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ANIMAL PRODUCTS |
,1
* Department of Animal Science, Oklahoma State University, Stillwater 74078;
and
Intervet Inc., Millsboro, DE 19966; and
Department of Animal and Food Science, Texas Tech University, Lubbock 79409
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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0.008) sub-primal yields of shoulder clod, chuck tender, knuckle, top round, outside round, eye of the round, strip loin, top sirloin butt, bottom sirloin butt ball tip, full tenderloin, and flank steak than steers not fed zilpaterol. In addition, zilpaterol hydrochloride treatment decreased (P = 0.002) trimmable fat. Zilpaterol hydrochloride increased (P 0.006) estimated carcass protein and moisture and decreased (P 0.007) estimated carcass and LM fat percentage. For LM WBSF there was a zilpaterol hydrochloride x postmortem aging interaction (P < 0.01). The β2-adrenergic agonist increased (P = 0.001) LM WBSF at 7, 14, and 21 d postmortem and decreased (P < 0.001) trained sensory-panel juiciness, tenderness, and flavor intensity of LM steaks aged for 14 d. A consumer sensory panel also found LM steaks from zilpaterol-fed steers were (P = 0.03) less tender than steaks from steers not fed zilpaterol; however, tenderness acceptability and overall acceptability were not affected (P 
0.26). For the main effect of monensin and tylosin, withdrawal of monensin and tylosin decreased (P = 0.01) consumer juiciness scores, although other yield and compositional measurements were not affected (P 0.07). Zilpaterol is a strong repartitioning agent that increases meat yield through increased protein and decreased fat deposition.
Key Words: β-adrenergic agonist • beef cattle • carcasses cutability • tenderness • yield • zilpaterol hydrochloride
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Moloney et al., 1990; Chikhou et al., 1993). Zilpaterol was recently approved in the United States by the Center for Veterinary Medicine in 2006 (FDA, 2006); therefore, little has been reported on this new repartitioning agent. Plascencia et al. (1999) is the only report to date to indicate beef carcass cutability was increased by zilpaterol.
Other βAA such as clenbuterol, L-644,969, and cimaterol have been shown to have negative effects on beef shear force (Miller et al., 1988; Boucqué et al., 1994; Moloney et al., 1994). Studies conducted with zilpaterol on cattle in the Republic of South Africa have shown that zilpaterol may increase beef LM Warner-Bratzler shear force (WBSF) and decrease sensory tenderness scores (Strydom et al., 1998; Strydom and Nel, 1999).
Monensin (Rumensin, Elanco Animal Health, Indianapolis, IN) and tylosin (Tylan, Elanco Animal Health) are commonly fed in combination to finishing cattle. In a review of 228 trials, Goodrich et al. (1984) found that carcass characteristics were not significantly influenced by monensin. However, Depenbusch et al. (2008) reported monensin and the combination of monensin and tylosin reduced carcass LM area. Thus, the use of monensin and tylosin may affect carcass and cutability factors, whereas effects on beef palatability have not been investigated. At the time of approval of zilpaterol in the United States, a cross-clearance for combined use with monensin and tylosin did not exist. The objective of this experiment was to determine effects of feeding zilpaterol with and without monensin and tylosin on carcass cutability, meat quality, and palatability.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Cattle
A feedlot experiment was conducted using 3,757 steers to evaluate the effects of feeding zilpaterol hydrochloride with or without monensin and tylosin on feedlot performance and carcass characteristics (Montgomery et al., 2009b). The number of steers assigned to each pen averaged 94, which provided approximately 24 cm of bunk space and 13.9 m2 of pen space per animal. Cattle were allotted to 4 pens per block for a total of 10 blocks per treatment using a total of 40 pens. Because of the large number of steers in the study and the fact that the animals arrived at the site in 2 large groups, the steers were blocked based on arrival, processing, and allotment dates. There 4 treatments were arranged as a 2 (zilpaterol hydrochloride; 0.0 or 8.3 mg/kg, DM basis) x 2 (monensin and tylosin; 0.0 and 0.0 or 33.1 and 12.2 mg/kg, respectively, DM basis) factorial. A concentrate finishing diet containing monensin and tylosin was fed through d 130 or 125 of the experiment for the first and last 5 blocks, respectively. Feeding of the treatment diets commenced on d 131 or 126 for the first and last 5 blocks, respectively. Zilpaterol, monensin, and tylosin were supplemented at 8.3, 33.1, and 12.2 mg/kg (DM basis), respectively. Zilpaterol hydrochloride was included in the diet for 30 d at the end of the finishing period, and withdrawn from the diet for the last 5 d cattle were on feed. Monensin and tylosin remained withdrawn to slaughter for steers on the monensin/tylosin withdrawal treatments. After the withdrawal period, cattle were weighed and shipped to slaughter.
