Grazing on irrigated pastures for finishing sheep is a feeding strategy that is widely used in different parts of the world, including Mexico, because of the high prices that present both oilseeds and cereal grains on the market. In regions where the alfalfa (Medicago sativa) is produced and the availability of water is abundant, it is common the planting of this forage as a monoculture or mixed with grasses for grazing sheep¹. The alfalfa is characterized as a forage of high nutritional value, since its digestibility and protein levels (18 to 20 %) are high². However, growth rate, feed efficiency and some carcass characteristics in lambs under this grazing system are lower than those usually seen in feedlot lambs with grain-based diets³.

Some recent studies suggest that addition of zilpaterol hydrochloride (ZH) in finalization diets for feedlot lambs, improves productive performance^4,5 and some carcass characteristics of economic importance (i.e. weight and carcass yield, Longissimus dorsi muscle area)^6,7. However, the use of ZH in grazing finished lambs has been sparsely studied⁸, and the available information is null on irrigated pastures of alfalfa. Supplementation with ZH in lambs that graze alfalfa could be a strategy to improve their productive performance and carcass characteristics. Alfalfa has a high content of rumen degradable protein (~ 25%)², and some studies have reported that the use of sources of non-protein nitrogen in the diet of ruminants Fed ZH, could enhance the positive effect of this agonist on the growth and carcass yield of fattening sheep⁵ and steers⁹. This, considering that the agonists, apart from stimulating the muscle development¹⁰, also stimulate the growth of the population and microbial activity in the rumen⁹. Therefore, the objective of the present study was to evaluate the effect of ZH on the growth and carcass characteristics of lambs finished in an alfalfa pasture with supplementation.

Standards adopted in Mexico for care (NOM-051-ZOO-1995) and the humanitarian slaughter of animals (NOM-033-ZOO-1995) were used to handling lambs. The study was conducted from October 28 to December 15, 2011, at the ranch "Galvan", located in the Valley of Mexicali, Baja California, Northwest of Mexico (32° 25' N and 115° 06' W). Weather conditions during the experimental period were very fluctuating with the presence of rainwater into the final days (44 mm). The averages for temperature, relative humidity and solar radiation were 12.7 °C (1.4 to 33.8 °C), 57.7 % (12.5 to 95.3 %) and 11.5 W/m² (0 to 62.4 W/m² respectively.

The experiment was developed using 40 Dorper x Pelibuey entire male lambs, with an average age of 4.5 mo accustomed to grazing pastures of alfalfa since the first month of age. They were injected with 1.0 ml of vitamin A-D-E (Vigantol, Bayer, of Mexico City, Mexico) and treated with 0.5 ml of invermectin (Invermectin, Sanfer Laboratory, city of Mexico) 15 d before the experiment (adaptation period). In general, in this period, all lambs were adapted to grazing on the schedule from 0700 to 1500 h, and the rest of the day to remain in a pen (10 x 10 m), provided with shade and drinking. Additionally, lambs were adapted into individual pens, where they consumed 450 g/head/d of supplement divided into morning and afternoon, and thus, ensure a 100 % intake. The supplement was formulated with contained 83 % ground wheat, 10 % soybean meal, 5 % molasses, 0.5 % orthophosphate, 0.3 % dicalcium phosphate and 1 % common salt (Table 1).

After the adaptation period, individual weight of all the lambs was recorded to select 24 animals of similar body weight (BW) and adaptation level to the supplement. A lamb that come easily to the individual pen and consumed 100 % of the supplement without any problem of diarrhea was considered adapted to the process of supplementation.

Selected lambs (BW= 25.0 ± 0.24 kg and age= 5 mo) were assigned to 1 of 2 treatments (n= 12) under a randomized complete block design, where blocking factor was the initial BW. The treatments consisted in offering 450 g/head/d of supplement mixed with 0 (control) or10 mg/d/head of ZH (Zilmax, Intervet Mexico, DF, Mexico) during 30 d. To ensure the ZH intake in the corresponding group, 75 g of the Zilmax commercial product was mixed with 10,725 g of concentrate; 30 g/d/lamb of this mixture was offered in the morning and the remainder concentrate was given immediately, corresponding to the morning feeding. It is noteworthy that lambs were introduced in individual pens while taking the supplement. In general, the lambs were fed 60 % of the supplement before and 40 % after pasture. In the case of the group supplemented with ZH, the agonist was withdrawn two days before the end of the test. Water was offered ad libitum. The individual BW of lambs at the beginning, middle and end of the experimental period was recorded to calculate daily and total weight gain.

Alfalfa pasture was 1.5 yr of established and a dry matter production (DM) of 2,569 kg/ha considering 5 cm of remnants. Before starting the experiment, DM intake that would have the group of lambs during the 30 d it was calculated considering a daily intake per animal of 4.5 % of their BW. Thus, 931.5 kg of DM was determined to be the requirement of alfalfa to feed the lambs throughout the experimental period. Based on this information, three paddocks of 1,209 m² were delimited with sheep mesh (310.6 kg with remnants of 5 cm) each, considering an occupation period of 10 d per paddock. To ensure that the three paddocks were in the same phenological state (beginning bloom) at the time of start grazing, each pasture was watered and fertilized (6 kg of urea) at d 35 and 10 before joining the lambs to graze.

Alfalfa and supplement samples collected weekly in paper bags were taken to the Nutrition laboratory of the Instituto de Ciencias Agricolas (ICA), of the Universidad Autonomous de Baja California (UABC). They were dried in a forced air oven at 60 °C for 48 h, and stored for later bromatological analysis by the AOAC methodology¹¹ and Van Soest et al¹². The amounts of total digestible nutrients¹³ and digestible energy (DE)¹⁴ were calculated in order to know the metabolizable energy (ME) according to NRC¹⁴.

