Changes of muscle fibre profile and fat cell size around first parturition in cows differing in lactation performance

• F2 generation cows of a Charolais×Holstein cross had an overall low lactation performance.
• Cows accreted tissue and gained weight during lactation depending on lactation performance.
• Changes in fat cell size reflected continued tissue accretion during lactation.
• Proportion of fast fibers increased during transition from late pregnancy to early lactation.

The presented study investigated changes of fat cell and muscle fibre size, and muscle fibre type composition around first parturition in cows with a combined beef and dairy genetic background. In contrast to high-merit dairy cows, these cows can serve as a model for beef cows because of the very low milk production level. Fifty heifers used in this experiment were F2 offspring originating from mating Charolais bulls to German Holstein cows and a following intercross of F1 individuals. They were assigned to 3 groups according to their lactation performance, namely high lactating (HL), low lactating long (LLL; duration of first lactation at least 100 days), and low lactation short (LLS; duration of first lactation shorter than 100 days). Biopsy samples were taken from semitendinosus muscle (MST) and subcutaneous fat (SCF) at three time points, 10 days before first parturition, 30 and 100 days after calving. These samples were investigated by histology and computer image analysis for muscle fibre profile, as well as intramuscular and subcutaneous fat cell size to estimate the impact of lactation on body reserves. Most cows continued growth and gained weight during lactation. Continuous tissue accretion was indicated on the cellular level by an increase in intramuscular fat cell size. Subcutaneous fat cells were larger in low lactating cows with a short lactation period. There were no morphological signs of fat mobilization from fat cells. There was also no protein mobilization indicated by changes in muscle fibre size. However, the increase in percentage of fast, glycolytic muscle fibres indicated alterations in muscle metabolism during transition from late pregnancy to early lactation. Cows remained obviously in a positive energy balance in this study and continued tissue accretion during lactation. This was reflected on the cellular level in skeletal muscle and subcutaneous fat.