|dc.description.abstract||Lamb meat export is a multi-billion-dollar New Zealand industry. Therefore, strategies to maintain and improve quality have immediate commercial benefit. Consumers judge quality by colour, tenderness, and flavour, all of which are affected by ultimate pH (pHu). Ideal meat pHu is 5.5 and is influenced by muscle glycogen concentration. Insufficient muscle glycogen at slaughter results in high pHu (>5.8) which results in tough meat, dark colour, and shorter storage life. The decrease in meat pH post-slaughter is caused by the build-up of H+ ions as a consequence of anaerobic glycolysis of glycogen for ATP formation. Physical stress depletes muscle glycogen, prompting consumers to conclude animal mistreatment. However, stress, does not necessarily equate to abuse as all animals experience some level of pre-slaughter stress and any negative effects depend on duration, type, intensity, and susceptibility. Studies have focused on post-farm-gate stressors (e.g. transport and lairage) but less clear is the role of on-farm stressors on the incidence of high pHu. The aim of this study was to investigate the variation of muscle glycogen of lambs on-farm, examine the use of heat shock protein 20 (HSP20) as a marker for physiological stress, and determine any relationship between residual glycogen level and meat quality attributes such as water binding capacity (WBC), drip loss, microbial abundance, fat, protein, and water content.
Two observations were conducted. Firstly, 60 animals from 2 farms with different production systems: wethers (castrated males) were raised with ewes in Farm 1, whereas, intact rams, wethers, cryptorchids, and ewes were all together and introduced at different times to the group in Farm 2. These animals were biopsied two months before slaughter, a month before, and post-rigor. On-farm, large inter-individual variation in muscle glycogen was observed in both farms but no significant difference in true glycogen between farms. Muscle glycogen levels seemed to fluctuate from one biopsy to another, contrary to previous belief that it remains constant throughout. There was no significant difference in HSP20 between farms and no correlation between muscle glycogen and HSP20 was observed. Post-rigor, 66% of carcasses from Farm 2 had high pHu (≥5.8), compared to none from Farm 1. This may be due to stress brought about by exaggerated behaviours of rams (e.g. aggression and chasing) during close confinement at lairage. However, more observations on-farm and at lairage are needed. Lastly, 69 loin samples of varying levels of residual glycogen were tested for drip loss, (WBC), fat, protein, and microbial content a day post-rigor (Day 1) and again after eight weeks (Week 8). Results showed no correlation between residual glycogen content and any of the parameters tested at either time points. A decrease in total glycosyl units at Week 8 was observed but no decrease in true glycogen levels. This was likely due to microbes use of free glucose as a food source.
To conclude, the results of this study suggests that the co-habitation of intact rams and ewes on-farm causes stress, which is compounded by the close confinement at lairage, leading to high pHu of their carcasses. It is, therefore, recommended that, although intact rams have higher saleable weight, these rams be castrated before mixing with other rams and ewes to avoid high pHu and, hence, provide good quality meat and less stressed animals (better animal welfare). The fluctuation in muscle glycogen levels over time does also need closer scrutiny.||