The impact of different Hormonal Growth Promotants on the eating quality of steer carcasses finished in a feedlot or on pasture

Abstract

<p>Hormonal Growth Promotant (HGP) implants have been used to increase productivity and profitability of beef production for over 40 years. The use of HGPs can improve average daily liveweight gain by 10-30% and feed conversion efficiency by 5-8%. Whilst HGP implants have a positive impact on beef production, their negative impact on beef eating quality has been an important industry and research topic over the past two decades.</p> <p>The Meat Standards Australia (MSA) beef grading model predicts muscle by cooking method eating quality outcomes, based on inputs from production, carcass and processing variables, which have been correlated to untrained consumer sensory scores. A body of Australian research conducted in the mid-2000s identified that the use of HGP implants, results in a decrease in consumer sensory scores and objective measures of tenderness, such as shear force. This resulted in a common HGP adjustment introduced to the MSA model in 2008 to account for the negative impact on eating quality. However, there was concern from industry stakeholders that a single HGP adjustment in the MSA model may not account for the different HGP formulations used in Australia, particularly oestradiol only (OES) formulations used in northern Australia for pasture growing and finishing cattle. This formed the basis of this research and the hypothesis that OES implants have minimal or no impact on eating quality when assessed by the MSA consumer sensory protocols.</p> <p>The feedlot experiment allocated 300 cross-bred steers to three HGP treatment groups; untreated control (CON-100-F), 100 day oestradiol only HGP implant (OES-100-F) and a combination trenbolone acetate and oestradiol HGP implant (TBA+OES-100-F), which were finished on a grain ration for 73 days. The use of a stronger anabolic formulation (TBA+OES100-F) significantly increased live weight gain but had negative impacts on carcass traits such as ossification and marbling, when compared to CON-100-F treatment. Similarly, the use of the TBA+OES-100-F implant resulted in a marked decrease in consumer sensory eating quality points (MQ4) of the <i>m. longissimus lumborum</i> (LL, ~8 MQ4 points), as well as an increase in shear force, though ageing for 35 days mitigated much of these impacts. The OES100-F treatment significantly increased liveweight gain when compared to the CON-100-F treatment and had moderate effects on carcass traits. The OES-100-F treatment impact on consumer sensory scores and shear force in the LL, was not different from that of the use of CON-100-F at five or 35 days ageing. However,<i> m. gluteus medius</i> (GM) samples at 35 days ageing were significantly different among treatments for consumer sensory scores. The HGP treatment impact was greater in the LL than in the GM, which supports previous findings that HGP implants have the greatest impact on muscles that have the greatest ageing potential. There was an increase in the inhibitor of post-mortem proteolysis, calpastatin, in both HGP treatments, and the TBA+OES-100-F treatment was significantly different from the CON-100-F. This inhibition of post-mortem proteolysis assisted in explaining part of the reduced eating quality. Further analysis of a subset of the carcasses and muscles, indicated that the TBA+OES decreased desmin degradation post-mortem in the LL. The OES-100-F treatment only decreased desmin degradation in the GM, which possibly explains the consumer sensory score impact at 35 days ageing. None of the HGP treatments had an impact on shear force for the GM, m. infraspinatus (IS), or m. semitendinosus (ST), or the LL sarcomere length or collagen content in the LL from these subsets of animals.</p> <p>For the pasture experiment, 200 <i>Bos taurus/Bos Indicus</i> composite steers were allocated to two HGP treatment groups; untreated control (CON-400-P) or a 400 day oestradiol only HGP (OES-400-P), and finished on pasture for 386 days. The OES-400-P treatment increased liveweight gain, carcass weight and ossification when compared to the CON-400-P treatment. The HGP treatment had a large negative impact on consumer sensory scores in the LL (~10 MQ4 points) and increased shear force. This negative impact of the OES-400-P treatment was reduced by ageing for 35 days, though still significantly different from the CON-400-P treatment. The OES-400-P impact was greater in the LL, when compared to the GM. The OES-400-P treatment increased calpastatin activity, which explained part of the negative impact on eating quality. On further analysis of a subset of carcasses, the OES-400-P treatment had no impact on shear force, desmin degradation, sarcomere length or collagen content, in the LL and<i>m. biceps femoris</i> (BF). This means that consumers in the pasture experiment detected a significant impact on eating quality, which could not be explained by reduced desmin degradation in this subset of animals</p> <p>The MSA model uses both the common HGP adjustment combined with the indirect effects on carcass traits such as marbling, ossification and hump height, to account for differences in HGP formulation impacts on eating quality. To test the accuracy of the MSA model for predicting the eating quality of different HGP formulations, residual consumer MQ4 scores were generated (predicted MQ4 minus actual MQ4) for all muscle samples across the two experiments and ageing periods. The MSA model accurately predicted (5/18), or under predicted (11/18), the majority of the HGP treatment samples. The majority of the underprediction was predominately for 35 day ageing and the GM HGP treatment samples. This could be seen as a safeguard for the consumer, as it would ensure that the beef eating experience was as expected or better than expected. Some over-prediction was observed in the CON-100-F and TBA+OES-100-F treatment samples, which may be due to factors such as the genetic propensity for the cross-bred cattle used in this experiment to have tougher LL muscles than the population in the MSA model. As a result, it was decided by the MSA Pathways Committee, an independent scientific reference group, that the use of a common HGP adjustment, combined with the indirect effects on carcass traits, provided a reasonable prediction of eating quality of different HGP formulations. There was an intention to further examine the HGP muscle ageing algorithms in the MSA model, as it was not fully accounting for these improvements. Similarly, consumers scored GM samples higher than the MSA model accounted for. Additional research GM sample data generated in future eating quality experiments may increase the accuracy of prediction GM samples in the model.</p> <p>The results of these experiment concluded that different HGP formulations had different impacts on eating quality when steers were finished in a feedlot or on pasture. However, the MSA provides a reasonable prediction of eating quality for these different formulations through a common HGP adjustment, combined with the different impacts on carcass traits. One of the mechanisms that explains the negative HGP impact on eating quality is through an increase in calpastatin, and therefore a decrease in muscle degradation post-mortem.</p>