Growth, development and nutritional manipulation of marbling in cattle

This review discusses our current understanding of the growth and development of intramuscular fat and proposes a simple growth model to explain the accumulation of intramuscular fat in cattle. Potential effects of nutritional manipulation are discussed in relation to the proposed growth curve for intramuscular fat. The scope for nutritional control during both the pasture (or backgrounding) and intensive grain finishing phase are discussed. Additional discussion on nutritional triggers of the cellular and early life events associated with the development of the intramuscular fat are discussed in the companion paper by Harper and Pethick (2001). Development of fat and muscle in cattle during growth A common conclusion from animal developmental studies is that intramuscular fat is late developing (Vernon 1981). Indeed the usually quoted developmental order is abdominal, then intermuscular, then subcutaneous, then finally intramuscular. However, because fat is deposited at a greater rate than lean tissues later in life the concentration of fat in muscle will inevitably increase later in an animal’s life. Therefore the commercial trait, marbling, visible intramuscular fat or simply % fat is late maturing. As we will show this does not mean that the rate of fat accretion in intramuscular adipocytes is also late maturing. To determine if there is a difference in fat deposition over time, it is informative to express the data as proportions of total carcass fat that develop within various depots, because changes in these proportions would indicate if intramuscular fat develops at a different rate from other fat depots. When fat deposition have been described in this way (Johnson et al. 1972), the proportional distribution of fat between carcass pools is found to be constant over a wide range of carcass fat contents (in the range from 5 to over 150 kg total fat; Fig. 1). Other data (Cianzio et al. 1982 &1985, Trenkle et al. 1978) are also consistent with this observation. There is substantial variation in intramuscular fat content between different muscles of the carcass at the same carcass fatness (Johnson et al. 1973). However, it has become customary to assess intramuscular fat content in the m. longissimus thoracis et lumborum (striploin). On average the intramuscular fat content of the striploin is a little lower or about the same as the average carcass muscle intramuscular fat content. Importantly there is a high correlation between fat measured in the m. longissimus thoracis et lumborum and other muscles over a wide range of intramuscular fat content (Johnson et al. 1973; Brackebush et al. 1991).