Consequences of selection for higher growth rate on muscle fibre development in cattle

The aim of this study was to analyse the consequences of selection on the development of muscle fibres in the semitendinosus muscle of two groups of Charolais bulls with a high or low muscle growth index. Muscle fibre kinetics were analysed during foetal life (110, 180, 210, 260 days of gestation) and after birth (3 weeks, 9, 15, 19 months). The weight, length and area of muscle were measured. The total number of fibres and muscle fibre area were calculated for each stage. The contractile properties of fibres were revealed by immunohistochemistry. Oxidative capacity of individual fibres was determined on serial sections stained for Succinate Dehydrogenase. Finally, whole muscle metabolic properties were measured on homogenates of muscle with analysis of the activity of Lactate Dehydrogenase (glycolytic) and Isocitrate Dehydrogenase (oxidative). The main results showed muscle hypertrophy in bulls with a high growth potential due to a higher total number of fibres (indicating hyperplasia). From the last trimester of gestation onwards, this is associated with particular contractile and metabolic properties, with mainly a higher proportion of IIX fibres (fast glycolytic). However, the delayed physiological maturity of these bulls induced a delay in the plasticity of muscle fibres after birth. Consequently, the effects of selection on growth rate on muscle properties are not evident at each stage.