Seasonal variation in muscle glycogen in beef steers

Abstract

Dark-cutting in beef carcasses is a quality and economic problem for the grass-fed beef industry in Australia, with ~10% of carcasses graded as dark-cutting. Dark-cutting results from low muscle glycogen levels at the time of slaughter. An experiment was designed to examine the relationship between season and muscle glycogen levels for cattle at pasture. Sixty steers were allocated to 2 stocking rate treatments, low and high (1.5 and 2.5 steers/ha, respectively) with 3 replicates for each treatment and grazed in 6 separate paddocks. Monthly samples of the M. semimembranosus (SM) and M. semitendinosus (ST) were taken by biopsy from all cattle and analysed for glycogen and lactate content. Significant differences in muscle glycogen were found between seasons. Average muscle glycogen levels for autumn, winter, spring and summer were 1.24, 1.00, 1.15 and 0.82 mg/g SM and 0.85, 0.91, 1.05 and 0.76 mg/g ST, respectively. The seasonal effects on muscle glycogen were not influenced by stocking rate, but it is postulated that they were influenced by nutrition, with the peak in muscle glycogen level generally coinciding with the peak in pasture quantity and quality in spring.