After the grazing test, all lambs were fasted 12 h and slaughtered by the disgorgement method in the ICA-UABC meat laboratory. The blood was collected in plastic bags and weighted. In addition, skin, head, foot, testicles and pelvic-renal-heart (KPH) fat and visceral organs (heart, lungs, liver, kidneys, spleen, rumen and small intestine) were removed from the body to register their weights, as well as the hot carcass weight (HCW). Subsequently, the carcass were chilled at 4 °C for 24 h to register the following features: cold carcass weight (CCW), carcass length, thorax depth, leg length and perimeter, Longissimus dorsi muscle area (LDM) and fat thickness⁴. Between the 12^th and 13^th rib, carcasses were transversely cut to measure with a grid point (64 mm) the LDM area, as well as the fat thickness with a vernier scale. It is worth mentioning, that a person entered in the cold room at 45 min and 24 h post-mortem to measure pH of the LDM using a portable potentiometer, which was equipped with a penetration electrode (Hanna Instruments, model HI 98140, Woonsocket, RI, USA). Finally, carcasses were divided into wholesale cuts⁴.

The forequarter was divided into neck, ribs, loin and shoulder, while the hindquarter on legs, plain loin and the back skirt. Weights of wholesale cuts and KPH fat were expressed as a percentage of HCW. Hot carcass weights and visceral organs were expressed as a percentage of the slaughter weight.

All variables were subjected to a variance analysis under a randomized complete block design using the GLM procedure of the SAS statistical software¹⁵. Comparisons between means were performed with t-student test to a α= 0.05.

The growth of lambs in the alfalfa pasture was not affected (P>0.05) by supplementation with ZH (Table 2), noting that, on average, both groups had a weight increase of 5.4 ± 0.5 kg after the 30-d trial. Similarly, in Pelibuey lambs finished by 30 d in Dichantium aristatum grass, dietary supplementation of 10 mg/head/d of ZH did not increase total and daily weight gain⁸. Some studies also found evidence that in lambs fattened in feedlots with grain-diets, 6.7 mg¹⁶ or 10 mg¹⁷ of ZH did not improve growth; however, some research on lambs in pen indicates a positive effect of this β-agonist on the final BW, weight gain and feed efficiency, without altering DM intake^5,18,19. Therefore, these results suggest that zilpaterol may improve the growth of the hair sheep breeds, only when they are finished in pens with diets rich in grains, but not under grazing conditions.

Carcass characteristics (Table 3) and wholesale cut yields (Table 4) were similar (P>0.05) between lambs fed with or without ZH. No studies were found that evaluated these characteristics in lambs supplemented with ZH under grazing conditions. However, the results of this research are contrary to expectations, since most of the works done in finishing feedlot lambs found a positive effect of zilpaterol (variable doses of 6.7 to 17.5 mg/head/d) on carcass characteristics,^16,17,19 and wholesale cut yields^4,7,17 of economic importance (HCW, CCW, dressing percentage, LDM area, and percentage of ribs and legs).

Possibly, the lack of effect of zilpaterol on growth, carcass characteristics and wholesale cut yields, was due to failures in the body transport of ZH molecules, or to negative changes in the β₂-adrenergic receptor at striated muscle level due to environmental factors (temperature, exposure to direct sunlight, high activity, stress by predators, other)^6,10. Thus, coupling between molecules of ZH and receptors did not occur properly, causing that the β₂-agonist did not promote protein synthesis and avoid proteolysis at muscle level, and also, it would reduce lipogenesis and increased lipolysis in fat tissue²⁰. This situation did not allow the ZH increase muscle mass formation in parts of the body such as legs and loin, which was reflected in a null effect of this agonist on the weight gain, HCW, dressing percentage, LDM area, body fat, wholesale cut yields, others. Reports show that the mechanism of action in vivo of β₂ receptors is extremely sensitive to other secondary events that cause endocrine or physiological responses within the target tissue¹⁰.

Administration of ZH reduced (P<0.05) the percentage of blood, skin and heart in the lambs, without effect (P>0.05) on the percentage of other visceral organs (Table 5). The percentage of liver tended to decrease (P=0.08) by effect of ZH. Results published in relation to the effect of zilpaterol on the development of visceral organs are contradictory. So far, no studies had reported a reduction in the percentage of blood, skin and heart by feeding ZH in lambs. Some researchers¹⁷⁾ reported only an increase in the percentage of peritoneum and head by addition of 10 mg/ head/d of zilpaterol; although subsequently other authors⁴ determined a contrary effect on these same components of the body using the same dose. Meanwhile, in other study²¹ indicated that administration of 7.4 mg/head/d only reduced the percentage of rumen, liver and visceral fat. It is noteworthy that by using other β-agonists such as L-644, 969²², reported a reduction in the weight of heart and liver in lambs. Possibly, the reduction in the weights of those organs is due to greater wear and tear, as a result of an increase in the metabolic activity by the action of β-agonists, which would explain the decrease in the percentage of heart weight in the present study. Recently, it was reported a reduction in the skin weight of goats when they were supplemented in pen with ZH for 42 d²³, which coincides with results from this report.

In conclusion, the ZH at a dose of 10 mg/ head/d does not alter the growth and carcass characteristics in entire male lambs fattened in alfalfa pasture plus nutritional supplement. However, under these conditions, apparently the ZH reduces the amount of circulating blood and the weight of the heart. In general, results of this study suggest not supplement with zilpaterol hydrochloride to lambs fattened during the finishing stage under grazing conditions, at least in alfalfa grasslands.