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J. Anim Sci. 2006. 84:2795-2800. doi:10.2527/jas.2005-614
© 2006 American Society of Animal Science
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ANIMAL NUTRITION

Effects of ractopamine and protein source on growth performance and carcass characteristics of feedlot heifers1

D. K. Walker, E. C. Titgemeyer2, J. S. Drouillard, E. R. Loe, B. E. Depenbusch and A. S. Webb

Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506


   Abstract
Top
 Abstract
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 IMPLICATIONS
 LITERATURE CITED
 
An experiment was conducted to determine the relationship between feeding ractopamine and different amounts of MP on growth and carcass characteristics of feedlot heifers. Seventy-two crossbred heifers (475 kg of initial BW) were fed individually a diet based on steam-flaked corn for ad libitum intake for 29 d. Heifers were implanted with 140 mg of trenbolone acetate and 14 mg of estradiol-17ß 60 d before the experiment. Treatments were arranged as a 2 x 3 factorial and included 0 or 200 mg of ractopamine-HCl (23 ppm)/ d, and urea, solvent soybean meal, or expeller soybean meal (ESBM) as the predominant protein supplement. The amounts of MP supplied by the urea, solvent soybean meal, and ESBM diets were 688, 761, and 808 g/ d, respectively, calculated according to level 1 of the NRC model. Body weights were obtained 1 d before ractopamine feeding and at slaughter. Blood samples were obtained 1 d before starting the experiment and 13 d later. Ractopamine improved ADG, efficiency of gain, carcass-adjusted ADG, and carcass-adjusted efficiency of gain (P < 0.01). For ADG, heifers demonstrated a ractopamine x protein source interaction (P < 0.05); heifers not fed ractopamine had greater ADG when fed ESBM than when fed urea, whereas for heifers fed ractopamine there were no differences (P ≥ 0.10) among protein supplements. This interaction was not observed for carcass-adjusted ADG (P = 0.60). Final live weights (P = 0.02) and carcass weights (P = 0.01) were greater with ractopamine feeding. Carcass marbling scores and yield grades were not affected by ractopamine or protein source (P ≥ 0.39). Plasma total {alpha}-amino N and glucose concentrations decreased more from pretreatment concentrations when heifers were fed ractopamine (P < 0.05). Feeding ractopamine to heifers for 28 d before slaughter improved ADG and efficiency of gain without any large effects on carcass characteristics. The MP supply does not need to be increased from that provided by finishing diets based on steam-flaked corn with urea as the primary N supplement to allow the maximal response to ractopamine by finishing heifers.

Key Words: heifer • metabolizable protein • protein requirement • ractopamine


   INTRODUCTION
Top
 Abstract
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 IMPLICATIONS
 LITERATURE CITED
 
Diets for feedlot cattle typically contain 12 to 14% CP, with a majority of the protein as degradable intake protein (DIP; Galyean, 1996Go). Milton et al. (1997)Go showed that the addition of undegradable intake protein (UIP) to a diet based on dry-rolled corn, which increased CP from 10.6 to 12.6%, did not improve performance of finishing steers and that DIP derived from corn and urea was sufficient to maximize performance. Furthermore, increasing CP in a diet based on steam-flaked corn from 13 to 14.5% had no effect on feedlot performance, and urea was more effective in improving efficiency than was cottonseed meal (Gleghorn et al., 2004Go). Taken from these data, feedlot diets containing urea as the predominant protein supplement supply adequate amounts of DIP and are sufficient to optimize gain and efficiency of feedlot cattle.

