Introduction
Ruminants, classified in the order Arteriodactyla and suborder ruminantia, are very important domesticated animals for the human society. Approximately 155 species of ruminants can be found around the globe but only about 6 of them are domesticated, cattle, sheep, goats, buffaloes, reindeer and yaks (Van Soest, 1994). Ruminants are different from all other mammals because of its digestive anatomy composed by four stomach compartments (reticulum, rumen, omasum and abomasum). Another unique characteristic is the interaction between animals, plant and microorganisms present inside the gastrointestinal tract resulting in a symbiotic relationship through gastro-enteric microbial fermentation. Plants consumed by ruminants are utilized as substrates by the microorganisms and the products from fermentation and microorganisms provide energy and protein to the host animal. Animal products such as milk and meat have always been an important component of human diets, therefore technologies to enhance production efficiency and increase economic return for producers are important. Ruminant nutrition research has focused on strategies to improve animal growth performance and carcass quality. Different feed additives and mineral supplementation strategies have been used to influence several characteristics on ruminants and some are used to modify growth. Ractopamine Hydrochloride is one of them, and consists of a metabolic growth modifier used to increase animal performance and has been used in cattle legally since 2003 in the United States of America (Gruber et al., 2007). Therefore, utilization of feed additives including ractopamine in association with Zinc and other minerals should be evaluated and validated considering protein synthesis effect and also economical feasibility before any decision-making.
Mineral Supplement utilization in USA confined cattle Inorganic elements known as minerals are detrimental to animal health and production performance. Some minerals are considered essential and others non-essential, nonetheless they play important roles nutritionally and biochemically for those animals if consumed in ideal amounts (NRC, 2005). However, negative effects on performance may be observed if mineral supplementation present excess mineral, and also in some cases, death for toxicity may occur. Besides that, incorrect mineral supplementation may cause health issues and mineral residues in different tissues (NRC, 2005). Livestock research has been focused in several different areas, however mineral supplementation feeding strategies have been stagnated. Vasconcelos and Galyean, 2007, reported on their survey that feedlot nutritionists are utilizing NRC (1996) basal levels of mineral recommendation as a guideline, however majority of them exceed those levels in feedlots diets and some issues may be appearing recently. Past research results on zinc requirements necessary to meet optimal levels for different variables response measured range from 7 to 28 mg/kg DM. NRC (1996) recommend 30 mg Zn/kg diet for beef cattle, however a safety margin for that value should be considered. Underwood and Suttle (1999) reported minimum requirements for growth and normal zinc plasma values ranging from 10-14 mg Zn/kg, and majority of past research observed stagnation of performance response with zinc concentrations between 17 and 23 mg/kg DM. Some characteristics that should be considered when analyzing mineral metabolism in ruminants are sensitivity and specificity of the elements in feeds, bioavailability and mechanisms of action, toxicity levels and mineral association with different compounds.