Slaughter
At slaughter, HCW was collected. Five carcasses (n = 200) meeting the selection criteria of a weight range between 305 and 421 kg of HCW and free from bruises, major trim loss, or other slaughter defects were selected randomly from each pen. Selection of carcasses within the described weight range was conducted to reduce derived effects between cutability measurement and carcass weight by selecting carcasses of very similar weight when carcasses were from the same block of pens. Thus there were no differences (P = 0.38) in the treatment carcass weights of the carcasses selected for the cutability study. A 30-g LM sample was removed from each carcass at 30 min postmortem for calpastatin and calpain determination per sample according to the procedures of Koohmaraie (1990). Carcass LM pH was measured on the right carcass side with a model 230A Orion temperature-compensated pH meter (Orion Research, Cambridge, MA) between the 10th and 11th ribs at 3 h postmortem. Carcass LM temperature also was measured on the right carcass side at 3 h postmortem using a Hantover model TM99A-H digital thermometer (Hantover, Atlanta, GA).
Carcasses were spray-chilled at 0°C for 36 h. After chilling, carcasses were ribbed at the 12th rib, and quality and yield grade traits were recorded (USDA, 1997). Carcass trait results for the 3,757 steer experiment were reported by Montgomery et al. (2009b). For the 200 selected carcasses, Commission Internationale de l’Eclairage (CIE) L* (muscle lightness), a* (muscle redness), b* (muscle yellowness), saturation index, and hue angle values were collected from the LM of each carcass between the 12th and 13th ribs with a Minolta Spectrophotometer Meter model CM-2002 with a D65 illuminant with a 1-cm-diameter aperture (Minolta Camera Co. Ltd., Osaka, Japan). The percentages of myoglobin, oxymyoglobin, and metmyoglobin were calculated using the specific-wavelength method described by Krzywicki (1979). Additionally, cold carcass weight was collected for each carcass side.
WBSF and Tenderness Determination
The left side of the 200 selected carcasses was fabricated into subprimals as per Institutional Meat Purchase Specifications (IMPS) as described by NAMP (1997) and USDA (1996). For each left side, strip loins (LM; IMPS # 180) and beef ribs (IMPS # 104) were collected, vacuum-packaged, transported to the Texas Tech University Meat Laboratory and stored at 2°C until further processed. Strip loins were cut into 2.54-cm thick steaks, placed in Cryovac B160 beef bags, and wet-aged (in anaerobic conditions) at 2°C. Steaks were randomly allotted to aging treatments of 7-, 14-, and 21-d postmortem for WBSF determinations and for sensory and consumer evaluations of steaks aged for 14 d. After the appropriate aging period, steaks were frozen at –20°C until further analyses.
Sensory panel evaluations and WBSF determinations were conducted according to AMSA (1995) guidelines. Steaks for sensory and WBSF determinations were thawed slowly in a 2°C cooler for 18 to 24 h and cooked on a MagiGrill belt grill (model TBG-60 electric conveyor grill; MagiKitch’n, Quakertown, PA) for 5 min and 40 s with a grill-plate thickness of 2.16 cm and grill-plate temperature of 163°C to an internal steak temperature of 71°C. Temperature was monitored with a Cooper Instruments (model SH66A, Middlefield, CT) digital meat thermometer. Individual steaks were weighed before and after cooking to determine cooking loss on WBSF steaks. Once cooked, steaks for WBSF evaluation were placed on plastic trays, covered with polyvinyl chloride film, and chilled for 18 to 24 h at 2°C. Six 1.3-cm diameter round cores were removed from each LM steak parallel to the muscle fiber orientation, and sheared once with a WBSF machine (G-R Elec. Mfg. Co., Manhattan, KS). The 6 shear force determinations for each steak were then averaged for statistical analysis.