Requirements for MP differ depending on rate and composition of gain. Thus, MP requirements could be increased by growth promotants that cause cattle to grow faster and deposit more lean tissue. Ractopamine is a ß1-adrenergic agonist recently introduced for use in cattle in the United States. Feeding ß-agonists (clenbuterol, cimaterol, zilpaterol) to cattle leads to marked alterations in metabolism that increase leanness and muscle accretion, with only moderate effects on fat deposition (Mersmann, 1998Go), which could increase MP requirements. Alternately, ß-agonists could enhance the efficiency with which cattle use MP, thereby leading to no change or even a decrease in MP requirements. Feeding ractopamine to pigs increased lean tissue deposition, but pigs fed ractopamine required no change in dietary protein to achieve optimal response to ractopamine (Mitchell et al., 1991Go; Xiao et al., 1999Go).

Our objectives were to determine the impact of feeding ractopamine to heifers for 28 d before slaughter and to determine whether or not the response of heifers to ractopamine is influenced by supplemental protein sources supplying differing amounts of MP.


   MATERIALS AND METHODS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 IMPLICATIONS
 LITERATURE CITED
 
Procedures for this study were approved by the Kansas State University Institutional Animal Care and Use Committee.

Mixed-breed medium- to large-framed heifers that were predominantly continental crosses (n = 72; 475 kg of initial BW, range 393 to 583 kg) were used in a randomized complete block design with a 2 x 3 factorial arrangement of treatments to evaluate the effects of ractopamine and supplemental protein source (supplying differing amounts of MP) on growth and carcass characteristics. Dietary treatments (Table 1Go) consisted of finishing diets based on steam-flaked corn, formulated to contain 13.7% CP, with urea, solvent soybean meal (SSBM), or expeller soybean meal (ESBM) as the primary supplemental protein source and with 0 or 200 mg of ractopamine-HCl/d (23 ppm; provided as Optaflexx, Elanco Animal Health, Indianapolis, IN). Heifers were implanted with 140 mg of trenbolone acetate and 14 mg of estradiol-17ß (Revalor H; Intervet, Millsboro, DE) 60 d before the experiment started. All heifers had been fed and housed individually for 49 d before initiation of our trial, and all were fed a common diet based on steam-flaked corn for ad libitum consumption for 27 d before initiation of our trial. Dietary treatments, which had little impact on heifer performance, had been applied between 27 and 49 d before initiation of our trial, and these prior treatments were randomly distributed among our treatments.


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  		Table 1. Composition of the diets fed to finishing heifers
 
Heifers were allotted to 1 of 12 blocks based on frame size (score of 1 to 3, each representing roughly one-third of the heifers as the largest, middle, and smallest frame sizes), body condition (2 groups, with 1 containing the thinnest one-third of the heifers and the other containing the remainder), and BW and were housed in individual pens (1.5 x 6 m). Heifers were fed individually and were given ad libitum access to their diet for 28 d, which represents the shortest feeding time for which ractopamine usage is labeled as well as the most typical feeding period for ractopamine by the feedlot industry. Heifers were fed once daily at 0900 with bunk management designed to result in empty bunks at feeding. Initial BW were measured 1 d before initiation of ractopamine feeding at 0800, and final BW were obtained at 0500 directly before heifers were shipped to a commercial abattoir.

Hot carcasses were weighed at slaughter. Percentage of KPH, 12th-rib fat thickness, marbling score, LM area, USDA yield grade (calculated), and marbling score of the carcasses were measured after a 24-h chill. Marbling scores were determined by a USDA grader.

Jugular blood samples were collected 1 d before initiation of ractopamine feeding at 0800 and 13 d later at 0800. Blood was collected into vacuum tubes (Becton Dickinson, Franklin Lakes, NJ) containing sodium heparin, immediately placed on ice, and centrifuged for 20 min at 1,000 x g to obtain plasma. Plasma samples were stored (–20°C) for later analysis of plasma urea (Marsh et al., 1965Go), total {alpha}-amino N (Palmer and Peters, 1969Go), glucose, and lactate (glucose/lactate auto-analyzer; YSI 2300 STAT Plus, YSI Inc., Yellow Springs, OH).

