Comparative evaluation of supplemental zilpaterol hydrochloride sources on growth performance, dietary energetics and carcass characteristics of finishing lambs

Objective We compare the effects of three different approved sources of supplemental zilpaterol on growth-performance responses and carcass characteristics of finishing lambs. Methods Twenty four Pelibuey×Katahdin lambs (46.75±2.43 kg) were used in a 33-day feeding trial. Lambs were fed a dry rolled corn-based finishing diet. Treatments consisted of the non-supplemental basal diet (Control) versus the basal diet supplemented with 125 mg zilpaterol/kg of diet (as fed basis) from three commercial sources marketed in Mexico: Zilmax (ZIL), Grofactor, and Zipamix. Results Compared to controls, zilpaterol (ZH) supplementation did not affect dry matter intake (DMI), but increased carcass adjusted daily weight gain (ADG, 36.7%), gain efficiency (34.2%), and dietary net energy (26.0%), and decreased (23.4%) the ratio of observed:expected DMI. Compared to controls, supplemental ZH increased hot carcass weight (6.4%), dressing percentage (3.2%), m. longissimus thoracis (LM) area (15.6%), and shoulder muscle:fat ratio (28.7%), but decreased kidney-pelvic-heart fat, and fat thickness. Supplemental ZH increased 10.9% and 14.3% whole cut weight of loin and leg, respectively, and the proportion (as percentage of cold carcass weight) of leg (4.3%). These increases were reflected in greater forequarter and hindquarter weights. Lambs fed ZH increased (4.6%) empty body weight (EBW) and reduced (14.7%) liver/spleen weight (as g/kg EBW). Likewise, ZH supplementation tended (p = 0.08) to lower (8.9%) visceral fat. Growth performance, energetic efficiency, hot carcass weight, dressing percentage, LM area and whole cuts were not different across supplemental ZH sources. However, compared with non-supplemented controls, only ZIL appreciably decreased carcass fat distribution, including fat thickness, percentage kidney pelvic and heart fat, shoulder fat, and visceral fat. Conclusion Supplemental ZH increases ADG, gain efficiency, carcass dressing percentage, and LM area. The magnitude of these responses was similar among ZH sources. Nevertheless, compared with non-supplemented controls, only ZIL appreciably decreases carcass fat. The basis for this is uncertain, but indicative that some practical differences in zilpaterol bio-equivalency may exist across commercial sources tested.


INTRODUCTION
Zilpaterol hydrochloride (ZH), a beta adrenergic agonist, is an FDAapproved feed additive for beef cattle [1]. It was originally patented and marketed under the trade name Zilmax (ZIL; MSD, Summit, NJ, USA). However, following patent expiration outside the USA, additional "generic" forms of the compound have been approved for mar keting in countries where the use of ZH as a feed additive is authorized. Notwithstanding potentially lower product costs, industry acceptance of "generic" forms of feed additives is limited [2]. The basis for this include: perceived differences in quality control during manufacturing and marketing; unifor mity; purity; drug particle size and carrier, and their associated effects on product distribution during feed mixing and deli very; manufacturer product support; and demonstrated bio equivalency. Supplementing finishing lambs with ZH at the rate of 6 mg/kg diet dry matter (DM) increase average daily gain (ADG), gain efficiency [35], carcass weight and dressing percentage [5,6]. These effects help to increase gain efficiency, particularly during the late finishing phase when cost of gain is greatest. The objective of the present study was to compare the effects of three approved sources of ZH (MEX SAGARPA, 2016; registration Q0042401, Q7833242, and Q0273205) marketed under the trademark Grofactor (GRO; Laboratorios Virbac México, Guadalajara, Mexico), Zipamix (ZIPA; Pisa Agropecuaria, Guadalajara, Mexico), and ZIL (MSD, Salud Animal Mexico, Estado de Mexico, Mexico) on growth perfor mance, dietary energetics and carcass characteristics of finishing lambs.