Sensory steaks were cut into 1-cm3 cubes immediately after cooking and stored in warming pans (approximately 5 min) until served (approximately 50°C) to the trained sensory panel (AMSA, 1995). Samples were evaluated by a 6- to 8-member panel trained according to the standards of Cross et al. (1978). Steaks were evaluated for initial juiciness, sustained juiciness, initial tenderness, sustained tenderness, flavor intensity, beef flavor, and overall mouthfeel (8 = extremely juicy, tender, intense, characteristic beef flavor, and beef-like mouthfeel; 1 = extremely dry, tough, bland, uncharacteristic beef flavor, and nonbeef-like mouthfeel), as well as off flavor (5 = extremely off flavor; 1 = none).
The consumer panel subjects were selected from consumers who sampled steaks at supermarkets in Lubbock, TX, as described by Miller et al. (2001). Three stores within the same chain were targeted in different areas of the city to obtain consumers with variations in income, ethnicity, education level, sex, and age. Consumers tested samples of steaks near the meat counter. The steaks had been broiled on an Open Hearth electric broiler (Farberware, Bronx, NY) to an internal temperature of 71°C according to AMSA (1995) guidelines. Temperature was monitored with a model SH66A (Cooper Instruments) digital meat thermometer. Each consumer panelist rated two 1-cm LM sample cubes from the 4 different treatments (n = 8 cubes/panelist) for overall acceptability and tenderness acceptability (acceptable or unacceptable), overall quality, beef flavor, juiciness, and tenderness (1 = extremely dislike, uncharacteristic beef flavor, extremely dry, extremely tough; 8 = extremely like, extremely characteristic beef flavor, extremely juicy, and extremely tender). Each consumer was given samples from each of the 4 different treatments within the same block. Each steak was rated by 5 to 9 different panelists. There were a total of 564 consumers who participated in the study.
9th, 10th, 11th-Rib Dissection and Chemical Analyses
Collected beef ribs were fabricated into 9th, 10th, 11th-rib sections and then dissected to predict carcass chemical composition (Hankins and Howe, 1946). Soft tissue moisture, protein, and fat were determined in triplicate according to AOAC (1990) techniques. Tissue moisture was determined using an approximate 4-g sample and drying samples at 100°C for at least 16 h in a drying oven. Tissue protein was estimated on 1-g samples using Kjeldahl procedures. Tissue fat was determined using an approximate 4-g sample using ether extraction. Additionally, LM samples were taken from the beef ribs at the 12th rib and LM moisture, protein, and fat were determined according to AOAC (1990) techniques.
Cutability
After carcass grading, 3 carcasses (n = 120) of the original 5 carcasses from each pen were selected based on quality grade percentages that were most similar to the average quality grade for each individual pen. This was to reduce any possible confounding effect of marbling scores on yield characteristics. The right sides of the 3 carcass sides per pen were weighed and then fabricated into subprimals. The subprimals contained standard packer fat trim levels associated with commodity-boxed beef, which varied by subprimal. The ribeye roll, chuck roll, chuck tender, knuckle, eye of the round, full tenderloin, and flank steak were peeled and denuded. The blade meat and bottom sirloin subprimals were trimmed to 3 mm of trim. And the shoulder clod, brisket, short plate, top round, outside round, strip loin, and top sirloin butt were trimmed to 6 mm of trim. The subprimals collected from each fabricated carcass were the blade meat (IMPS # 109B), ribeye roll (IMPS # 112A), shoulder clod (IMPS # 114), chuck roll (IMPS # 116A), chuck tender (IMPS # 116B), brisket (IMPS # 120), outer skirt steak (IMPS # 121C), inner skirt steak (IMPS # 121D), boneless short plate (IMPS # 121G), short ribs (IMPS # 123), back ribs (IMPS # 124), boneless short ribs (IMPS # 130A), knuckle, peeled (IMPS # 167A), top round (IMPS # 169), outside round (IMPS # 171B), eye of the round (IMPS # 171C), strip loin, short-cut, boneless (IMPS # 180), top sirloin butt (IMPS # 184), bottom sirloin butt, flap (IMPS # 185A), bottom sirloin butt, ball tip (IMPS # 185B), bottom sirloin butt, tri-tip (IMPS # 185C), full tenderloin, defatted (IMPS # 189A), flank steak (IMPS # 193), neck meat, deep pectoral meat, hanging tender meat (diaphragm), rose meat (cutaneous omobrachialis), fat, bone, kidney, 90/10, 80/20, and 50/50 trimmings. Each subprimal was weighed and expressed as a percentage of the cold carcass side weight.