Data were analyzed by using the MIXED procedure of SAS (Release 8.1, SAS Inst. Inc., Cary, NC). The model included the effects of protein source, ractopamine, and ractopamine x protein. Block was included as a random effect. For final BW and HCW, initial BW was included as a covariate. Treatment means were computed with the LSMEANS option. When an F-test for protein or for the ractopamine x protein interaction was significant (P < 0.05), individual treatment means were separated with pairwise t-tests among all means.

Diets were evaluated with level 1 of the NRC (1996)Go model and with the Cornell Net Carbohydrate and Protein System 5.0 (CNCPS, Cornell University, Ithaca, NY; Fox et al., 1992Go; Russell et al., 1992Go; Sniffen et al., 1992Go). Inputs included DMI of 8.7 kg/d and BW of 480 kg (averages for our experiment). For the NRC (1996)Go evaluation, ESBM was assumed to contain 48% CP, with 45% of CP being DIP; SSBM was assumed to contain 55% CP, with 65% of CP being DIP. For both models, concentrated separator by-product was considered similar to beet molasses, with 20% ash and 17.6% CP, with 100% of CP being DIP. Nutrient contents of other feedstuffs were as specified by the models’ databases.


   RESULTS AND DISCUSSION
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
IMPLICATIONS
 LITERATURE CITED
 
Performance data for heifers are presented in Table 2Go. Dry matter intake was not affected by ractopamine or protein source (P ≥ 0.13). Average daily gain was increased 18% by ractopamine (P = 0.02). When daily gains were calculated from carcass weights, ractopamine increased ADG by 25% (P = 0.008). Basing daily gains on carcass weights removes potential differences in gut fill, perhaps allowing a more accurate evaluation of heifer growth. Similarly, Carroll et al. (1990)Go demonstrated an 11% increase in ADG in response to feeding 20 ppm ractopamine for 38 to 45 d in feedlot steers. Preston et al. (1990)Go reported a 25% increase in ADG when feeding 20 ppm ractopamine to finishing steers for 46 d.


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  		Table 2. Effect of ractopamine and dietary protein source on growth and carcass characteristics of finishing heifers
 
Ractopamine increased (P = 0.002) efficiency of gain by 17%. Gain efficiencies calculated on the basis of carcass-adjusted BW were increased (P = 0.003) 26% by feeding ractopamine. Anderson et al. (1989)Go, Carroll et al. (1990)Go, and Preston et al. (1990)Go showed similar improvements in gain efficiencies due to ractopamine. Schroeder et al. (2003a)Go and Laudert et al. (2004)Go demonstrated a 16% improvement in feed efficiency when feedlot steers were fed 20 ppm ractopamine for 28 or 42 d. In a similar study, Schroeder et al. (2003b)Go reported a 14% improvement in feed efficiencies for feedlot heifers.

Carcass-adjusted daily gain, efficiency of gain, and carcass-adjusted efficiency of gain were not affected (P ≥ 0.52) by protein source. For ADG, heifers demonstrated a ractopamine x protein source interaction (P = 0.04). Heifers not fed ractopamine had greater ADG when fed ESBM than when fed urea, whereas heifers fed ractopamine showed no response (P ≥ 0.10) to protein supplementation. The response of heifers not receiving ractopamine suggests that performance was improved by increasing MP supply. However, heifers receiving ractopamine showed numerical decreases in daily gains as MP was increased. For heifers fed ractopamine, carcass-adjusted gains were numerically greater when they were fed the urea-supplemented diets, suggesting that there was no benefit to increasing the MP supply. Feedlot steers implanted with trenbolone acetate and estradiol-17ß had gain efficiencies that were numerically greater when the diet was supplemented with a combination of 75% feather meal: 25% soybean meal rather than 50:50 or 25:75 of feather meal and soybean meal (Cecava and Hancock, 1994Go). This suggested that aggressively implanted finishing steers might respond to increases in MP supply, but this response was not observed in our implanted heifers fed ractopamine. Anderson et al. (1989)Go observed no interaction between dietary ractopamine-HCl (0 to 80 ppm) and dietary protein concentration (11 or 14% CP) for performance of finishing steers over a 56-d period, suggesting that, similar to our results, ractopamine feeding did not markedly increase the MP requirement.

