Summary
This study is a meta-analysis of the effects of the beta-agonists (β-AA) zilpaterol hydrochloride (ZH) or Zilmax and ractopamine hydrochloride (RAC) or Optaflexx on feedlot performance, carcase characteristics of cattle and Warner Bratzler shear force (WBSF) of muscles. It was conducted to evaluate the effect of the use of these agents on beef production and eating quality and to provide data that would be useful in considerations of the effect of these agents on beef in Meat Standards Australia evaluations.
A comprehensive literature search was conducted using more than four databases, examination of literature in retrieved papers and contact with authors of papers. These papers were examined for suitability for inclusion in meta-analysis using predetermined study quality assessment criteria by two experienced examiners. More than 30 papers were identified for both ZH and RAC and these contained more than 50 comparisons. These data, however, provided challenges because the unit of evaluation was not always clear, nor consistent. A number of papers contained pseudo-replicated studies (where the effective number of observations was one per group), some were evaluated at the pen level, some at the animal, carcase or muscle level. The structure of data in studies was carefully evaluated in this study. Data were extracted from more than 50 comparisons for both agents and analysed using meta-analysis and meta-regression. The sensory information available did not clearly identify the unit of analysis and some studies were purposively sampled making these unsuitable for meta-analysis.
Both ZH and RAC markedly increased average daily gain, resulting in a higher final weight before slaughter for treated cattle and increased hot carcase weight (HCW), dressing percentage and longissimus muscle area (LMA). The ZH treatment more markedly increased HCW than final feeding weight as assessed either by effect size or weight mean difference. In particular, ZH dramatically increased outcomes in some cases by more than 2 standardised mean differences (SMD). For ZH the fat thickness, marbling and kidney, pelvic and heart fat percentage and USDSA yield grade were decreased by treatment. These effects were also large (approaching or exceeding a standardised mean difference (SMD) of 0.8) and marbling was markedly reduced. It appears that ZH has a very large effect in partitioning of nutrients to muscle and away from other tissue pools. In general, the effects of RAC were similar, but less pronounced than ZH on feedlot efficiency. The effect of RAC treatment was not significant on fat thickness, marbling or kidney, pelvic and heart fat percentage. However, USDA yield grade was significantly decreased.
Treatment with ZH markedly increased WBSF by 1.2 standard deviations and more than 0.8 kg, while RAC increased WBSF by 0.43 standard deviations and 0.3 kg. In the case of ZH, the results were influenced by the unit of evaluation, i.e. pen or animal, cases and muscle. Reasons for this are not entirely clear, but significant differences existed in the raw means of the groups defined by unit of evaluation. These findings were consistent with similar observations of the effect of unit of evaluation for ZH studies on LMA and marbling. In all cases (LMA, marbling and WBSF), the pen studies showed a more pronounced effect of treatment than individual studies and the similarly observed effects in unweighted evaluations of the raw means of the effects suggest a biological, rather than statistical methods basis for the observation. These effects were not observed for the RAC trials. The ZH results for WBSF were also influenced by the aging of beef. Increased aging reduced the effect size of the increase in WBSF. All ZH trials showed an increase in WBSF.
These findings support the previously identified physiological roles of the β-AA and provide a strong evidence for producers and others to examine and consider the effects of ZH and RAC on beef cattle production. Once these results have been critically reviewed by others, they can be immediately applied and used to formulate strategies to make best use of agents that markedly improve the efficiency of production.
This work provided critically needed information on the effects of ZH and RAC on production, efficiency and meat quality. The goals of providing more precise and robust estimates of the effects of the β-AA on efficiency of production and carcase quality measures were achieved. Meta-regression identified that the method of feeding cattle may influence responses to ZH and identified that the process of aging can reduce the effects of ZH on WBSF.
