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ANIMAL NUTRITION |
Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-1600
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Key Words: ractopamine-hydrogen chloride • beef cattle • heifer
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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; Byrem et al., 1998). β-Agonists such as clenbuterol, cimaterol, and L-644,969 have elicited increased ADG, improved feed efficiency, and larger LM area when fed to beef cattle (Mersmann, 1998). Walker et al. (2006) observed improved ADG, G:F, and HCW in heifers fed ractopamine compared with those not fed ractopamine.
The use of ractopamine-HCl as a means of increasing growth and lean tissue gain in swine has been well documented (Olayiwola et al., 1990; Watkins et al., 1990; See et al., 2004). Generally, the increased growth associated with this agent manifests as improved feed conversion and weight gain. However, existing data on the effects of ractopamine-HCl on growth performance and carcass characteristics for finishing feedlot heifers need to be refined. Data released by Elanco Animal Health imply that differences by sex in response to ractopamine may exist; therefore, appropriate strategies for the administration of this compound must be defined for heifers independent of steers. Further research is needed to determine the most appropriate strategies for dosage or duration of ractopamine administration to achieve optimal growth response with heifers.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Exp. 1
Crossbred heifers (n = 302, 479 kg of initial BW) were used in a study with a randomized complete block design. Upon arrival, heifers were offered ad libitum access to long-stem prairie hay and water before processing. Twenty-four hours after arrival, the heifers were processed through the working facility. At arrival, individual BW were determined, and the heifers were given an internal/external parasiticide. Cattle observed with signs of lameness, respiratory dysfunction, or other illness at initial processing were removed from the experiment. Cattle were implanted with Revalor-H (140 mg of trenbolone acetate, 14 mg of estradiol; Intervet Inc., Millsboro, DE) 90 d before slaughter and gradually adapted to a diet comprised of approximately 80% steam-flaked corn and 6% alfalfa hay (DM basis, Table 1). Heifers were adapted to finishing diets for approximately 21 d and fed common diets for 54 d before initiation of the ractopamine-HCl (Optaflexx, Elanco Animal Health, Greenfield, IN) feeding. On d 0 of the Optaflexx period, heifers were stratified by initial BW and randomly allotted, within strata, to treatment in 24 dirt-surfaced feeding pens (10.4 x 26.8 m) containing 12 to 13 heifers each. Heifers receiving the Optaflexx treatment were fed diets formulated to provide 200 mg of ractopamine per heifer daily, beginning 28 d before slaughter. Finishing diets were fed once daily at approximately 1500 h. Dry matter intake, ADG, and efficiency of gain were determined for each pen of cattle. Final BW was calculated by dividing HCW by a common dressing percentage of 63.5%.
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After the carcass chill, loins were obtained from 3 animals randomly selected from each pen. The loins were allowed to age for 14 d in Cryovac packages (Cryovac Food Packaging, Duncan, SC) at 0 ± 1°C. After aging, the loins were cut into 2.54-cm-thick steaks. Loin steaks were analyzed for fatty acid composition (data not shown), purge loss measurements during a 7-d display period, and Warner-Bratzler shear force values. One steak from each loin was reserved for measurement of L*, a*, and b* colorimetric values during a 7-d retail display. After packaging, steaks were placed under retail display lighting at 1,614 lx, light intensity 40W Deluxe Warm White (Philips Lighting Company, Somerset, NJ). Temperature was maintained at 1.6°C throughout the display period. Measurements were taken on d 0, 2, 4, and 6 of the display period using spectrophotometry (HunterLab Miniscan XE Plus, model 45/0 LAV, 2.54-cm-diam. aperture, 10° standard observer; Hunter Associates Laboratory Inc., Reston, VA). Purge loss during retail