Protein source did not alter HCW (P = 0.65), but HCW was increased (P = 0.008) 6.9 kg when heifers were fed ractopamine. Schroeder et al. (2003b)Go observed a significant increase in HCW from heifers fed ractopamine, with an improvement of 2.9 kg. Carroll et al. (1990)Go, Schroeder et al. (2003a)Go, and Laudert et al. (2004)Go showed similar increases in HCW of feedlot steers fed 20 ppm ractopamine (6.4, 5.6, and 4.9 kg, respectively).

Final BW were 8.3 kg greater when heifers were fed ractopamine (P < 0.02). Heifers also demonstrated a ractopamine protein source interaction (P = 0.04) for final BW that was similar to that observed for ADG. Heifers not fed ractopamine had greater final BW when fed ESBM than when fed urea, whereas heifers fed ractopamine showed no response (P ≥ 0.10) to protein supplementation. Data reported from previous studies showed a 7.2-kg (Schroeder et al., 2003aGo) and 6.7-kg (Laudert et al., 2004Go) increase in final BW of feedlot steers and a 6.6-kg (Schroeder et al., 2003bGo) increase in feedlot heifers in response to 20 ppm ractopamine fed for 28 or 42 d.

Carcass data for heifers are shown in Table 2Go. Dressing percent, LM area, 12th-rib fat thickness, USDA yield grade, and marbling score were not affected by ractopamine or protein source (P ≥ 0.07). This is in agreement with Schroeder et al. (2003b)Go, who showed no response to ractopamine in feedlot heifers for dressing percent, 12th-rib fat thickness, LM area, marbling score, USDA yield grade, or USDA quality grade. The KPH of control heifers was least when SSBM was fed, whereas the KPH of heifers fed ractopamine was least when ESBM was fed (ractopamine x protein source interaction, P = 0.002). These differences are unlikely to be of biological importance. In our experiment, modest responses to the treatments for carcass characteristics would not be detected due to the number of heifers studied. Previous research showed no effect of ractopamine on KPH in feedlot steers (Schroeder et al., 2003aGo; Laudert et al., 2004Go) or feedlot heifers (Schroeder et al., 2003bGo). Carroll et al. (1990)Go and Schroeder et al. (2003a)Go reported that ractopamine fed to feedlot steers significantly increased dressing percent and LM area. These same responses in dressing percent and LM area were also observed in feedlot heifers (Laudert et al., 2004Go). With ractopamine feeding, lean muscle deposition increases, potentially leading to increases in LM area (Mersmann, 1998Go).

Plasma metabolites are presented in Table 3Go. Because differences among treatments existed before the experiment, changes in plasma metabolite concentrations after implementation of treatments were calculated and considered most appropriate for assessing treatment responses. The decrease in plasma glucose concentrations from before the experiment to d 13 was greater when heifers were fed ractopamine (P < 0.05). Changes from pretreatment concentrations of plasma lactate were not different (P ≥ 0.14) among treatments.


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  		Table 3. Effect of ractopamine and dietary protein source on plasma metabolites in finishing heifers
 
Treatment effects on changes from pretreatment concentrations were not observed for plasma urea (Table 3Go). Shifting dietary N from DIP to UIP might be expected to decrease plasma urea concentrations because less ammonia is absorbed across the gut. Urea production, however, is related to N intake above anabolic requirements, and our diets were isonitrogenous and yielded no effect on heifer growth. The effects of ractopamine on plasma urea, if any, may have been difficult to detect because the increase in N deposition in response to ractopamine was only about 5% of N intake (the 0.29 kg/d increase in ADG would correspond to increases in deposition of N of about 9 g/d). Total {alpha}-amino N could be expected to increase with increases in MP supply, but differences among diets were not observed. Decreases from pretreatment concentrations for plasma total {alpha}-amino N were greater for heifers fed ractopamine (P = 0.004), suggesting that more AA were taken up by tissues in response to this growth promotant.

