INTRODUCTION

The meat industry is continually looking for feed additives to promote fast and efficient growth of livestock, as well as to improve the carcass yield and quality of farm animals (^{Smith et al., 2020}). One feed additive used in sheep is zilpaterol hydrochloride (ZH), a β-adrenergic agonists (β-AA) used as growth promoter and improver of carcass characteristics of feedlot lambs (^{Brand et al., 2013}; ^{Macías-Cruz et al., 2013}).On the same manner ZH affect the ruminal microbiome of wethers (^{Dufy et al., 2018}). Activation of β- receptors in muscle and fat results in decreased lipogenesis increased lipolysis, and increased muscle accretion or the combination of these effects (^{Abney et al., 2007}; ^{Scramlin et al., 2010}; ^{Miller et al., 2012}; ^{Johnson et al., 2014}). Also, β-AA reduce the frequency and intensity of ruminal contractions (^{Leek, 2001}), which are important to digestion, and these growth promoters increase absorption in the digestive tract (^{McIntyre & Thompson, 1992}) and the amount of gram-negative bacteria species which are vital for fermentation (^{Walker & Drouillard, 2012}). These changes in ruminant digestion attributed to β-AA may lead to changes in VFA production. In addition, β-AA may influence the production of VFA directly to increase the efficiency of digestion and provide more energy to the animal.

Currently, the only β-AA approved for livestock in the United States is ractopamine HCl (RHCl), a β1 agonist, and ZH, a β2 agonist, (^{Delmore et al., 2010}; ^{Boler et al., 2012}). β-AA have been approved for use as a growth promoter in food-animal production in several countries including Mexico (^{Johnson et al., 2013}).

Because β-AA can promote skeletal muscle growth without increasing serum hormone concentration, both swine and cattle producers have broadly adopted this technology to increase meat production (^{Centner et al., 2014}). However, there is concern regarding the potential harm of β-AA for human consuming meat from animals treated with these growth promoters (^{Centner et al., 2014}). For this reason, China, the European Union, and Russia have constrained and banned the use of β-AA, and the importation of meat with detectable concentrations of ractopamine (^{Bories et al., 2009}).

In FDA analyzes for beta-agonist residues in porcine and bovine meat, positive results are rarely found, but even in such cases, the levels found are below the maximum residue limits established by the FDA itself and the International Food Code Commission for safe human consumption. At the same time, no foodborne illness or human side effects have been reported that are attributable to the use of approved beta-agonists in the production of meat products.

In Mexico, the use of β-AA as feed additive for livestock is allowed and the maximum residue limits have been established exclusively for cattle, contemplating 10 ng/g in muscle tissue, 12 ng/g in kidney, and 15 ng/g in liver. Therefore it was considered pertinent to ascertain the residues of β-AA in feedlot lambs. The objective of the present trial was to evaluate the effect of the time of supplementation of ZH on residues of this growth promoter in muscle, liver, and kidney of feedlot hair lambs.

MATERIALS AND METHODS

The experiment was carried out in the experimental area for small ruminants of the Veterinary school of the Autonomous University of Baja California, in Mexicali, Mexico. The climate in this region is arid dry with extreme temperatures, both in winter and summer (0 °C to 50 °C), and the average annual rainfall is 80 mm (^{INEGI, 2018}).

RESULTS AND DISCUSSION

The effect of the treatment on residues of ZH is shown in Table 2. Regarding muscle, a linear increase (P≤0.05) in ZH concentrations was observed as days of supplementation increased. There was a significant difference in ZH concentrations between the control and all other treatments.

Likewise, a linear increase (P≤0.10) was observed in ZH concentration in the kidney as the days of treatment increased; the control group differed compared to all other treatments. Regarding the liver, a linear increase (P≤0.10) of ZH residue concentration was observed as days of treatment increased. No significant difference (p>0.05) existed between the control group compared to all other treatments.

ZH has a minimum three-day withdrawal period before slaughter to ensure residues in the muscle tissue are eliminated before consumption (^{Merck, 2017}). ZH and other β- adrenergic agonists residues in feedlot produced farm animals can have high trade implications because markets outside the countries where ZH is legally permitted, must comply with ZH residues established by the importing country. In the United States, where the use of ZH is approved for administration to cattle only, maximum residues levels for ZH in beef liver and muscle are 12 and 10 ng/g, respectively. In the case of Mexico, the maximum residues of this growth promoter are 10 ng/g in muscle tissue, 12 ng/g in kidney, and 15 ng/g in liver (^{SENASICA, 2017}). In the case of ZH, residual concentrations in sheep have not been established, but there are reports on ZH concentration in lamb´s tissues. A study in sheep have shown that levels of ZH in muscle, liver, and kidney in lambs at different withdrawal times were 1.5 ng/g in muscle, 0.86 ng/g in liver, and 1.10 ng/g in kidney (^{Shelver & Smith, 2006}), which were higher than values found in the present study. It is important to note that the observed levels of ZH in lambs were lower than those established by ^{SENASICA (2017)} for cattle. Also, the tissue in which the greatest amount of CZ residues were found was the kidney, which is because ZH is a highly soluble substance in water, therefore its excretion is mainly through the urinary system. In fact, a friendly rapid detection method for ZH is the determination of this growth enhancer in the urine (^{Shelver & Smith, 2018}).

For other ruminants other than cattle and other livestock, ^{SENASICA (2017)} establish that animal tissues must present a “not detected” quantity of ZH (zero tolerance limit). Under this criteria, ZH could not be used in sheep in Mexico, which is contradictory because cattle meat can have some ZH residues. It is important to mention that this limit was established basically because of the lack of studies on residues of ZH in sheep meat. Therefore, more studies must be carried out to confirm whether these amounts are dangerous to humans and, if applicable, establish maximum permissible limits that guarantee the health of the consumer.

CONCLUSIONS

The observed residual concentrations of zilpaterol hydrochloride in muscle, kidney, and liver of hair lambs increased linearly with the days of administration of this growth- enhancer. The residues of zilpaterol hydrochloride in various tissues of hair lambs were lower than those established by SENASICA (Mexico) for cattle. Given that this institution has not established formal limits for the use of use zilpaterol hydrochloride in sheep, these results are a good point of reference to establish maximum permissible limits for feedlot hair lambs.