Diets, animals and experimental design
This experiment was conducted at the Universidad Autónoma of Sinaloa Feedlot Lamb Research Unit, located in the Culi acán, México (24° 46′ 13″ N and 107° 21′ 14″ W). Culiacán is about 55 m above sea level, and has a tropical climate. Average daily minimum and maximum air temperature during the trial was 25.9°C and 33.9°C (average = 29.9°C), respectively. Average daily relative humidity was 78.8%. All animal man agement procedures were conducted within the guidelines of locallyapproved techniques for animal use and care: NOM051 ZOO1995: Humanitarian care of animals during mobilization of animals; NOM062ZOO1995: Technical specifications for the care and use of laboratory animals. Livestock farms, farms, centers of production, reproduction and breeding, zoos and exhibition halls, must meet the basic principles of animal wel fare; NOM024ZOO1995: Animal health stipulations and characteristics during transportation of animals, and NOM 033ZOO1995: Humanitarian care and animal protection during slaughter process.
Twentyfour Pelibuey×Katahdin (46.7±2.4 kg initial shrunk weight) crossbred intact male lambs were used in a 33d growth performance experiment to evaluate the treatment effects on growth performance, dietary energetics, carcass traits, and visceral organ mass. Prior to initiation of the study, lambs were treated for endoparasites (Albendaphorte 10%, Animal Health and Welfare, México City, México), and injected with 1×10 6 IU vitamin A (SyntADE, Fort Dodge, Animal Health, Mexico City, México). Lambs then adapted to the control diet (Table  1) for a period of 7 weeks. Upon initiation of the experiment,  lambs were individually weighed (electronic scale; TORREY  TIL/S: 107 2691, TOR REY electronics Inc, Houston TX, USA), grouped by weight into six uniform weight blocks, and assigned to pens (1 lamb/pen). Individual pens were 6 m 2 with over head shade, automatic waterers and 1 m fenceline feed bunks. Dietary treatments (Table 1) consisted of a cornbased finish ing diet supplemented with no zilpaterol (Control), or the, same basal diet plus the label dosage (125 mg of product/kg diet, asfed basis) as ZIL (MSD Salud Animal Mexico, Santiago Tianguistenco, Mexico), GRO (Laboratorios Virbac México, Mexico), or ZIPA (Pisa Agropecuaria, Mexico). According to the label, all products tested contained 4.8% ZH. Thus, the dosage of 125 mg of product/kg diet corresponds to a dietary ZH concentration of 6 mg/kg (as feed basis). Supplemental ZH was handweighed using a precision balance (Ohaus, mod AS612, Pine Brook, NJ, USA), and premixed for 5 min with the other minor dietary ingredients (urea, limestone, and trace mineral salt) before incorporation into complete mixed bas Zilmax  [3]. Dietary NE was estimated by means of the quadratic formula: , and c = -0.877 DMI in accordance to Zinn et al [11].

nd visceral mass data
were harvested on the same day. After humanitarian sacrifice, lambs were skinned, and the gastrointestinal ere separated and weighed. After carcasses (with kidneys and internal fat included) were chilled in a cooler 1°C for 48 h, the following measurements were obtained: i) fat thickness perpendicular to the m. longissimus )/2c, where x = NE m , a = -0.41EM, b = 0.877EM+ 0.41DMI+EG, and c = -0.877 DMI in accordance to Zinn et al [11].

Carcass and visceral mass data
All lambs were harvested on the same day. After humanitarian sacrifice, lambs were skinned, and the gastrointestinal organs were separated and weighed. After carcasses (with kidneys and internal fat included) were chilled in a cooler at -2°C to 1°C for 48 h, the following measurements were obtained: i) fat thickness perpendicular to the m. longissimus thoracis (LM), measured over the center of the ribeye between the 12th and 13th rib; ii) LM surface area, measure using a grid reading of the crosssectional area of the ribeye between 12th and 13th rib, and iii) kidney, pelvic and heart fat (KPH). The KPH was removed manually from the carcass, and then weighed and reported as a percentage of the cold carcass weight [12]. Each carcass was split into two halves. The left side was fabricated into wholesale cuts, without trimming, according to the North American Meat Processors Association guidelines [13]. Rack, breast, shoulder and foreshank were obtained from the fore saddle, and the loins, flank and leg from the hindsaddle. The weights of each cut were subsequently recorded. The tissue composition of shoulder was assessed using physical dissec tion by the procedure described by Luaces et al [14].
All tissue weights were reported on a fresh tissue basis. Organ mass was expressed as g/kg final empty body weight (EBW). Final EBW represents the final SBW minus the total digesta weight. Full visceral mass was calculated by the sum mation of all visceral components (stomach complex+small intestine+large intestine+liver+lungs+heart), including di gesta. The stomach complex was calculated as the digestafree sum of the weights of the rumen, reticulum, omasum and abomasum. The weights of the heart and lungs, and the weights of liver and spleen were recorded together.

Statistical analysis
Growth performance (weight gain, feed intake, gain efficiency), dietary energetics, carcass data and visceral mass data were analyzed as a randomized complete block design using the MIXED procedure of SAS [15], where initial weight was the blocking criterion (blocks = 6), and lamb was considered as the experimental unit. The treatment means were separated using the least significant difference test (Tukey's Test). Treat ment effects were considered significant when the value of p≤ 0.05, and were identified as trends when the value of p>0.05 and ≤0.10.
Treatment effects on growth performance and dietary energetics are shown in Table 2. Compared to controls, ZH supplementation did not affect (p>0.10) DMI, but increased (p<0.05) carcass adjusted ADG (36.7%), gain efficiency (34.2%) and dietary NE (26.0%). Accordingly, ZH supplementation markedly decreased (23.4%, p<0.05) the ratio of observed: expected DMI. At comparable levels of ZH intake, growth per formance and energetic efficiency responses of feedlot lambs were not different (p>0.10) across supplemental ZH sources.

