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ANIMAL PRODUCTION |
* Department of Animal and Food Sciences, Texas Tech University, Lubbock 79409; and
Intervet Inc., Idalou, TX 79329
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Key Words: β-adrenergic receptor agonist • cattle • feedlot • performance • zilpaterol
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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; Bell et al., 1998). These compounds have been studied extensively in livestock because of their marked effects on rate or composition and growth, most notably increased accretion of skeletal muscle and decreased accretion of fat (Bell et al., 1998; Mersmann, 1998). The effects of β-AA are pronounced in ruminants, with stimulation of β-adrenergic receptors on cell surfaces causing substantially increased skeletal muscle mass, cross-sectional area of individual muscles, or both (Chung and Johnson, 2008).
Zilpaterol hydrochloride (ZH) is a potent β-AA that has increased ADG, G:F, yield of trimmed cuts (Plascencia et al., 1999), and dressed yield (Avendaño-Reyes et al., 2006) in feedlot cattle. Although the use of ZH to improve feedlot cattle performance has been approved for more than a decade in Mexico and South Africa (Avendaño-Reyes et al., 2006), ZH was only recently approved for use in the United States, and limited research has been published relative to its effects on performance and carcass traits of feedlot cattle in the United States. Physiological maturity might alter the response of cattle to β-AA; however, Winterholler et al. (2007) reported that the response of yearling steers to another β-AA [ractopamine hydrochloride (RH)] was similar across days on feed (DOF) before slaughter. Nonetheless, no data are available on responses to ZH as affected by DOF and thereby compositional maturity of feedlot cattle. Thus, the objectives of the present study were to evaluate the effects of both duration of ZH feeding and DOF before slaughter on performance and carcass characteristics of feedlot cattle.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Cattle
Five hundred eighty-seven steers (British and British x Continental) were purchased in 5 different loads (received between September 26 and October 1, 2006) in Oklahoma and Kansas, and transported to the Texas Tech University Burnett Center at New Deal, Texas (average BW on arrival = 340 kg). The cattle were housed in soil-surfaced pens with access to limited quantities of sudangrass hay and a 65% concentrate diet. Initial processing of cattle included measurement of BW [C & S squeeze chute, Garden City, KS; set on 4 electronic load cells (Rice Lake Weighing Systems, Rice Lake, WI; readability of ± 0.45 kg); scales were calibrated with 454 kg of certified weights (Texas Department of Agriculture) before use], shoulder height, hip height, rump width, and rump length, which were collected on each steer to predict the DOF required to reach a targeted compositional endpoint (Performance Cattle Company LLC, Amarillo, TX). Additional routine processing included: 1) placement in the ear of a uniquely numbered tag; 2) recording of coat color; 3) vaccination with a modified live virus vaccine (Vista 5 SQ, Intervet Inc., Millsboro, DE) and clostridial bacterin toxoid (Vision 7 with SPUR, Intervet Inc.); and 4) treatment with Safe-Guard (Intervet Inc.).
Experimental Design and Treatment and Pen Assignment
Cattle that would not be used in the experiment because of breed type (Brahman or dairy influence), eye problems, or temperament issues were excluded, leaving 560 steers for use in the experiment. Sixteen treatments arranged in a 4 x 4 factorial were assigned to pens in a completely random design (112 pens total; 7 pens/treatment; 5 steers/pen). Factors included duration of ZH (8.33 mg/kg; Zilmax, Intervet Inc.) feeding (0, 20, 30, or 40 d) and DOF before slaughter (136, 157, 177, and 198 d). The different slaughter groups were designed to evaluate the effect of feeding ZH when cattle were somewhat under-finished (137 d), somewhat over-finished (198 d), and finished to approximately the desired degree (157 and 177 d). All slaughter groups had the same projected mean number of days (DOF) to attain a Small degree of marbling. Predicted DOF to reach this marbling endpoint were generated for each steer based on proprietary data of Performance Cattle Company. The 560 steers selected for the experiment were stratified by predicted DOF to reach a Small degree of marbling and assigned randomly within strata to the 4 slaughter groups. Within slaughter group, the data were stratified by days to finish, and ZH duration treatments were assigned randomly within strata. Pens were then assigned randomly to treatments. Before the start of the experiment, each pen of steers was brought through the Burnett Center working facilities, and each steer was treated with ivermectin pour-on (Aspen Veterinary Resources Ltd., Liberty, MO), implanted with Revalor IS (80 mg of trenbolone acetate + 16 mg of estradiol, Intervet Inc.), and given a colored ear tag to uniquely identify steers within the slaughter group x ZH treatment combinations. Steers were then housed in the concrete, partly slotted floor pens (2.9 m wide x 5.6 m deep; 2.4 m of linear bunk space) assigned to each treatment.