After fabrication into the various subprimals, muscle pH was measured on 7 different subprimals, muscle pH was measured with a model 230A Orion temperature-compensated pH meter (Orion Research) on the semitendinosus, bicep femoris, semimembranosus, LM, gluteus medius, pectoralis, and supraspinatus muscles. Additionally, the 120 strip loins (IMPS # 180) from the fabricated right carcass sides were collected for display color analysis, vacuum-packaged, transported to the Texas Tech University Meat Laboratory, and stored at 2°C until further processed.
Display Color Analysis
The 120 collected strip loins were aged in vacuum packages to 14 d postmortem, and then a steak was removed for color analysis from the anterior end of each strip loin after squaring the subprimal steak surface. Steaks for color analysis were placed on Styrofoam trays, covered with polyvinyl chloride film, and placed in a Tyler (model DGC6, Niles, MI) retail display case at 2 to 4°C for 5 d to simulate retail display conditions. The steaks were under 24-h exposure of 8 Sylvania 40 W GRO-LUX light bulbs with 2,000 lm each. The illumination intensity was 2,000 lx at the surface of the steaks. During the 5-d display period, the steaks were evaluated daily by a trained panel, consisting of at least 6 members, for beef color (8 = extremely bright cherry red; 1 = extremely dark red), color uniformity (5 = extreme 2-toning; 1 = uniform), surface discoloration (7 = 100%; 1 = 0%), and lean browning (6 = dark brown; 1 = none) according to AMSA (1991) color guidelines.
Statistical Analyses
Carcass composition, LM cooking loss, LM WBSF, and sensory traits were analyzed using a 2 (zilpaterol treatment) x 2 (monensin/tylosin treatment) factorial arrangement of treatments in a randomized complete block design, where a pen of steers was the experimental unit. There were a total of 10 blocks for each of the 4 treatments. Data for the 2 x 2 factorial were analyzed according to Steel and Torrie (1980), and least squares means were calculated using the MIXED procedure (SAS Inst. Inc., Cary, NC). When a zilpaterol x monensin/tylosin interaction was significant (P 0.05), differences among treatment means were determined using least significant difference.
For LM WBSF and display color analysis, a split-plot arrangement was used. The main plot was as described previously, whereas the subplot consisted of aging treatment for WBSF and display day for display color analyses. All interactions were tested; the error term for the main plot was zilpaterol treatment and monensin/tylosin treatment nested within pen and display day, whereas the error term for the subplot was the residual error. For consumer overall acceptance and overall tenderness acceptance data the binomial proportions of consumer acceptance were transformed using the Arcsin transformation for proportions as explained by Snedecor and Cochran (1973). Consumer acceptance data were then analyzed as explained above.
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RESULTS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The carcasses described in the present study are a subsample of the cattle and carcasses reported in Montgomery et al. (2009b). Results of monensin/tylosin and zilpaterol effects on carcass variables at grading in the present experiment are reported in Table 1. There were no effects (P 0.17) of withdrawing monensin/tylosin during the last 35 d on feed on carcass characteristics. In general, results of the effects of zilpaterol in the present experiment were similar to studies of Montgomery et al. (2009a, b) for marbling, quality grade, fat thickness, LM area, KPH fat percent, and calculated yield grade. There was a zilpaterol x monensin/tylosin interaction (P = 0.05) for quality grade. Zilpaterol decreased (P < 0.001) quality grade regardless of monensin and tylosin treatment, although withdrawal of monensin and tylosin for 30 d decreased quality grade to a greater extent (0.71 vs. 0.29 quality grade on average). Carcasses were selected to equalize weight; therefore, no treatment effect (P = 0.38) was detected for HCW. In addition, there were no effects (P 0.11) of zilpaterol treatment on skeletal, lean, and overall maturity, color score, CIE L* values, or hue angle. The CIE a* (lean redness indicator) and b* (lean yellowness indicator) values and Chroma values were decreased (P 0.04) by zilpaterol treatment. Percent myoglobin, oxymyoglobin, and metmyoglobin in the LM at grading were estimated using reflectance wavelength data. Feeding zilpaterol to steers increased (P < 0.001) LM myoglobin percentage, whereas LM oxymyoglobin and metmyoglobin percentage were decreased (P 0.008) by zilpaterol compared with carcasses from cattle not fed zilpaterol. Carcass pH and temperature measured in the LM at 3-h postmortem were not affected (P 0.33) by zilpaterol treatment; however, there was a tendency (P = 0.053) for a zilpaterol x monensin/tylosin interaction for carcass pH.
