The effect of magnesium oxide supplementation on muscle glycogen metabolism before and after exercise and at slaughter in sheep

This study was a series of experiments designed to test the influence of su pplemental magnesium oxide (MgO) on muscle glycogen concentration in sheep exposed to stress (exercise) and the commercial slaughter process, and to t est the effectiveness of this supplement in the commercial scenario. In Expt 1, Merino wethers maintained on a mixed ration (metabolisable energ y 11 MJ/kg and crude protein 16.3% in DM) were supplemented with MgO at the rate of 0%, 0.5%, or 1% of their ration for 10 days prior to a single bout of exercise and for 10 days prior to slaughter at a commercial abattoir. T he exercise regimen consisted of 4 intervals of 15 min, with muscle biopsie s taken by biopsy drill from the m. semimembranosis (SM) and m. semitendino sis (ST) pre-exercise and immediately post-exercise, and at 36 and 72 h pos t-exercise. Muscle biopsies were also taken 1 week prior to slaughter from the SM and ST, with further samples taken approximately 30 min post-slaught er. Ultimate pH (pHu) of the SM, ST, and m. longissimus dorsi (LD) was meas ured 48 h after slaughter. Sheep supplemented with MgO lost less muscle gly cogen in the ST during exercise, and repleted more muscle glycogen in the S M during the post-exercise repletion phase, than unsupplemented sheep. The supplemented animals also had higher muscle glycogen concentrations in the ST at slaughter. In Expt 2, MgO was administered to Merino wether lambs for 4 days prior to slaughter in the form of a waterborne slurry at a rate equivalent to 1% of their ration. This treatment resulted in significantly reduced muscle glyco gen concentrations in both the SM and ST at slaughter. In Expts 3-5, MgO was used as an `in-feed' supplement in the commercial sce nario. In each case, slaughter-weight Merino lambs were supplemented with M gO at the rate of 1% of their ration for 4 days prior to commercial slaught er. Positive responses were seen in 2 of the 3 experiments, with increased glycogen concentrations and a reduced pHu. The animals that demonstrated no response to MgO had the lowest pHu after slaughter, suggesting a minimal s tress load, thus providing very little scope for an effect of the MgO suppl ement. We conclude that MgO can reduce the effects of exercise, leading to a subse quent reduction in glycogen loss, and an increase in the rate of glycogen r epletion in skeletal muscle following exercise. The results support MgO sup plementation as a viable option for reducing the stress associated with com mercial slaughter.