On October 17, 2006, the cattle were weighed, and the experiment was started for the d 136 and 157 slaughter groups; cattle in the d 177 and 198 slaughter groups were started on the following day. To decrease the effects of gut fill at the time of the initial BW measurement, cattle were fed approximately 25% of their regular daily allotment of feed, and feed was removed from the feed bunk at 1200 h on the day before the initial BW measurement.
Management, Feeding, and Weighing Procedures
Estimates of the approximate quantity of unconsumed feed remaining in the feed bunk were made in each of the 112 pens from 0700 to 0730 h daily. Cattle were fed once daily in the morning, and adjustments to the feed delivery for each pen were made to ensure ad libitum access to feed, with the target being to leave from 0 to 0.5 kg of feed in the bunk before the next daily feeding. The feeding order was established from the beginning of the experiment and was based on the duration of ZH feeding, with the 40-d group fed first, followed by the 30-, 20-, and 0-d duration groups. Diets were mixed in a 1.27-m3-capacity paddle mixer (Marion Mixers Inc., Marion, IA) and transferred by a drag-chain conveyor to a tractor-pulled mixer/delivery unit (Rotomix 84-8, Dodge City, KS; scale readability of ± 0.45 kg), which was used to deliver feed to each pen. Cattle were fed the 65% concentrate receiving diet at the beginning of the experiment, after which they were stepped up to 75, 85, and 90% concentrate diets 4, 6, and 7 d later, respectively. Feed was offered at 95% of the previous day’s delivery on each step-up day. The 90% concentrate diet was fed thereafter until the time that each slaughter group was designated to receive the various ZH duration of feeding treatments. Once ZH feeding started, 2 diets were fed: 1) control (no ZH); and 2) a diet that provided ZH at 8.33 mg/kg (DM basis). The ZH was included by means of an intermediate, ground corn-based premix (Table 1). In addition, the standard trace mineral, vitamin, and feed additive supplement was modified to remove monensin (Rumensin, Elanco Animal Health, Indianapolis, IN) and tylosin (Tylan, Elanco Animal Health) from ZH-containing diets. To reflect the widespread use of monensin and tylosin in typical feedlot diets, the control treatment diet contained these feed additives (Table 1). Feeding ZH requires a 3-d withdrawal period before slaughter; thus, during the 3-d withdrawal period, all cattle in each slaughter group were fed the control diet, which contained monensin and tylosin. The ingredient and chemical compositions of the 2 diets and supplements are shown in Table 1. Diets were formulated to meet or exceed NRC (1996) recommendations for nutrients.
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The DMI by each pen during various periods of the study was calculated by subtracting the quantity of dry feed refused at the end of each period from the total dietary DM delivered to each pen during that period. The number of animals housed per pen was multiplied by the number of days in the period to determine animal days, which were then divided into the corrected total DM delivered to the pen to obtain average DMI per steer.
