Graham Gardner

Dark cutting meat is a major economic problem within the cattle and sheep meat industries, and is linked to a shortage of muscle glycogen at slaughter. This study investigated various nutritional aspects involved in the regulation of muscle glycogen metabolism in sheep and cattle. Initially, a repetitive muscle biopsy protocol was developed such that changes in muscle glycogen concentration over time could be measured. An exercise model for cattle (similar to an existing model for sheep) was then developed facilitating the controlled depletion of muscle glycogen such that rates of glycogen repletion could be measured. This model involved trotting cattle at 9 km/h for five 15-min intervals, with 15-min rest between each interval, depleting muscle glycogen by approximately 50%. The model was then used to assess the impact of various aspects of nutrition on muscle glycogen metabolism, and in a number of experiments the results were supported by data collected during commercial slaughter. The first experiments carried out in sheep and cattle, compared the rate of glycogen repletion in animals maintained on roughage and cereal grain diets. In cattle there was a positive linear relationship between metabolisable energy (ME) intake and rate of muscle glycogen repletion following exercise. Muscle glycogen repletion following exercise in sheep was independent of ME intake, with animals on all diets repleting at the same rate, suggesting that physiologically sheep place a greater emphasis on recovery of the glycogen depot following stress. An experiment was then run to determine the influence of dietary nitrogen on muscle glycogen metabolism. Cattle were maintained on grain based diets, formulated with 1, 2 or 3% urea, or 17, 35, 50 or 70% lupin grain. Following exercise there was no difference between any of the dietary treatments, however at slaughter muscle glycogen concentration demonstrated a negative linear relationship with increasing levels of urea inclusion in the diet. There were no differences between lupin treatments, thus the contrasting dietary urea effect may be associated with an increased rate of ammonia absorption from the rumen. Therefore, diets containing high levels of readily hydrolysed rumen degradable nitrogen may lead to reduced muscle glycogen concentration at slaughter. The next trial investigated the impact of supplemental grain on basal muscle glycogen concentration in cattle previously maintained on roughage diets. After 6 days of supplementation, cattle receiving 6 kg/head/day of barley grain demonstrated the most marked increase in muscle glycogen concentration, however after 16 days all supplemented groups had equilibrated to a similar level of muscle glycogen concentration, all at a higher level than the control. This suggests that over a longer period, 2 kg/hd/day of barley grain can be as efficient as 6kg/hd/day for promoting higher muscle glycogen concentrations. The use of carbohydrate supplements added to the water were trialed in sheep and cattle in an attempt to provide water-borne substrate for muscle glycogen repletion. Initially, sheep were drenched with various carbohydrate supplements showing that a combination drench of 70% glycerol and 30% propylene glycol produced the greatest hyperglycaemic response. Cattle and sheep were then offered the supplement at the rate of 3.5% glycerol and 1.5% propylene glycol, increasing the rate of muscle glycogen repletion following exercise, although not affecting muscle glycogen concentration at slaughter. The supplement increased water intakes during lairage, and reduced pHu in both species. Magnesium oxide was supplemented in the feed of sheep and cattle, with an aim to reduce glycogen loss at slaughter, and to increase the rate of repletion following exercise. In sheep, the rate of muscle glycogen repletion was increased following exercise, and muscle glycogen concentration was increased at slaughter, however in cattle there was minimal response. Thus MgO appears to be a viable option for reducing glycogen loss in sheep at slaughter. Lastly, the metabolism of muscle glycogen in Merino lambs was compared to first and second cross Merinos at slaughter. Increasing proportions of Merino genetics were associated with increased muscle glycogen loss during commercial slaughter. This suggests that Merino’s are more sensitive to stress than cross bred lambs, highlighting that extra care must be taken with the management and handling of this breed prior to slaughter.