Our diets were formulated to provide different amounts of MP to the heifers, with the urea diet providing the least and the ESBM diet providing the most. Metabolizable protein balances for the urea, SSBM, and ESBM diets were calculated with models from level 1 of the NRC (1996)Go and the CNCPS, and the results are shown in Table 4Go. Both models predicted energy allowable gains that were less than those observed. This is likely due to our heifers being individually fed in small pens, thereby minimizing activity of the heifers and reducing their nonproductive energy usage. Because gains for the heifers were underpredicted, MP requirements would also be underpredicted, and MP balance would be overpredicted. Prediction of MP balance by the NRC model showed that the urea diet was slightly deficient, but MP balances were positive with SSBM and more so with ESBM. Predictions from the CNCPS showed all diets to be in a positive MP balance. Lesser MP supply from the urea diet than from the SSBM and ESBM diets was also predicted. The more positive MP balances predicted by the CNCPS than by the NRC can be attributed to 1) the lesser gain predicted by CNCPS, which results in lesser MP requirements, and 2) greater predictions of MP supply by the CNCPS, which resulted from greater predictions of microbial protein supply. The NRC and CNCPS predicted the greatest ruminal N balance for the urea diet and the least for the ESBM diet, although the NRC predicted a 121 g/d difference among diets and the CNCPS predicted only a 19 g/d difference. Because ruminal N balance was predicted by the CNCPS to be negative for the ESBM diet, MP balance predicted by the CNCPS did not differ much between the SSBM diet and the ESBM diet. In contrast, the NRC model, which predicted positive ruminal N balances for all diets, predicted a greater difference between the SSBM and ESBM diets. Predictions for MP requirements are based on heifers fed no ractopamine; MP requirements for heifers fed ractopamine would be greater if the efficiency of MP use for tissue gain is not affected by the ractopamine.


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  		Table 4. Metabolizable protein balances of heifers fed diets with different protein supplements
 
When not fed ractopamine, finishing steers generally do not respond to increases in MP supply above that provided by typical finishing diets containing little UIP supplementation (Milton et al., 1997Go; Gleghorn et al., 2004Go). McCoy et al. (1998)Go demonstrated that steer calves fed a diet based on dry-rolled corn or dry-rolled corn and wet corn gluten feed for 168 d showed no improvements in performance when supplemented with UIP (80% feather meal and 20% blood meal). Barajas and Zinn (1998)Go observed minimal improvements in performance of heifers fed diets based on dry-rolled or steam-flaked corn when they were supplemented with cottonseed meal (10% of diet DM) during a 110-d finishing trial. Although research evaluating the effect of MP supply on performance in finishing heifers is limited, MP requirements of heifers are predicted to be less than those of steers because of the lesser amount of protein deposited (NRC, 1996Go).

It was unknown whether heifers fed ractopamine might respond to increases in MP supply because ractopamine increases growth rate and lean tissue deposition, which in turn might lead to a greater need for MP. Our study demonstrates that heifers fed ractopamine did not respond to increased MP supply. Thus, provision of adequate DIP to the diet by the addition of urea seems sufficient to meet the MP needs of ractopaminefed heifers consuming typical finishing diets based on steam-flaked corn.


   IMPLICATIONS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 IMPLICATIONS
LITERATURE CITED
 
Our results show that ractopamine fed for 28 d before slaughter to heifers implanted with trenbolone acetate/ estradiol can improve daily gains and feed efficiency with little impact on carcass characteristics. Our data do not support the concept that dietary metabolizable protein supply needs to be increased above that present in feedlot diets based on steam-flaked corn with urea as the primary nitrogen supplement to maximize the response of finishing heifers to ractopamine.