Individual BW measurements were generally obtained using the hydraulic squeeze chute described previously. On d 63, all cattle were weighed (unshrunk) and implanted with Revalor S (Intervet Inc.; 120 mg of trenbolone acetate and 24 mg of estradiol). The unshrunk BW at the start of the ZH feeding period for each slaughter group was measured at approximately 0700 to 0800 h, before the morning feeding. The final unshrunk BW measurements for each slaughter group also were obtained in the morning just before cattle were shipped to the Cargill facility in Friona, Texas. Personnel from the Texas Tech University Meat Laboratory collected carcass data. Because some cattle were too large to fit through the standard alley leading to the described previously squeeze chute, the final BW of the d 198 slaughter group was obtained using a squeeze chute suspended on weigh bars attached to Mettler-Toledo (Mettler-Toledo Inc., Columbus, OH) load cells (4 load cells on weigh bars) and indicator. This scale and squeeze chute had a more spacious alley leading to the chute and could accommodate larger, wider cattle than the described previously chute. As before, the scale was calibrated with 454 kg of certified weights before use.
Carcass Evaluation
Carcass measurements were assessed by Texas Tech University Meat Laboratory personnel and included HCW, LM area (determined from video image analysis), estimated percentage of KPH, and preliminary yield grade. The 12th-rib fat was calculated from an adjusted preliminary yield, and the final yield grade was calculated from HCW, LM area, 12th-rib fat, and KPH (USDA, 1997). Quality grade factors were marbling score, skeletal maturity score, lean maturity score, overall maturity score, and final USDA quality grade (based on measurements of USDA, 1997). In addition, liver condemnations were classified for presence of abscesses as either not condemned, as A-, A, or A+ (Brink et al., 1990), and for occurrence of distoma, telangiectasis, or contamination during the slaughter process.
Statistical Analyses
The unshrunk live BW data measured at d -43 and the final BW measured just before shipment to slaughter were multiplied by 0.96 before use in calculations, whereas the initial BW was not mathematically adjusted. Data were initially tested to ensure that the assumption of normality of model residuals was satisfied and to determine whether treatment variances were homogeneous. In cases of heterogeneous variance, the Repeated/Group option (group = ZH duration x slaughter group) of the MIXED procedure (SAS Inst. Inc., Cary, NC) was used. Performance and carcass data were analyzed as a completely random design using the Mixed procedure, with pen as the experimental unit. The duration of ZH feeding, slaughter group, and duration of ZH feeding x slaughter group interaction were the fixed effects. Carcass count and frequency data (USDA quality grade, and distributions of HCW, liver abscesses, and calculated yield grades) were analyzed on a pen basis as binomial proportions using the Glimmix procedure of SAS, with the same model as for performance and carcass measurements. For all analyses, specific orthogonal contrasts were used to test 1) control vs. the average of the 3 duration of ZH feeding groups and 2) linear and quadratic effects of duration of ZH feeding and DOF before slaughter.
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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Analyzed chemical composition data for the 2 treatment diets are shown in Table 1
. Values were in agreement with expectations from formulation and differed little between the control and ZH treatments.
Performance
Performance data are presented in Table 2. No duration of ZH feeding x slaughter group interactions were noted for any of the performance measurements analyzed (P > 0.10); as a result, only main effects will be discussed. No differences were detected among the duration of ZH feeding treatments for ADG, DMI, and G:F for the periods between initial BW and reimplant and from reimplant to the final 43 d on feed (d -43; P > 0.29). Final live BW did not differ (P = 0.15) between the 0-d (control) group and the average of the 3 ZH-supplemented groups, but ADG differed between the 0-d group and the average of the 20-, 30-, and 40-d ZH groups during the period in which treatment diets were imposed (P < 0.01) and for the entire experimental period (P = 0.05). Dry matter intake during the 40-d ZH feeding and 3-d withdrawal period decreased in a linear manner (P = 0.01) among the 3 ZH-supplemented groups. Thus, with increased ADG and decreased DMI, there was a linear increase in G:F (P < 0.01) as duration of ZH feeding increased, as well as a difference in G:F during the treatment period between the 0-d group and the 3 ZH-supplemented groups (P < 0.01). The linear decrease in DMI with increasing duration of ZH feeding likely contributed to the lack of linear responses in final BW and ADG as duration of ZH feeding increased beyond 20 d.