DISCUSSION
Optimal daily ZH dosage for feedlot cattle is between 0.15 to 0.165 mg/kg body weight (BW) [16,17]. In practice, this cor responds to a dietary concentration of 6.7 mg/kg diet DM (6 mg/kg airdry feed, 90% DM basis). Zilpaterol is not currently labeled as a feed additive for lambs. The numerous studies conducted thus far in feedlot lambs adopted the labeled dosage as indicated for feedlot cattle [3,5,6,18].
Enhancements in ADG, gain efficiency and dietary ener getics when lambs were supplemented with ZH are consistent with previous reports [3,19]. In previous studies, ZH supple mentation of finishing diets for feedlot lambs increased ADG by 20.1% to 40.6% and gain efficiency by 16.5% to 43.3%. The observed enhancements in ADG and gain efficiency in the present study fall within those ranges. However, the decrease (13.3% to 16.2%) in observed:expected DMI ratio due to ZH supplementation (23.4%) was greater than previously observed [3,5,19,20].
Increased HCW, LM area, and dressing percentage, and reduced backfat thickness with ZH supplementation has been a consistent response in feedlot lambs [18,21]. Increased LM area is partially explained by the greater ADG [22], whereas the increased dressing percentage is expected due to greater  carcass protein accretion with no change in digestive tract fill [6,23]. The increased LM area is also consistent with increased shoulder muscle [21]. Increased carcass cutability due to ZH supplementation has been consistent response in feedlot cattle [16], with the more pronounced effect occurring in the hindquater [17]. However, in lambs, the effects of ZH supplementation on carcass cutability is less consistent. Whereas in some studies [4,24] supplemental ZH increased loin and leg (as observed in the present experiment), in others [25,26] supplemental ZH did not affect lamb carcass wholesale cuts. The basis for this is uncertain. EstradaAngulo et al [3] observed that factors such as diet energy density, age, genetics, and ZH dosage level may influence response to supplemental ZH. Comparative effects of different ZH sources on carcass cutability has not been pre viously reported. However, because the dietary concentration of ZH was similar for each of the three sources, absence of effects of ZH source on carcass cutability is expected.
The effects of βagonist on noncarcass organs has received limited attention. The βagonist salbutamol decreased liver mass in pigs [27]. Likewise, supplemental ZH decreased liver mass in feedlot cattle [28]. Decreased liver mass has also been a consistent observation in feedlot lambs receiving ZH [5,23].
Effects of ZH on visceral organs has been attributed to the differences in the abundance of βagonist receptors subtypes in these tissues [17]. In as much as an appreciable proportion of energy expenditure can be attributed to maintenance of visceral organs, especially the liver and gastrointestinal tract [29], reductions in visceral organ mass could contribute to the increased energy efficiency observed when dietary βagonists are fed.
Increased carcass dressing percentage explained 66% (65 g/d) of the increase (98 g/d) in carcass adjusted ADG in ZH supplemented lambs. Enhanced daily gain accounts for 55% of the net economic value of ZH supplementation (benefit to the feeder), while increased carcass yield accounts for 45% of the net value (benefit to the meat packer and retailer). Thus, the economic benefit to ZH supplementation is optimized through integrated production and meat purveying systems (Table 6).
AvendañoReyes et al [30] compared GRO vs ZIL in cross breed bulls (75% Bos indicus and remainder Bos taurus) in a 34d finishing trial (30 days of ZH supplementation and 4 days withdrawal). Bulls ingested an average of 0.134 and 0.139 mg ZH/kg BW of GRO and ZIL, respectively. Supplemental ZH enhanced ADG and gain efficiency. However, growth perfor mance responses were not affected by ZH source. Likewise, in the present study differences in growth performance re sponses in lambs fed different ZH sources were not appreciable.
AvendañoReyes et al [30] did not detect an effect of sup plemental ZH source on carcass characteristics and organ weights of feedlot cattle. Likewise, in the present study differ ences among ZH sources on carcass characteristics and organ weights were not appreciable. In contrast, ZH sources differed in their effects on fat distribution. Compared with control, supplemental GRO and ZIPA did not affect (p>0.10) fat dis tribution. Whereas supplemental ZIL decreased (p<0.05) carcass fat thickness, KPH, and shoulder muscle:fat ratio. In as much as all ZH products provided a similar zilpaterol dos age, this outcome was not expected.

CONCLUSION
Supplemental ZH increases ADG, gain efficiency, carcass dress ing percentage and LM area, while reducing observed to expect DMI in feedlot lambs. The magnitude of these responses were similar among ZH sources. Nevertheless, compared with non supplemented controls, only ZIL appreciably decreases carcass fat. The basis for this is uncertain, but indicative that some prac tical differences in zilpaterol bioequivalency may exist across commercial sources tested.