction effects.

Effective use of RAC (Optaflexx) for managing growth of finishing cattle requires a more thorough understanding of how various kinds of cattle respond to its inclusion in the finishing diet. This study compared responses of British, Continental crossbred, and Brahman crossbred calf-fed steers supplemented with RAC at 200 mg · steer^–1 · d^–1 during the final 28 d of the finishing period. Results showed that 28 d of supplementation with RAC at the specified dosage rate elicited consistent responses (measured as increases in ADG, G:F, HCW, and LM area compared with a negative control) among 3 very diverse biological cattle types. Larger-scale commercial studies would assist in making broader inferences concerning responses of various biological cattle types to RAC supplementation.

¹ Corresponding author: J.Daryl.Tatum{at}Colostate.edu

Received for publication September 15, 2006. Accepted for publication April 4, 2007.

	LITERATURE CITED

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION LITERATURE CITED

 


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