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fed 0 or 6 mg/kg of ZH to 140 crossbreed yearling steers (BW = 373 kg) in a 42-d finishing trial and did not observe effects of ZH on DMI, although ADG and G:F were increased approximately 28% by ZH supplementation. Comparing the effects of 2 different β-AA (ZH and RH) using 54 steers (BW = 424 kg) in a 33-d feeding study, Avendaño-Reyes et al. (2006) observed that steers fed ZH had a 26% greater ADG than control steers. Moreover, steers fed β-AA consumed less DM than control steers, resulting in increased G:F. These data are consistent with the results of the present study, although the magnitude of the response was somewhat less in our study. The 0-d group in the present study had a 12.5% lower ADG than the average of the 3 ZH groups during the period in which treatment diets were imposed, and the 3 ZH groups had 15.3% greater G:F. Differences in the magnitude of performance responses between the present and previous studies, particularly in final BW and ADG, might reflect the lesser final BW of the cattle used in the Plascencia et al. (1999) and Avendaño-Reyes et al. (2006) studies. Nonetheless, the lack of an interaction between duration of ZH feeding and slaughter group in the present study indicates that with the range in final BW we considered, the response to ZH is consistent across cattle with varying degrees of fatness and physiological maturity.
As expected, final BW increased linearly (P < 0.01) as days on the finishing diet before slaughter increased. From d 0 to slaughter (P = 0.04) and from d -43 to slaughter (P < 0.01), there was a quadratic response in ADG among the 4 slaughter groups (Table 2). No differences were detected among slaughter groups for DMI for the entire trial period; however, a quadratic response was observed during the ZH supplementation period (P = 0.02). In the experimental results submitted for FDA approval of ZH (FDA, 2006), feeding ZH decreased DMI, but effects of length of ZH feeding and P-values for changes in DMI were not reported. A quadratic response was also detected from d 0 to slaughter (P = 0.02) and during the ZH supplementation period (P < 0.01) for G:F. The G:F was greatest for the 136 d group, followed by a decrease in G:F to the 177 d group, with no further change in G:F for the d 198 slaughter group. Similar results for decreased G:F by feedlot steers fed for extended periods were reported by Van Koevering et al. (1995).
Carcass Characteristics
Carcass data are presented in Tables 3 through 8. With the exception of KPH (P = 0.022), no duration of ZH feeding x slaughter group interactions were detected (P > 0.10). Evaluation of KPH simple-effect means indicated, however, that this interaction did not preclude evaluation of the main effects (i.e., changes in magnitude, not direction, of the response). The ZH-treated cattle had greater HCW (P < 0.01), greater dressing percent (P < 0.01), less fat at the 12th rib (P < 0.01), a larger LM area (P < 0.01), less KPH (P = 0.03), and a lower yield grade than the 0-d cattle, regardless of the duration of ZH feeding (P < 0.01; Table 3). With the exception of dressing percent, 12th-rib fat, and KPH, no linear responses (P > 0.10) were detected for duration of ZH feeding. Dressing percent increased linearly (P = 0.05) with increased duration of ZH feeding, whereas 12th-rib fat (P = 0.05) and KPH (P = 0.03) decreased linearly with increased duration of ZH feeding. The decreased carcass fatness (12th-rib fat and KPH) and increased HCW, along with only a tendency for a difference in FBW, suggest a shift in how nutrients were partitioned between lean and fat depots. Avendaño-Reyes et al. (2006) reported that feeding ZH for 33 d resulted in a slightly greater increase in HCW (approximately 22 kg) than in final BW (approximately 19.5 kg). In addition, carcass lean tissue tended to increase when ZH was fed in their study, whereas carcass fat was decreased by approximately 1 percentage unit, but the difference was not significant. Additional research is needed to understand how ZH affects repartitioning of nutrients between lean and fat tissue depots. In particular, the extent to which fat associated with the intestinal tract that would be discarded as offal in the slaughter process and thereby affect estimates of dressing percent needs to be evaluated. Relative to slaughter group, there was a linear increase (P < 0.01) in HCW, dressing percent, and 12th-rib fat as DOF increased. This increase in dressing percent was expected because dressing percent generally increases with increased carcass fatness, which increases with DOF. Similarly, Winterholler et al. (2007) observed that increased DOF increased HCW and dressing percent. Van Koevering et al. (1995) also reported greater HCW and 12th-rib fat with increasing DOF; however, dressing percent was not altered by slaughter group in their study. As in the present study, Bruns et al. (2004) observed that dressing percent increased linearly with increasing DOF, but in their study, 12th-rib fat increased in a quadratic fashion as DOF before slaughter increased.
