This experiment was conducted at the Universidad Autónoma of Sinaloa Feedlot Lamb Research Unit, located in the Culiacán, 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 management procedures were conducted within the guidelines of locally-approved techniques for animal use and care: NOM-051-ZOO-1995: Humanitarian care of animals during mobilization of animals; NOM-062-ZOO-1995: 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 welfare; NOM-024-ZOO-1995: Animal health stipulations and characteristics during transportation of animals, and NOM-033-ZOO-1995: Humanitarian care and animal protection during slaughter process.

Twenty-four Pelibuey×Katahdin (46.7±2.4 kg initial shrunk weight) crossbred intact male lambs were used in a 33-d 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 1×10⁶ IU vitamin A (Synt-ADE, 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 m² with overhead shade, automatic waterers and 1 m fence-line feed bunks. Dietary treatments (Table 1) consisted of a corn-based finishing diet supplemented with no zilpaterol (Control), or the, same basal diet plus the label dosage (125 mg of product/kg diet, as-fed 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 hand-weighed 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 basal diet using a 2.5 m³ capacity paddle mixer (mod 30910-7, Coyoacán, México). To avoid contamination, the mixer was thoroughly cleaned between each treatment. The ZH was supplemented for 30 d followed by a 3-d preharvest withdrawal when all lambs received the non-supplemented basal control diet. The four dietary treatments were randomly assigned to pens within each weight block in a randomized complete block design. Lambs were allowed ad libitum access to dietary treatments. Fresh feed was provided twice daily at 0800 and 1400 h in a 40:60 proportion (as feed basis). Daily feed allotments to each pen were adjusted to allow for approximately 5% residual feed remaining in feed bunk at time of the morning feeding. Feed bunks were visually assessed between 0740 and 0750 h each morning, residual feed was collected and weighed for determination of daily feed intake. Adjustments, in daily feed delivery were provided at the afternoon feeding. Lambs were individually weighed at the beginning of the trial and at harvest. The initial shrunk body weight (SBW) was determined as full body weight×0.96 (adjustment for gastrointestinal fill). Upon completion of the study, all lambs were weighed following an 18 h fast (feed but not drinking water was withdrawn) to obtain final SBW. Final SBW was adjusted for hot carcass weight (HCW) by dividing the individual HCW by the average dressing percentage (0.6120) for all lambs.

Dietary treatments were subjected to the following analyses [7]: DM (oven drying at 105°C until no further weight loss; method 930.15); crude protein (N×6.25, method 984.13); ash (method 942.05), and ether extract (method 920.39). The neutral detergent fiber (NDF) fraction was determined according to Van Soest et al [8] [corrected for NDF-ash, incorporating heat stable α-amylase (Ankom Technology, Macedon, NY, USA) at 1 mL per 100 mL of NDF solution (Midland Scientific, Omaha, NE, USA)]. Feed and orts were sampled daily for DM analysis (oven-drying at 105°C until constant weight, method 930.15) [7]. The ZH concentrations for the various sources (blind samples) were assayed by MSD Quality Control Laboratory (MSD Salud Animal Mexico, Mexico).

Average daily gain was determined as the difference in initial SBW and carcass adjusted final SBW divided by 33 (days on test). Gain efficiency was determined as the ADG divided by corresponding dry matter intake (DMI). The estimation of expected DMI was performed based on observed ADG and average SBW according to the following equation: expected DMI, kg/d = (EM/NE_m)+(EG/NE_g), where EM is the energy required for maintenance, Mcal/d (0.056×SBW^0.75) [9], EG is the energy gain, Mcal/d (0.276×ADG×SBW^0.75) [9], and net energy for maintenance (NE_m) and net energy for gain (NE_g) of the diet are 2.11 and 1.44 Mcal/kg, respectively (derived from tabular values based on the ingredient composition of the diet [10]. The coefficient (0.276) is based on a mature weight of 113 kg for Pelibuey×Kathdin male lambs [3]. Dietary NE was estimated by means of the quadratic formula: x=(-b-b2-4ac)/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].

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 cross-sectional 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 foresaddle, 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 dissection 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 summation of all visceral components (stomach complex+small intestine+large intestine+liver+lungs+heart), including digesta. The stomach complex was calculated as the digesta-free 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.

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). Treatment 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.