Zinc Characteristics, Deficiency and toxicity in Cattle Zinc (Zn) present 30 as its atomic number and generally it is considered in the divalent state (NRC, 2005). As an electron acceptor, Zn is susceptible to form complexes with amino acids, peptides, proteins, and nucleotides. Its essentiality and importance was first described by Kielin and Mann (1940), when zinc was found necessary for the carbonic anhydrase activity. Later, the importance of zinc on protein synthesis was discovered on the zinc fingers protein domains and it was described by Vallee et al. (1991). Zinc deficiencies reported in the past are directly related with reduced growth and protein accretion, decrease in feed intake and feed efficiency, reproductive performance and negative effects on animal’s immune system (Mills et al., 1967; Perryman et al., 1989; Engle et al., 1997). Zinc toxicity is uncommon in cattle and its signs were observed after really high concentrations (between 500 and 900 mg/kg DM). Zinc toxicity on these levels was not lethal for the animals, however reduced gain, feed intake variations and decrease in feed efficiency were observed (Jenkins and Hidiroglou, 1991). Zinc Mineral Role in Protein Metabolism Additionally to critical roles on enzymatic roles throughout the mammalian body, zinc also plays a very important nonenzymatic role in protein synthesis and genes regulations. Rhodes and Klug (1993) presented the concept of zinc fingers, which consists of the pattern of amino acids folding on gene transcription of the factor TFIIIA, and those fingers then promote the DNA binding process in promoter regions of the gene. Therefore, zinc is essential for gene transcription and mRNA polymerase. β- adrenergic agonists in Livestock Livestock research has been focused in several different groups of feed additives with the objectives of enhanced production, maximizing profit, and improving feed efficiency. Beef cattle production research has shown interest in different metabolic modifiers that influence or modify growth rate and composition of growth (NRC, 1994). Ractopamine Hydrochloride is a β- adrenergic agonist, that can be defined as a repartitioning agent that redirects and increases nutrient flow from fat deposition towards muscle deposition (Ricks et al., 1984). Ractopamine hydrochloride, a phenethanolamine derivate also described as β- adrenergic agonist that chemically can be related to catecholamines, epinephrine, and norepinephrine (Bell et al., 1998), have been approved for use in beef cattle to enhance growth performance in feedlots. As part of a synthetic group of anabolic steroids, this compound generally increases protein accumulation, enhances growth performance, and may affect adipose tissue deposition, depending on the dose and diet by ractopamine interactions (Xiao et al., 1999; Abney et al., 2007). In 2003, the USDA approved Ractopamine hydrochloride for use in commercial beef cattle production in the United States. Subsequently, many studies have been performed to improve understanding about its effects on finishing cattle.
To understand the results of metabolic and performance studies, it is first necessary to know how the β- adrenergic agonists work (especially ractopamine hydrochloride). When ractopamine or other β- adrenergic agonists are administered to an animal, a physiological response occurs due to the binding reaction between β- adrenergic agonist and the β- adrenergic receptors. There are three types of β- adrenergic receptors, β1, β2 and β3. All types are generally present on most mammalian cells, with varied distribution depending mainly on given tissue and species. Therefore, differences in physiological response may be observed due to the large number of effects involving the role of β- adrenergic receptors, dietary factors, and animal characteristics (Mersmann, 1998). Animal species that are closer to the biological maximal growth rate, such as some swine and chicken breeds due to intensive selection for growth, may present less response than ruminants which may possess a higher potential to increase growth or simply have better response by particular β- adrenergic receptors (Mersmann, 1998).
The compounds known as β- adrenergic agonists are organic molecules that have the ability to bind to β- adrenergic receptors and start biochemical reactions that will result in different outcomes. The β- adrenergic agonist in discussion, ractopamine hydrochloride, is more specific and generally used to improve the performance of finishing animals (Abney et al., 2007). Byrem et al., 1998, reported protein accretion in vivo due to a direct response to the β- adrenergic agonist (cimaterol). This anabolic response was temporary, with a peak time occurring during the first 14 days. The response was considerably attenuated by 21 d of treatment. This phenomenon may be explained by the desensitization of the β-adrenergic receptor, due to β-adrenergic agonist duration of exposure (Hausdorff et al., 1990). This phenomenon have been studied data available confirm that depending on administration time and dose of β-adrenergic agonists, different responses are observed on growth performance and carcass characteristics such as weight gain, fat deposition and longissimus dorsi muscle throughout the time (e.g., Sainz et al., 1993, Williams et al., 1994). Desensitization of β-adrenergic receptors mainly affects two pathways: Gs protein and adenylyl cyclase (cAMP). These pathways catalyze cyclic adenosine phosphate (major intracellular signaling molecules) formation from ATP, leading to a plateau of the levels of cAMP after constant stimulation (Hausdorff et al. 1990). Two main different desensitization processes were described by Hausdorff et al. (1990), short or long-term, and both suggested a decrease in response by the receptors. The short-term desensitization is caused by the rapid attenuation of the adenylyl cyclase response that disappears in minutes after removal of the desensitization agonist and do not require new protein synthesis. Long-term desensitization is a more complex process that mostly requires new protein synthesis and may take several days for total recovery.