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). A tendency (P = 0.09) also was noted for an increased overall maturity score for the ZH-fed steers vs. the control cattle. Yield grade (P < 0.01), marbling (P = 0.01), LM area (P = 0.03), and KPH (P = 0.01) responded quadratically to increased DOF before slaughter. Skeletal maturity and overall maturity scores also responded quadratically to slaughter group (P 
0.05). In general, these measurements increased with increased DOF. In addition, a linear increase was noted for lean maturity score with increased DOF (P = 0.01). Bruns et al. (2004) also reported that skeletal maturity increased linearly as DOF before slaughter increased, and Van Koevering et al. (1995) noted that maturity score (calculated by averaging lean and skeletal maturity) increased at a decreasing rate with DOF before slaughter, resulting in both linear and quadratic responses.
With ZH supplementation, we observed an increase of 4.5% in HCW, an increase of 3.1% in dressing percent, and an increase of 10.6% in LM area (control vs. the average of the 3 ZH groups). Compared with our findings, Plascencia et al. (1999) observed a similar increase in HCW (4.8%) and dressing percent (3.6%), but a smaller increase in LM area (2.7%). Moreover, no ZH effects were observed on 12th-rib fat, KPH, retail yield, marbling score, or liver abscesses in the Plascencia et al. (1999) study. Avendaño-Reyes et al. (2006) observed a greater increase in HCW (7.5% heavier than control carcasses) but similar increases in dressing percent (3.3%) and LM area (12.7%) compared with our findings. Although the results of Plascencia et al. (1999) and Avendaño-Reyes et al. (2006) are somewhat comparable to the present results, it is important to note differences between these experiments and the present study. In Mexico, consumer preferences result in cattle being slaughtered at a lesser BW than cattle in the United States (Avendaño-Reyes et al., 2006). Plascencia et al. (1999) slaughtered cattle weighing approximately 433 to 455 kg, whereas cattle from the experiment conducted by Avendaño-Reyes et al. (2006) weighed 488 kg at slaughter. Cattle in the present experiment were slaughtered at weights ranging from 569 to 644 kg. As noted previously, lack of an interaction between duration of ZH feeding and DOF before slaughter suggests that ZH has positive effects on HCW and LM area in cattle of widely different slaughter weights.
Data for percentage of carcasses in various quality grades (assigned based on marbling score and maturity data) are shown in Table 4. The percentage of carcasses grading Choice or greater (Choice, Premium Choice, and Prime combined) was less (P < 0.01) for cattle fed ZH than for control cattle, with no effect of duration of ZH feeding. No differences between the 0-d group and ZH-supplemented groups were observed (P = 0.39) for percentage of carcasses grading Choice; however, the 0-d cattle had a lesser percentage of carcasses grading Select than carcasses of cattle in the average of the 3 ZH duration treatments (P = 0.01). Moreover, the percentage of cattle grading Select increased linearly (P = 0.04) as duration of ZH feeding increased. No differences were observed between the control and the 3 ZH-supplemented groups for the percentage of carcasses grading Standard, Utility, and Commercial (all 3 grades combined, P = 0.97). A tendency for a linear increase in the percentage of carcasses grading Standard, Utility, and Commercial, however, was observed with increasing days on ZH (P = 0.10; Table 4). The percentage of cattle grading Choice increased quadratically (P < 0.01) with increased DOF before slaughter, peaking at 157 d, whereas the percentage of cattle grading Select decreased quadratically (P < 0.01) with increasing DOF, being least for 157 d. Regressing marbling score against DOF before slaughter, May et al. (1992) and Van Koevering et al. (1995) observed that marbling score responded quadratically before reaching a plateau at approximately 112 to 119 d on feed. In contrast, Bruns et al. (2004) observed that marbling scores increased linearly with increasing time on feed before slaughter.
