Microstructure of skeletal muscles of growing calves fed silage-based vs hay-based diets. I. Fibre diameters.

Samples of musculus longissimus dorsi, semimembranosus and semitendinosus were obtained post-slaughter from 2-week, 3-month and 10-month-old bull calves. The 2-week-old calves were fed milk only. All the remaining animals were fed grass silage or hay ad lib and a restricted amount of concentrate from 2 weeks of age onwards. Muscle fibres were differentiated according to Ziegan into fast-twitch glycolytic, fast-twitch oxidoglycolytic and slow-twitch oxidative fibres (FTG, FTO and STO, respectively). In all calves, marked fibre growth took place between 3 and 10 months of age. From the 2nd week to the 3rd month, growth was observed virtually only in the FTG fibres, irrespective of feeding. Feeding calves with a silage-based vs hay-based diet resulted in thicker muscle fibres for each type of fibre. Significant linear regression coefficients have shown the fibre diameters to be clearly related to live weight gain resulting from DM intake that was higher in the silage-based diet.