The objective of this study was to investigate the growth- and breed-related changes of marbling characteristics in cattle. Four cattle breeds with different growth impetus and muscularity were reared and slaughtered under experimental conditions. German Angus, as a typical beef cattle; Galloway, as a smaller, environmentally resistant beef cattle; Holstein-Friesian, as a dairy-type cattle; and double-muscled Belgian Blue, as an extreme type for muscle growth, were used. These 4 breeds were expected to have differences in muscle development and i.m. fat deposition. Between 5 and 15 bulls of each breed were slaughtered at 2, 4, 6, 12, or 24 mo of age. Marbling characteristics were determined and classified in LM and semitendinosus muscle by computerized image analysis. Among breeds, differences appeared in the quantity, structure, and distribution of the marbling flecks in both muscles. The deposition of fat in the double-muscled Belgian Blue bulls remained substantially inferior to that of the other breeds, up to the age of 24 mo. Marbling in German Angus bulls particularly showed larger (P < 0.05) marbling fleck areas. Galloway cattle had the greatest (P < 0.05) number and the most regular (P < 0.05) distribution of the marbling flecks in young animals. Furthermore, for marbling characteristics in Holstein-Friesian animals, a great number and slightly finer structure were observed compared with the other breeds investigated. Postnatal growth-related changes of marbling in LM were characterized by as much as a 40-fold increase in the number of marbling flecks from 2 to 24 mo of age but also by up to a 4-fold enlargement in the area of the marbling flecks. The structure of marbling flecks was determined by 2 development trends. On the one hand, the marbling flecks became larger (P < 0.05), and the structure became coarser, which was reflected by an increasing (P < 0.01) proportion of long marbling flecks as well as an increasing (P < 0.01) maximum skeleton line length. On the other hand, continually new small, round marbling flecks appeared. This caused a decrease (P < 0.01) in the proportion of the 3 largest marbling fleck areas. The distribution of the marbling flecks became more regular (P < 0.05) with increasing proportion and number of marbling flecks. The results suggest that hyperplasia of adipocytes plays an important role in marbling during growth of muscle in cattle.
