Cattle at risk for dark-cutting beef have a higher proportion of oxidative muscle fibres.

The aim of this investigation was to compare muscle fibre type characteristics in dark-cutting (DFD) and normal beef animals. Three categories of animal were used in this investigation: 9 DFD bulls, 10 DFD steers and 10 normal bulls. For each animal, frozen sections of longissimus dorsi muscle were stained histochemically. The results showed that there were proportionately more slow, oxidative (SO) fibres in DFD bulls and DFD steers than in normal bulls. As far as fast, glycolytic (FG) fibres were concerned DFD bulls contained fewer of this type than DFD steers and normal bulls. When the results on fibre proportions were combined with those on mean fibre cross-sectional areas it was found that both DFD groups contained a smaller proportion of muscle exhibiting FG characteristics than normal bulls. Furthermore, the combination of all oxidative fibres, i.e. SO and fast, oxidative, glycolytic (FOG) fibres, showed that, both by numbers and relative areas, the two DFD groups (especially DFD bulls) exhibited significantly more oxidative metabolism in the longissimus dorsi muscle than the normal animals.