Abstract
The quantity of glycogen stored in muscle at slaughter is a major determinant of meat quality and profitability. Muscle glycogen levels below ~0.6% can result in high ultimate pH (>5.7) of beef (Ferguson el al. 2001) leading to dark cutting or dark firm dry (DFD) meat. Meat which is DFD has a darker colour, reduced shelf life, bland flavour and variable tenderness (Ferguson el al. 2001). This condition significantly reduces the value of a carcase since an elevated ultimate pH (>5.7) makes it ineligible for grading by Meat Standards Australia and premiums will not be rewarded. Low muscle glycogen pre-slaughter is caused by low glycogen storage as a result of low metabotisable energy intake Knee el al, 2004) and/or stress between mustering and slaughter. Stress stimulates the release of endogenous adrenaline, causing the mobilisation and depletion of muscle glycogen stores. When glycogen stores are low, stressors have the greatest influence on the incidence of DFD meat.
The demand for more profitable, high yielding carcasses by the beef industry has increased selection for animals with more muscle and less fat. Animals that have increased muscle hypertrophy generally have more fast-glycolytic type IIX myofibres (Wegner et al., 2000). The adrenaline responsiveness of muscle with more fast- glycolytic fibres is likely to be greater due to their increased glycolytic and glycogenolytic capacity (Wegner et at. 2000), possibly extenuating the problem of DFD meat in high muscling selection line cattle. Therefore, we hypothesise that selection for muscling will decrease muscle glycogen storage and increase the response of muscle to adrenaline.