display was measured by weighing each steak before and after the display period. A second steak sample was cooked using a DFG 102 Blodgett oven (G. S. Blodgett Inc., Burlington, VT) for evaluation of meat tenderness and fatty acid profile of cooked samples. During the cooking period, internal temperatures were monitored using 30-gauge copper/constantine wire (Omega Engineering, Stamford, CT) connected to a Doric Minitrend 205 monitor (VAS Engineering, San Francisco, CA). Steaks were turned at an internal temperature of 33°C and removed from the oven at 70°C. Cooked steaks were allowed to cool and were weighed for determination of weight loss during cooking. Samples were then refrigerated at 3.3°C for 24 h. After the 24-h refrigeration period, six to eight 1.3-cm cores were removed from the steaks parallel to the fiber orientation for determination of tenderness. Shear force values were obtained using a model 4201 (Instron, Canton, MA) testing machine with a 50-kg compression load cell at a crosshead speed of 250 mm/min. Core samples also were analyzed by gas chromatography (Sukhija and Palmquist, 1988) using a Shimadzu GC-17A chromatograph (Columbia, MD) equipped with a flame ionization detector and a Supelco SP 2560 fused silica capillary column (Bellefonte, PA). High-purity helium was the carrier gas, with a hydrocarbon trap and carrier gas purifier at a 60 mL/min flow rate and 20 cm/s velocity, and a split ratio of 48:1, with a sample injection volume of 1.0 µL. Initial temperature was 140°C for 5 min, followed by an increase of 4°C/min to a final temperature of 240°C for 15 min. Injector and detector temperatures both were set at 260°C.
Exp. 2
In Exp. 2, nonimplanted crossbred heifers (n = 281, 477 kg of initial BW) were fed steam-flaked corn-based diets (Table 1
) and individually weighed on d –3. On d 0, heifers were stratified by individual BW and randomly assigned within strata to 1 of 5 experimental treatments. Heifers were housed in 50 partially covered, concrete-surfaced pens (4.3 x 8.5 m, 5 to 6 animals/pen, 10 pens/treatment) and assigned to treatments consisting of no ractopamine (control), 200 mg of ractopamine per heifer daily for 28 d before slaughter (200 x 28), 200 mg of ractopamine per heifer daily for 42 d before slaughter (200 x 42), 300 mg of ractopamine per heifer daily for 28 d before slaughter (300 x 28), and 100 mg for 14 d, 200 mg for 14 d, and 300 mg per heifer daily of ractopamine for the 14 d before slaughter (step-up).
Pens of cattle were weighed using a platform scale on d 0 and immediately before being transported to a commercial abattoir for slaughter. All cattle were allowed ad libitum access to a common finishing diet. The entire daily ration was delivered at approximately 1300 h each day. Unconsumed feed (orts) was weighed when in excess, and feed consumption was adjusted accordingly. Dry matter intake, ADG, and G:F were determined for each pen of cattle. To minimize the effects of gut fill, initial carcass weights (on d 0 of Optaflexx feeding) were used for performance calculations and were estimated by multiplying initial live BW by an assumed initial dressing percentage of 62%. Carcass data for Exp. 2 were collected as in Exp. 1.
Statistical Analysis
Experiments 1 and 2 were arranged as a randomized complete block, with pen as the experimental unit. Assumptions of normality were tested in Exp. 1 and 2 using the UNIVARIATE procedure (SAS Inst. Inc., Cary, NC). The GLM procedure of SAS was used to statistically analyze performance and carcass characteristics in Exp. 1 and 2. The effects of treatment and block were included in the model statement for each experiment. In Exp. 2, preplanned contrasts were performed for control vs. Optaflexx, 200 mg vs. 300 mg, 28 vs. 42 d, and 300 mg vs. all treatments. In both experiments, least squares means were generated and separated using the PDIFF option of SAS for significant main effects. For Exp. 1, data corresponding to L*, a*, and b* measurements were analyzed as repeated measures over the 7-d retail display period. The protected F-test was used to determine overall significance where P-values of 
0.05 were considered significant, whereas P-values between 0.05 and 0.10 were considered to indicate tendencies.