Early Research on Ractopamine Hydrochloride Fed to Livestock First starting in the late 1970’s with the first patents in mid-1980’s, β- adrenergic agonists have been studied intensively throughout the years. Several authors have reported results on utilization of ractopamine hydrochloride and other β- adrenergic agonist in livestock since 1983 with the majority of work done using swine as the study specie. This was because of the first objective of β- adrenergic agonist research, which was attempt to solve the problem of excessive fat deposition in the livestock. Early studies with β- adrenergic agonists utilized many different compounds such as clenbuterol, cimaterol, ractopamine, salbutamol, zilpaterol, etc. The majority of these studies reported divergent results due specifically to different compounds, animal genetic lines, and dosage of β-agonist. Baker et al. (1984) used clenbuterol in high-concentrate diets for lambs on three, 8- week treatment experiments. One of the studies showed no effect on weight gain, but an improvement in feed efficiency. No statistical performance differences were observed on the second experiment, although gain and efficiency were numerically higher for the treated groups compared to controls. The third experiment showed an increased rate of gain of 24.1% and improved feed conversion of 19.1% when compared to controls. Heavier carcasses and increased dressing percentages were observed in all experiments for treated animals receiving β-agonists compared to controls. Decrease in kidney and pelvic fat were also reported with a range of 10 to 34% less than the control. Increases of 25 to 45% in Longissimus dorsi muscle area were also observed for treated groups. Fat thickness decreased in one of the experiments by 37%. Veenhuizen et al. (1987) reported an increase in the rate of gain and feed conversion for pigs fed phenethanolamines for 10 days. Animals were harvested at an approximate equal weight and larger Longissimus dorsi muscle area was observed in treated groups compared to controls. Less fat depth on the 10th rib was reported. Phenethanolamines were considered effective for growth and improving carcass composition in pigs. Effects of ractopamine were studied by Anderson et al. (1987) in 8 trials with finisher pigs. Ractopamine increased nitrogen retention in a range of 12 to 19% when compared to controls and blood urea nitrogen was reduced by 10 to 13% for treated groups and digestibility was not affected. Animals receiving ractopamine showed increases in ADG, feed efficiency, Longissimus dorsi muscle area, dressing percentage, and a decrease in fat thickness on the 10th rib. Smith et al. (1987) found that ractopamine had some effects on specific genes that stimulate protein synthesis, explaining several results of increases in Longissimus dorsi muscle area. The same improvements and trends on performance and carcass characteristics were confirmed by Crenshaw et al. (1987) and Hancock et al. (1987). Different levels of ractopamine hydrochloride were tested by Watkins et al. (1990) on performance and carcass characteristics of finishing swine. Nine studies were conducted in different geographical areas of the United States and results showed an increase of ADG and feed efficiency in all of them. Dressing percentage, Longissimus dorsi muscle area, estimated fat-free muscle and dissected lean muscle were also improved for all treatments receiving ractopamine. In 1994, Williams et al. evaluated the impact of ractopamine on pig growth and carcass merit and these results confirmed that ractopamine improved ADG for both barrows and gilts. Pigs treated with ractopamine additionally had faster weight gain with less feed than non-treated groups. Maximum response for ractopamine was observed between test days 7 and 21. A plateau was reached and a linear decline in response was observed at this time. Ractopamine reduced carcass fat thickness at the 10th rib and improved Longissimus dorsi muscle area. Xiao et al. (1999) studied the effects of ractopamine at different dietary protein levels on growth performance and carcass merit in finishing pigs and reported an increase in ADG of 9% and feed efficiency of 14% for the high protein group compared to the control, however the low protein group did not differ from the control. Higher carcass lean proportion of 4.5%, increase of Longissimus dorsi muscle area, decrease in fat composition and fat depth on the10th rib, were observed in all ractopamine treatments independent of the dietary protein group. Present Research on Ractopamine Hydrochloride Fed to Beef Cattle After 2004, research data has been published on the effects of ractopamine on beef growth performance and carcass characteristics (e.g., Vogel et al., 2005; Van Koevering et al., 2006a, 2006b; Schroeder et al., 2005a, 2005b; Crawford et al., 2006; Laudert et al., 2005a, 2005b), and in general increase of body weight was observed for treatments that administered ractopamine against the controls with no ractopamine.