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There were no effects (P 0.10) of withdrawing monensin/tylosin during the last 35 d on feed on percentage yield of subprimal cuts (Table 2). For subprimals from the forequarter of carcasses, zilpaterol increased (P 0.02) yield of the blade meat, shoulder clod, chuck tender, neck meat, and deep pectoral meat compared with controls. Zilpaterol had a greater impact on yields of subprimals from the hindquarter of carcasses. Feeding steers zilpaterol increased (P 0.003) hindquarter sub-primal yield of the knuckle, top round, outside round, eye of the round, strip loin, top sirloin butt, bottom sirloin butt, ball tip, full tenderloin, and flank steak. In addition, 90% lean trimmings were increased (P = 0.001) by zilpaterol hydrochloride, whereas total trimmable fat was decreased (P = 0.002) by the zilpaterol. For yield of the subprimal bottom sirloin butt, tri-tip there was a zilpaterol x monensin/tylosin interaction (P = 0.024). Zilpaterol increased (P = 0.03) tri-tip subprimal yield only when monensin and tylosin were withdrawn compared with the other treatment groups.
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). In the present experiment, zilpaterol did not affect (P 0.12) yield of chuck roll, brisket, outer skirt steak, inner skirt steak, short plate, short ribs, back ribs, short ribs, hanging tender meat, rose meat, 50% trimmings, or percentage of bone. Calpain, Calpastatin Activity, Muscle pH, and Estimated Carcass Composition
There were no effects (P 0.10) of withdrawing monensin/tylosin during the last 35 d on feed on calpain, calpastatin, or muscle pH (Table 3). In addition, calpastatin, µ- and m-calpain activities were not affected (P = 0.76) by supplementation of zilpaterol hydrochloride to steers. Muscle pH measured after subprimal fabrication was decreased (P = 0.04) in the gluteus medius and supraspinatus in steers fed zilpaterol compared with steers not fed zilpaterol. For semimembranosus muscle pH there was a zilpaterol x monensin/ tylosin interaction (P = 0.04). Semimembranosus pH from steers with monensin/tylosin withdrawn the last 35 d and not supplemented with zilpaterol hydrochloride was less (P = 0.01) than semimembranosus muscle pH from steers supplemented with monensin/tylosin but not zilpaterol, and less (P = 0.04) than that from steers fed zilpaterol hydrochloride with monensin/tylosin withdrawn. However, muscle pH in the semitendinosus, biceps femoris, LM, and pectoralis profundi were not affected (P 0.15) by zilpaterol treatment compared with controls.
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. There were no zilpaterol x monensin/tylosin interactions (P 0.30) for estimated carcass composition. Withdrawing monensin/tylosin for 35 d at the end of the feeding period tended to decrease percentage estimated carcass protein (P = 0.07) and percentage LM CP (P = 0.09). No other effects (P 0.31) of withdrawing monensin/tylosin were observed. Feeding zilpaterol to steers for 30 d increased estimated carcass protein (P = 0.002) and moisture percentage (P = 0.006) and decreased (P < 0.001) estimated carcass fat percent compared with steers not fed zilpaterol. In addition, feeding zilpaterol decreased (P = 0.007) LM fat percentage, whereas LM protein and moisture percentage were not affected (P 0.12) by the zilpaterol treatment.
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Zilpaterol hydrochloride treatment resulted in a main effect (P = 0.032; Figure 1) on color scores, in which zilpaterol hydrochloride increased LM color scores throughout the 5-d display period. For color scores, the zilpaterol hydrochloride supplementation x monensin and tylosin supplementation interaction (P = 0.56) and the zilpaterol hydrochloride supplementation x monensin and tylosin supplementation x display day interaction (P = 0.83) were not significant as would be expected in most display color studies. Color scores did decline over the retail display days; the lack of a treatment x display day interaction may be attributable to the increased LM myoglobin seen at the time of carcass grading or storage conditions (2 to 4°C). Zilpaterol hydrochloride did not affect (P > 0.12) color uniformity, surface discoloration, or browning scores (data not shown). In addition, for color uniformity, surface discoloration, or browning scores, zilpaterol treatment did not interact (P 0.14) with withdrawal of monensin/ tylosin or retail display day.