   Footnotes
 
1 Contribution No. 06-85-J from the Kansas Agricultural Experiment Station. Back

2 Corresponding author: etitgeme{at}ksu.edu

Received for publication October 21, 2005. Accepted for publication May 22, 2006.


   LITERATURE CITED
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 IMPLICATIONS
 LITERATURE CITED
 


Anderson, D. B., E. L. Veenhuizen, J. F. Wagner, M. I. Wray, and D. H. Mowrey. 1989. The effect of ractopamine hydrochloride on nitrogen retention, growth performance, and carcass composition of beef cattle. J. Anim. Sci. 67(Suppl. 1):222. (Abstr.)

Barajas, R., and R. A. Zinn. 1998. The feeding value of dry-rolled and steam-flaked corn in finishing diets for feedlot cattle: Influence of protein supplementation. J. Anim. Sci. 76:1744–1752.[Abstract/Free Full Text]

Carroll, L. H., S. B. Laudert, J. C. Parrott, D. H. Mowrey, D. R. White, D. B. Anderson, and J. K. Merrill. 1990. Ractopamine HCl dose titration in feedlot steers: Performance and carcass traits. J. Anim. Sci. 68(Suppl. 1):294. (Abstr.)

Cecava, M. J., and D. L. Hancock. 1994. Effects of anabolic steroids on nitrogen metabolism and growth of steers fed corn silage and corn-based diets supplemented with urea or combinations of soybean meal and feather meal. J. Anim. Sci. 72:515–522.[Abstract]

Fox, D. G., C. J. Sniffen, J. D. O’Connor, J. B. Russell, and P. J. Van Soest. 1992. A net carbohydrate and protein system for evaluating cattle diets: III. Cattle requirements and diet adequacy. J. Anim. Sci. 70:3578–3596.[Abstract]

Galyean, M. L. 1996. Protein levels in beef cattle finishing diets: Industry application, university research, and system results. J. Anim. Sci. 74:2860–2870.[Abstract]

Gleghorn, J. F., N. A. Elam, M. L. Galyean, G. C. Duff, N. A. Cole, and J. D. Rivera. 2004. Effects of crude protein concentration and degradability on performance, carcass characteristics, and serum urea nitrogen concentrations in finishing beef steers. J. Anim. Sci. 82:2705–2717.[Abstract/Free Full Text]

Laudert, S. B., G. J. Vogel, A. L. Schroeder, W. J. Platter, and M. T. Van Koevering. 2004. The effect of Optaflexx on growth performance and carcass traits of steers. Optaflexx Exchange No. 4. Elanco Anim. Health, Greenfield, IN.

Marsh, W. H., B. Fingerhut, and H. Miller. 1965. Automated and manual direct methods for the determination of blood urea. Clin. Chem. 11:624–627.[Abstract]

McCoy, R. A., R. A. Stock, T. J. Klopfenstein, D. H. Shain, and M. J. Klemesrud. 1998. Effect of energy source and escape protein on receiving and finishing performance and health of calves. J. Anim. Sci. 76:1488–1498.[Abstract/Free Full Text]

Mersmann, H. J. 1998. Overview of the effects of ß-adrenergic receptor agonists on animal growth including mechanisms of action. J. Anim. Sci. 76:160–172.[Abstract/Free Full Text]

Milton, C. T., R. T. Brandt, Jr., E. C. Titgemeyer, and G. L. Kuhl. 1997. Effect of degradable and escape protein and roughage type on performance and carcass characteristics of finishing yearling steers. J. Anim. Sci. 75:2834–2840.[Abstract/Free Full Text]

Mitchell, A. D., M. B. Solomon, and N. C. Steele. 1991. Influence of level of dietary protein or energy on effects of ractopamine in finishing swine. J. Anim. Sci. 69:4487–4495.[Abstract]

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Palmer, D. W., and T. Peters, Jr. 1969. Automated determination of free amino groups in serum and plasma using 2, 4, 6-trinitrobenzene sulfonate. Clin. Chem. 15:891–901.[Abstract]

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