The influence of acute stress and feed deprivation on the meat quality and intermediary metabolism of Australian lamb

Understanding the impact of acute and chronic stress incurred during the pre-slaughter period is important to maximise lamb meat quality and yield as well as maintaining high levels of animal welfare. This thesis quantified the level of acute and chronic stress at slaughter utilising plasma indicators reflecting acute stress, feed deprivation, dehydration, muscle activity and damage and then analysed their association with meat quality and carcass yield traits. Finally, the impact of genetic selection for increased carcass yield on the metabolic response to feed deprivation was examined under resting and commercial slaughter conditions. This study utilised 2877 lambs of the Meat and Livestock Australia genetic resource flock from sites in Katanning, Western Australia and Armidale, New South Wales over a two year period. Production factors including site, year and kill group had the largest impact on all stress indicators at slaughter, the majority of which were elevated above normal range at slaughter. There was also considerable variation in the concentration of many indicators (particularly those reflecting fat turnover) which are likely to reflect differences in pre-slaughter management, on-farm nutrition, feed and water deprivation and various acute stressors. In addition, levels of metabolic indicators at slaughter were found to relate to lamb carcass phenotype, with leaner lambs demonstrating greater fat mobilisation and lower rates of glycogen turnover at slaughter. Similarly, site, year and kill group had the largest impact on ultimate pH, Warner-Bratzler shear force (WBSF) and carcass yield parameters in lamb. Importantly, ultimate pH in lamb loin was positively associated with slaughter levels of plasma glucose and lactate, indicating that acute pre-slaughter stress has an impact on glycogen turnover and increases ultimate pH in lamb. There was also a positive association between WBSF and kill-order, suggesting that lambs killed later within a kill group may have a greater duration of exposure to stress underpinning the link between kill-order, acute stress and the resultant decrease in WBSF tenderness. Feed deprivation up to 48 hours under resting conditions resulted in lower glucose concentrations and a greater non-esterified fatty acid (NEFA) and β-hydroxybutyrate (BHOB) response in Merino sired lambs compared to Terminal sired lambs. Selection for increased genetic growth and leanness also altered the NEFA, BHOB and glucose response to feed deprivation. Feed deprivation up to 48 hours under commercial slaughter conditions elicited a similar response, demonstrating that adipose tissue is highly sensitive to feed deprivation and stress in Merino genotypes. An important finding was that the NEFA response to feed deprivation was up to 35% higher under commercial slaughter conditions, highlighting that acute stress is a large driver of fat turnover in the pre-slaughter period. Feed deprivation did not affect loin ultimate pH, loin intramuscular fat content or tenderness but had a negative impact on ultimate pH of the M. semitendinosus in Merino lambs. Feed deprivation beyond 36hrs was also found to cause a 3% loss in carcass weight in both Merino and Terminal sired lambs, which on an industry level, could affect the profitability of lamb producers and processors. Further work is required to understand methods of mitigating acute pre-slaughter stress in order to minimise high ultimate pH and reduced tenderness. In addition, it was demonstrated that Merino genotypes have a significantly higher fat turnover in response to feed deprivation and stress. This indicates that a shorter duration of curfew and lairage may minimise carcass losses and improve meat quality.