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There were no effects (P 0.21) of withdrawing monensin/tylosin during the last 35 d on feed on WBSF or postmortem sensory scores (Tables 5 and 6). In addition, cooking loss percentage of LM was not affected (P = 0.13) by the βAA (Table 5). For LM WBSF there was a zilpaterol hydrochloride x postmortem aging interaction (P < 0.01). Longissimus muscle WBSF was increased (P = 0.001) by zilpaterol hydrochloride by 1.01 kg at 7-d postmortem, 0.39 kg at 14-d postmortem, and 0.25 kg at 21-d postmortem. Therefore, zilpaterol treatment effects on LM WBSF decrease with postmortem aging, and the effect of postmortem aging was significant (P < 0.001).
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). The β-agonist treatment decreased initial and sustained tenderness scores of trained panelists by approximately 6 to 7%. Zilpaterol hydrochloride treatment did not affect (P = 0.39) LM sensory panel off-flavor scores.
Demographic information for consumers participating in the study revealed that the consumers represented a wide range of consumer income, education, and ethnicity (Table 7). Sex was similarly represented, with females making up 56% of consumers surveyed. Consumer age was moderately dispersed, with individuals 20 to 29 yr of age making up the greatest percentage of the sample. A variety of ethnic backgrounds were represented in the study, with Caucasians representing the largest group. Some college or technical school training was represented by 29.2% of consumers, and the most common family income level was between $15,000 and $24,999.
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0.58) for LM consumer sensory ratings (Table 8). Feeding monensin/tylosin during the last 35 d on feed increased (P = 0.01) juiciness compared with when monensin/tylosin were withdrawn. When consumers were asked if steaks were acceptable for overall quality or tenderness, acceptability was similar (P > 0.25) between steaks from steers fed zilpaterol and steaks from steers not fed zilpaterol. However, when steaks were rated on an 8-point scale by consumers, zilpaterol treatment resulted in an approximate 4% decrease (P = 0.03) in consumer tenderness scores and tended to decrease (P = 0.07) overall quality scores by 3%. However, feeding zilpaterol did not affect (P 0.21) consumer rating of LM beef flavor, juiciness, or overall acceptability.
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DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, from which carcasses from the present experiment were subsampled, and similar to the study reported by Montgomery et al. (2009b). Zilpaterol dramatically increased carcass muscling including LM area and decreased USDA yield grades, indicative of a repartitioning agent. Because carcasses in the present experiment were selected to equalize weight, there was no treatment effect on HCW in the present study in contrast to the larger main study (Montgomery et al., 2009a) from which carcasses were selected. In the present experiment color was studied and measured in greater detail than in the main study presented by
Montgomery et al. (2009b). Zilpaterol treatment decreased CIE a*, chroma, estimated oxymyoglobin, and estimated metmyoglobin values, whereas estimated myoglobin content was increased compared with carcasses of steers not fed zilpaterol. Whereas carcass color scores in the present experiment were not affected by zilpaterol treatment, carcass LM color scores from the larger study in which the current study carcasses were subsampled indicated zilpaterol improved LM color, resulting in less dark ribeye with a more cherry red color. The results of the current study and Montgomery et al. (2009a) are indicative that zilpaterol has the potential to decrease metmyoglobin values and improve beef color. Additionally, when retail display was simulated on LM steaks from the present experiment, color scores were greater (lighter) throughout a 5-d display period. The improved shelf-life through color scores of steaks may be attributable to the increased LM myoglobin seen at the time of carcass grading. However, this hypothesis requires further research to substantiate. Strydom et al. (2000) reported zilpaterol supplementation improved color stability and decreased discoloration in LM and gluteus medius steaks for 5 display days and increased shelf life by approximately 1 d, and Avendaño-Reyes et al. (2006) reported similar carcass effects on CIE a* and chroma values. In agreement with the current study, Strydom et al. (2000) found zilpaterol treatment decreased metmyoglobin development in LM and gluteus medius steaks during simulated retail display. Research has shown only a slight (4%) increase in L* values using other βAA in cattle (Fiems et al., 1995) and a 3% increase in chroma values (Vestergaard et al., 1994) using cimaterol. Although further research indicated no difference between controls and cimaterol for muscle L*, a*, or b* values (Fiems et al., 1990; Boucqué et al., 1994; Vestergaard et al., 1994), it seems zilpaterol may have the capacity to affect meat color.