No duration of ZH feeding x slaughter group interactions were detected (P > 0.10) for liver score data (Table 5). Likewise, no differences were found in the percentage of livers with abscesses between ZH-fed steers and control steers (P = 0.97). Evaluation of the data indicated that the ZH groups had a numerical increase in the frequency of liver abscesses compared with the 0-d group, but with a limited number of cattle having abscesses, it is difficult to detect statistical differences. A difference in occurrence of liver abscesses might be expected as a result of the exclusion of tylosin from the ZH-containing diet. Increased DOF before slaughter did not influence the percentage of abscessed livers (P 
0.36). Similar data with increasing DOF for incidence of liver abscesses was reported by Van Koevering et al. (1995).
No differences among treatments (duration of ZH feeding or slaughter group) were noted in the distribution of HCW when categorized as <431, 431 to <454, 454 to <476, or 476 kg (P 0.31; Table 6). Again, small sample sizes in the various subclasses limited our ability to detect differences. The general trend noted in the distribution of HCW was for increased percentages of cattle in the heavier HCW groups when ZH was fed; this effect would be expected because of the significant increase in HCW with ZH feeding noted previously (Table 3). A significant increase (P < 0.01) was noted in the percentage of LM area >90 cm2 when ZH was fed (Table 7). Furthermore, the percentage of carcasses with LM area 103 cm2 tended to increase linearly (P = 0.10) as duration of ZH feeding increased. A quadratic response (P = 0.01) was observed in the percentage of LM areas between 77 and 90 cm2 with increasing DOF before slaughter.
The distribution of carcasses by calculated yield grade (calculated from measurements that are applied in the yield grade formula) is presented in Table 8. No differences between the 0-d group and the 3 ZH-supplemented groups were observed for percentage of carcasses with yield grade 3.5 to 4 (P > 0.95) or 4 (P > 0.97); however, ZH-supplemented groups had a greater percentage of carcasses with yield grades between 2 and <2.5 (P < 0.03) and <2 (P < 0.01). No duration of ZH feeding differences were observed for percentage of carcasses with a 2.5 to <3 yield, but percentage of carcasses grading between 3 and <3.5 tended (P = 0.09) to be greater for the 0-d group than for the 3 ZH-supplemented groups. A tendency (P = 0.10) for a linear decrease on percentage of carcasses with a yield grade of 3 to <3.5 was also noted as DOF before slaughter increased.
Overall, the results of the present experiment indicate that final BW increased linearly as DOF before slaughter increased, but quadratic responses were observed for ADG and G:F among the 4 slaughter groups. No differences were detected among slaughter groups for DMI for the entire feeding period. Hot carcass weight, dressing percent, and 12th-rib fat increased as DOF before slaughter increased, and marbling increased at a decreasing rate. Because no interactions were observed between duration of ZH feeding and DOF before slaughter, we conclude that ZH manifests its effects on performance and carcass characteristics regardless of the increased BW and degree of finish associated with greater time on the finishing diet before slaughter. Feeding ZH increased ADG without further improvements as duration of feeding increased beyond 20 d; however, G:F increased linearly as duration of ZH feeding increased from 20 to 40 d. Dry matter intake decreased in steers fed ZH for 30 and 40 d, which might explain the similar weight gain noted with increasing duration of ZH feeding beyond 20 d. With respect to changes in carcass characteristics across the 3 durations of feeding, ZH increased HCW, dressing percent, and LM area with concurrent decreases in 12th-rib fat and yield grade. Only dressing percent, 12th-rib fat, and KPH exhibited linear responses to ZH duration of feeding, indicating that there are no appreciable additive gains in carcass measurements with increased duration of feeding beyond 20 d. Feeding ZH had negative effects on marbling and shifted average USDA quality grade away from Choice toward Select.
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Footnotes |
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3 Present address: Panhandle Res. Ext. Center, Dept. Anim. Sci., Univ. of Nebraska, Scottsbluff 69361.
2 Corresponding author: jvasconcelos2{at}unl.edu
Received for publication March 13, 2008. Accepted for publication April 17, 2008.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
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