الفهرس 
Chapter 1 - IntroductionOnly 14 pages are availabe for public view
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Abstract
Dairy and beef cows differ in the way in which they utilise nutrients and in accretion or mobilization of body reserves during lactation. Thus far, little is known about the impact of lactation performance on body composition, meat quality, and the related muscle structure of cows with a defined, combined beef and dairy genetic background. In the described experiment, 50 F2 cows, originating from mating charolais bulls to German Holstein cows and a following intercross of F1 individuals, were slaughtered during the second lactation, 30 d after calving. Cows were assigned to 3 groups, each containing representatives of 3 families, according to lactation performance. Biopsy samples from live animal were taken from M. semitendinosus (MST) and subcutaneous fat (SCF) at three time points, 10 d before parturition, 30 and 100 d after parturition in the first lactation. These samples were investigated by histology and computer image analysis for muscle fiber profile, intramuscular and subcutaneous fat cell size. After slaughter, standard carcass and meat quality traits were determined. Additionally, samples from longissimus dorsi muscle (LDM) and SCF were investigated by histology and computer image analysis. Muscle fiber profile, intramuscular fat cell size, marbling traits, and subcutaneous fat cell size were measured to estimate the impact of lactation on body reserves. In live animals, the investigations can be summarized as follows; cows gained weight with advanced duration of lactation and with decreased amount of milk per day. Fat mobilization could not be detected, except in SCF. Subcutaneous fat cell size differed between groups, without any significant change during lactation, while intramuscular fat cell size was similar in different groups and at all investigated time points. There was no indication for protein mobilisation by lactation. Muscle fiber changes could not be detected, except minor changes in some characteristics. After slaughter, investigations revealed that the duration of the first lactation had no impact on body composition, meat quality or muscle structure. However, the amount of milk per day influenced body weight, body composition, and marbling traits. Relationships between traits were low, but showed consistently that increasing milk yield was negatively correlated with tissue accretion. Changes of muscle fiber and fat cell profile, indicating protein or fat mobilization by lactation, could not be detected. In the presented study, lactation had only minor consequences for meat quality.