Other βAA such as L644,969 and cimaterol have been shown to increase beef cutability. Moloney et al. (1990, 1994) showed that the βAA L644,969 increased cutability of low and high priced cuts including the brisket, strip loin, and inside and outside round. Additionally, Chikhou et al. (1993) observed cimaterol supplementation of Holstein steers resulted in increased cutability in brisket, flank, strip loin, and inside and outside round. Previous reports on the repartitioning effect of zilpaterol hydrochloride are limited. Leheska et al. (2008) reported a significant increase in carcass CP deposition due to zilpaterol supplementation. Plascencia et al. (1999) reported zilpaterol treatment of Mexican steers for 6 wk significantly improved carcass cutability of boneless, closely trimmed wholesale cuts, and sub-primal cuts. In both the present experiment and the study of Plascencia et al. (1999) zilpaterol hydrochloride improved the subprimal yield of the neck, flank, knuckle, and inside and outside rounds. However, in the present experiment steer cutability was increased in a greater number of subprimals than those reported by Plascencia et al. (1999), including the shoulder clod, chuck tender, deep pectoral, strip loin, and tenderloin. In addition, because carcasses were selected in a manner to equalize carcass weight before collection of the cutability data, the differences observed in the present study are indicative of only the enhancement of cutability yield. The effect of zilpaterol on in plant beef cutability yields under a commercial setting should be expected to be greater due to enhancement of cutability as shown in the present study plus the additional carcass weight typically seen with zilpaterol (Casey et al., 1997; Montgomery et al., 2009a,b).
Further evidence of the repartitioning capacity of zilpaterol hydrochloride was demonstrated in the present experiment by an increase in estimated carcass protein and moisture and a decrease in estimated carcass fat. Ricks et al. (1984) and Anderson et al. (1989) demonstrated similar results in the increase in estimated carcass protein with the use of clenbuterol and ractopamine. However, clenbuterol and cimaterol were shown to have a much greater effect on decreasing estimated carcass fat than zilpaterol in the present experiment (Ricks et al., 1984; Quirke et al., 1988; Fiems et al., 1993). When the effects on cutability and estimated carcass composition are taken into account, it seems that the present experiment substantiates the repartitioning effect of zilpaterol and indicates that zilpaterol functions to increase lean deposition and decrease fat deposition as shown in Leheska et al. (2008). Much attention has been given to whether other βAA function as repartitioning agents via increased protein accretion or reduced protein degradation (Ricks et al., 1984; Schiavetta et al., 1990; Mersmann, 1998). Zilpaterol clearly increases protein and lean deposition as seen in the increased carcass protein, LM area, and cutability yield in the present experiment. The lack of an effect of zilpaterol on calpastatin activity in the present experiment may indicate that zilpaterol has a much greater effect on increasing protein accretion than decreasing protein degradation.
Other βAA have been shown to increase beef shear force including clenbuterol (Schiavetta et al., 1990; Berge et al., 1993; Luño et al., 1999), ractopamine when supplemented at 300 mg·animal–1·d–1 (Schroeder et al., 2003), cimaterol (Fiems et al., 1990, 1995; Vestergaard et al., 1994), and L644,969 (Moloney et al., 1994). In addition, treatment of steers and heifers with 300 mg·animal–1·d–1 of ractopamine decreased trained sensory panel initial and sustained tenderness scores whereas juiciness, beef flavor, and off-flavor were not affected by treatment (Schroeder et al., 2003). However, at the decreased doses of 100 and 200 mg·animal–1·d–1 of ractopamine the compound did not affect WBSF or sensory scores (Schroeder et al., 2003). Luño et al. (1999) reported clenbuterol treatment of heifers dramatically decreased sensory panel scores for both tenderness and juiciness. Typically the negative effect of other βAA on meat tenderness and shear force have been associated with a dramatic increase in prerigor muscle calpastatin activity (Bardsley et al., 1992; Wheeler and Koohmaraie, 1992; Luño et al., 1999). However, in the current study zilpaterol did not affect calpastatin activity. In addition, Luño et al. (1999) showed an approximate 31% decrease in µ-calpain activity when heifers were treated with clenbuterol, although µ- and m-calpain activity were not affected by zilpaterol in the present study compared with animals not fed zilpaterol.