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Dry matter intake (10.1 and 9.86 kg/d for control and OPT, respectively) and ADG (1.69 and 1.80 kg for control and OPT, respectively) were not different between heifers fed Optaflexx for 28 d and control (P > 0.05, Table 2). Walker et al. (2006) also reported no difference in DMI for heifers fed Optaflexx but did observe increased carcass-adjusted ADG. However, in Exp. 1, there was a tendency for heifers fed Optaflexx to have improved G:F compared with heifers fed no Optaflexx (0.183 vs.0.160 kg/kg, respectively, P = 0.06, Table 2). In support of this observation, Gruber et al. (2007) and Winterholler et al. (2007) observed increased G:F in steers fed Optaflexx for 28 d before slaughter.
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). These results contrast with Walker et al. (2006) and Gruber et al. (2007), in which administration of ractopamine 28 d before slaughter to heifers and steers, respectively, resulted in increases in carcass weight. No treatment differences were observed for s.c. fat thickness, LM area, KPH, or marbling score among treatments (P > 0.31, Table 3). This would imply there was no increase in lean tissue accretion through feeding Optaflexx compared with control. Similar fat thickness and marbling scores suggest that Optaflexx had no effect on altering i.m. or s.c. lipid deposition. These observations are further reflected in the similarity among treatments for USDA yield and quality grades (P > 0.19, Table 3). These results are consistent with the findings of Gruber et al. (2007), in which steers fed ractopamine were not different with respect to average yield grade and quality grade; however, the steers fed ractopamine tended to have decreased marbling scores.
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). These observations have been made in calves fed the β-agonist clenbuterol and in finishing bulls fed cimaterol (Fiems et al., 1990). β-Agonistic effects have been variable with respect to postmortem muscle pH, water-holding capacity, or glycogen content of steaks. Warner-Bratzler shear force values obtained from cooked longissimus steak core samples in the current study were not different for heifers fed control and Optaflexx diets (P > 0.41, Table 4). Schiavetta et al. (1990) and Wheeler and Koohmaraie (1992) observed lower Warner-Bratzler shear force values in control animals compared with those fed the β-agonists clenbuterol and L-644,969, respectively. Furthermore, similar increases in LM toughness were observed with wethers fed the β-agonist L-644,969 (Pringle et al., 1993). When ractopamine was fed to finishing steers at 300 mg/steer daily, meat samples from those fed ractopamine had significantly greater shear force than steers that were not fed ractopamine (Avendano-Reyes et al., 2006). It should be noted that Avendano-Reyes et al. (2006) did report greater HCW and carcass yield in steers fed ractopamine compared with control steers, and in the current study, these same differences were not detected. However, given that in Exp. 1 there were no differences in LM area between treatments, it would be logical to expect that there would be no effect of Optaflexx on muscle tenderness. Results from Elanco’s Optaflexx exchange suggest that feeding Optaflexx to heifers has no effect on meat attributes. To date, few studies have investigated the effect of feeding beef heifers ractopamine and the effects on Warner-Bratzler shear force. Overall, feeding some β-agonists elicit much greater responses in tissue gain in cattle, and the potency of the β-agonist fed also affects meat tenderness. In instances in which there is a large tissue gain, meat tenderness may be more negatively affected than when less of a growth response is observed. Ractopamine is a less potent β-agonist than others such as clenbuterol and zilpaterol, and therefore, decreases in meat tenderness may not be significant. In studies in which there was a large response in growth to β-agonist feeding, it appears that there is more of an effect on muscle tenderness and meat quality.
