This study was a series of experiments designed to test the influence of
supplemental magnesium oxide (MgO) on muscle glycogen concentration in sheep
exposed to stress (exercise) and the commercial slaughter process, and to test
the effectiveness of this supplement in the commercial scenario.
In Expt 1, Merino wethers maintained on a mixed ration (metabolisable energy
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. The exercise regimen consisted of 4 intervals of 15
min, with muscle biopsies taken by biopsy drill from the
m. semimembranosis (SM) and
m. semitendinosis (ST) pre-exercise and immediately
post-exercise, and at 36 and 72 h post-exercise. Muscle biopsies were also
taken 1 week prior to slaughter from the SM and ST, with further samples taken
approximately 30 min post-slaughter. Ultimate pH (pHu) of the SM, ST, and
m. longissimus dorsi (LD) was measured 48 h after
slaughter. Sheep supplemented with MgO lost less muscle glycogen in the ST
during exercise, and repleted more muscle glycogen in the SM 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 water-borne slurry at a rate equivalent to
1% of their ration. This treatment resulted in significantly reduced
muscle glycogen concentrations in both the SM and ST at slaughter.
In Expts 3–5, MgO was used as an ‘in-feed’ supplement in the
commercial scenario. In each case, slaughter-weight Merino lambs were
supplemented with MgO at the rate of 1% of their ration for 4 days
prior to commercial slaughter. 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 stress load, thus providing very little scope
for an effect of the MgO supplement.
We conclude that MgO can reduce the effects of exercise, leading to a
subsequent reduction in glycogen loss, and an increase in the rate of glycogen
repletion in skeletal muscle following exercise. The results support MgO
supplementation as a viable option for reducing the stress associated with
commercial slaughter.
Adenovirus-mediated transfer of the muscle glycogen phosphorylase gene into hepatocytes confers altered regulation of glycogen metabolism.