Gruber et al. (2007) conducted a study with different biological types of steers (British, Continental crossbred and Brahman) examining the effects of ractopamine hydrochloride on growth performance and carcass characteristics. Ractopamine was fed during the last 28 days prior harvest in a dose of 200mg/head daily to the treatment group and no ractopamine was offered to the control. No interaction between biological type and ractopamine was observed. Ractopamine improved ADG and G:F and did not affect DMI of steers agreeing with Laudert et al. (2005a) and Schroeder et al. (2005a). No effect on dressing percentage, fat thickness, KPH and yield grade was observed. Heavier carcass and larger Longissimus dorsi muscle area was reported for animals receiving ractopamine, these results were also observed by Laudert et al. (2005b), Schroeder et al. (2005b), Vogel et al. (2005), Van Koevering et al. (2006a) and Crawford et al. (2006).
Greenquist et al. (2007) evaluated various durations of ractopamine in finishing steers. Treatments had 0 or 200 mg/head daily and 28 or 42 days immediately prior harvest. Results showed that feeding 200 mg of ractopamine per head daily increased live BW, ADG and feed efficiency compared to control. Most of the gain response to the β- adrenergic agonist (87%) was observed for the 28 days treatment when compared to 42 days receiving ractopamine. Improvement of HCW was observed, but no differences on dressing percentage, 12th rib fat thickness, Longissimus dorsi muscle area, marbling score and calculated yield grade were reported. Similar responses were reported by Walker et al. (2006) when feeding 200 mg/head daily during 28 days for feedlot heifers.
Also in 2007, Abney et al. presented a study that analyzed the effects of ractopamine on performance, rate and variation in feed intake and acid-base balance in feedlot cattle. Treatments consisted of doses of 0, 100 or 200 mg/steer daily and durations of 28, 35 or 42 days prior harvest. No interactions between dose and duration were detected. As ractopamine dose increased, a linear response for live BW, ADG and G:F was detected agreeing with past research. For longer feeding durations, ADG had a quadratic response and tendencies for live BW and G:F were also observed. HCW was increased linearly with increases of dose. Agreeing with Greenquist et al. (2007), optimum response to ractopamine was observed within the first 35 days of feeding the compound with little response from 35 to 42 days. For acid-base balance and intake, no difference on urine pH, blood gas measurements, or rate of intake were observed. For carcass characteristics, animals receiving ractopamine presented larger Longissimus dorsi muscle area and decreased yield grade. Optimal performance was provided by the 200mg/head daily during 35 days prior to harvest.

Zinc - Ractopamine Hydrochloride Association for Feedlot Cattle Based on the importance of zinc on proteins synthesis, and the role of β- adrenergic agonists on protein accretion, the hypothesis that the association of feeding of β- adrenergic agonists with different sources and concentrations of zinc could enhance response on growth performance was created. Research with β- adrenergic agonists have shown several benefits to producers, packers, processors, consumers, and environment. Zinc research has shown some performance improvements, however, when excess, no difference can be observed between different sources. More efficiency in beef production presents a large area of future concern without affecting carcass quality, therefore different strategies of technology utilization should be take in consideration. β- adrenergic agonists may be used as an important practice if economically viable at the present situation and its association with zinc should be tested to confirm research expectations. Consumers may benefit from leaner products with less cholesterol and reduced calories. Land productivity will improve and increased nitrogen retention in animal tissue growth may result in less nitrogen excreted as waste to the environment. Intermittent data are not available for beef cattle, however data reported from swine research have shown improvements on growth performance and carcass characteristics.

Objective of Present Research The objective of the present research is to evaluate the association between zinc and ractopamine hydrochloride on beef cattle protein synthesis.

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