In the present study LM WBSF was increased although the average increase of 28% at 7-d postmortem decreased to 8% when LM steaks were aged to 21-d postmortem. In the present experiment 14-d postmortem LM sensory variables of tenderness, juiciness, flavor intensity, and beef flavor were all decreased, but not to the extent as when animals were treated with clenbuterol (Luño et al., 1999). Effects on juiciness, flavor intensity, and beef flavor in the current study may have been attributable to decreased marbling (Montgomery et al., 2009a,b) by zilpaterol. Other reports on the effects of zilpaterol on tenderness have varied. Previous reports (Strydom et al., 1998; Morón-Fuenmayor et al., 2002; Avendaño-Reyes et al., 2006) of zilpaterol hydrochloride effects on LM shear force and sensory panel factors were similar to the results of the present study, whereas semitendinosus shear force seemed unaffected by zilpaterol treatment. In contrast, Casey et al. (1997) reported zilpaterol hydrochloride did not affect LM WBSF. Strydom and Nel (1999) reported that LM and semitendinosus sensory tenderness scores were increased, although treatment effects were reduced with postmortem aging and electrical stimulation. The β2-agonist L644,969 has been shown to prevent or greatly decrease postmortem aging of muscle, and the decrease of WBSF over the postmortem aging period depended on treatment duration in lambs (Pringle et al., 1993). Reductions in postmortem aging of lamb muscle due to L644,969 treatment have been attributed to decreases in muscle proteolytic capacity limiting postmortem myofibril degradation of desmin and troponin-T via decreased µ-calpain and increased calpastatin activities (Kretchmar et al., 1990; Koohmaraie et al., 1991). Postmortem aging and muscle proteolysis have been shown to be greatly reduced in veal calves when treated with clenbuterol (Geesink et al., 1993). In heifers and steers the βAA clenbuterol has been reported to prevent postmortem aging of muscle as measured by sensory tenderness, WBSF, and myofibril fragmentation index (Wheeler and Koohmaraie, 1992; Luño et al., 1999). Thus, zilpaterol hydrochloride seems to be unique to other βAA repartitioning agents in that muscle µ-calpain and calpastatin activities are not affected and meat ages postmortem.
Shackelford et al. (1997) classified 14-d postmortem LM as intermediate in tenderness when steaks were less than 4.8 kg in WBSF. In the present study, 14-d postmortem LM steaks from zilpaterol-treated steers averaged 4.00 kg in WBSF, were considered slightly to moderately tender by the trained sensory panel, and were rated as moderately tender by consumers on average. Whereas consumers reported differences in tenderness scores of 14-d postmortem LM steaks in the current study, consumer overall acceptability, tenderness acceptability, overall quality, beef flavor, and juiciness were not affected by zilpaterol treatment. Shackelford et al. (1991) reported the threshold LM WBSF limit was 4.6 kg for 88.6% consumer acceptance, whereas Miller et al. (2001) reported a LM WBSF threshold of 4.00 kg for 94% consumer satisfaction. Thus, zilpaterol effects on shear force diminish with postmortem aging and tenderness of LM aged for 14 d does not appear to adversely affect consumer acceptance of beef from zilpaterol-treated cattle compared with cattle not fed zilpaterol.
In conclusion, monensin and tylosin can be withdrawn from the diet during the zilpaterol feeding period (last 30 d on feed) with minimal to no impact on meat cutability or quality. Zilpaterol hydrochloride is a repartitioning agent that functions through increased protein and lean deposition and a decrease in fat deposition. Feeding zilpaterol hydrochloride for 30 d before slaughter to steers increases subprimal yield but decreases tenderness. However, this tenderness difference diminishes with increased postmortem aging and does not adversely affect overall consumer acceptance of LM aged for 14 d.
1 Corresponding author: jayden.montgomery{at}targacept.com
Received for publication May 11, 2008. Accepted for publication November 17, 2008.
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