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). These results differ from those reported by Geesink et al. (1993), in which clenbuterol-fed calves had significantly greater L* values in LM and semimembrinosis muscle samples. The authors explained that L* (a colorimetric measure of lightness in muscle) is associated with water-holding/binding capacity in muscle. The authors speculated that the increase in lightness may be correlated with a reduction in water-holding capacity due to protein denaturation in the muscle associated with pH drop from electrical stimulation. Reduced water-holding capacity may ultimately lead to greater Warner-Bratzler shear force values. Following this assumption, the lack of difference in L* values between the control and Optaflexx-fed heifers could potentially be attributed to the similarities in purge loss during retail display and cooking loss between treatments (P > 0.58, Table 4). In support of the latter observation, Avendano-Reyes et al. (2006) observed no difference in meat color during display from steers fed Optaflexx and steers not fed Optaflexx. Exp. 2
The dose and duration of Optaflexx feeding is summarized in Table 5. All values represented in tables were calculated based on the entire 4-d period. Dry matter intake was reduced (P < 0.05, Table 6) by 6.5% in 300 x 28 compared with control, 200 x 28, and 200 x 42 but not different from step-up. In addition, heifers fed Optaflexx at 200 mg had greater DMI compared with heifers fed Optaflexx at 300 mg (P < 0.05, Table 6). Calculated carcass weight gains were not different among treatments (P = 0.18, Table 6). However, carcass ADG was significantly improved in Optaflexx treatments compared with control (P < 0.05, Table 6). Carcass G:F was not different among treatments (P = 0.18, Table 6). Again, pens of heifers receiving Optaflexx had improved carcass G:F compared with control pens (P < 0.05, Table 6). The control treatment was not different with respect to carcass G:F for 200 x 28 and 300 x 28 treatments. This would suggest that duration of administration of Optaflexx may play a larger role in improving performance rather than dosage of the compound. This would contrast with observations made in rats fed cimaterol when increased BW gain was observed up to d 14 of the study, but rats exhibited little or no growth beyond that period (Kim et al., 1992). In support of Kim et al. (1992), Bridge et al. (1998) showed significantly reduced numbers of agonist-binding sites after 14 d of cimaterol treatment in bovine fetal skeletal muscle tissue cultures and would suggest there may be some receptor refractory period after a period of initial β-agonist treatment. Pringle et al. (1993) fed wethers the β-agonist L-644,969 and noted increased ADG during the first 2 wk of the study period, after which gains for treated animals returned to control values. Based on these observations, there may be some benefit in escalating the dosage of β-agonist feeding to compensate for decreased receptor binding, receptor density, or both. In contrast, Wheeler and Koohmaraie (1992) observed increased ADG during wk 3, 5, and 6 after initiation of feeding the β-agonist L-644,969 compared with untreated steers. This, conversely, would suggest that there is an adaptation period for the upregulation of receptors for the compound to elicit effects. However, in the current study, increasing dosage from 200 to 300 mg/d was not beneficial compared with increasing the length of Optaflexx feeding from 28 to 42 d.
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). However, there was a tendency for pens of heifers fed Optaflexx to have increased HCW compared with control pens (P < 0.10, Table 6). Similar results were observed by Walker et al. (2006), Gruber et al. (2007), and Winterholler et al. (2007) in which addition of ractopamine increased carcass weight gain in steers and heifers. No differences among treatments existed for dressing percentage, yield grade, marbling score, LM area, KPH, incidence of liver abscess, or quality grade (P > 0.27, Table 7). Heifers fed Optaflexx tended to have greater 12th rib s.c. fat compared with control (P = 0.06, Table 7). Therefore, feeding Optaflexx may increase carcass gain and efficiency without depressing i.m. and s.c. fat deposition. Miller et al. (1988) noted improved feed efficiency in 1 group of heifers fed 10 mg per animal clenbuterol for 50 d before slaughter. In this study, feeding clenbuterol decreased adjusted fat thickness, KPH, marbling scores, and yield and quality grades. Also, β-agonist-fed heifers in the Miller et al. (1988) study exhibited significantly greater Warner-Bratzler shear force values. The results of Miller et al. (1988) were not supported by carcass characteristics reported for the current study. In further support, Avendano-Reyes et al. (2006) reported increased shear force in samples from steers fed 300 mg/d of ractopamine compared with control steers. It must be noted that a number of the results from previous studies were derived from feeding β-agonists to steers, whereas the current experiments utilized heifers. In recent studies regarding ractopamine addition to steers and heifers (Walker et al., 2006; Gruber et al., 2007; Winterholler et al., 2007), inclusion of Optaflexx improved feed efficiency and carcass gain in both steers and heifers, but it is difficult to quantify expected differences between sexes.
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Footnotes |
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2 Corresponding author: jdrouill{at}ksu.edu
Received for publication February 21, 2007. Accepted for publication January 9, 2008.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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