A review of genetic and non-genetic opportunities for manipulation of marbling

The biology of marbling is a considerable issue for the Australian beef industry. Measurement of the trait is still a concern: subjective assessment based on the degree of visual fat deposition and its distribution is the 'industry standard' and the basis for payment of marbling grades. Yet this measurement may be subject to operator error and is influenced by chiller temperature. Chemical extraction gives an unequivocal measure of all fat in the muscle (intramuscular fat percentage: IMF%) and has higher heritability and genetic variation than marble score; but does this mirror exactly what the trade regards as 'marbling'? Progeny test results from the Cooperative Research Centre (CRC) for Cattle and Beef Quality breeding projects provide improved understanding of breed and genetic effects on IMF% and marble score. Estimated breeding values (EBVs) for IMF% have been released to the industry for 7 breeds. Heritability estimates confirm that genetic progress will be faster when selection is based on IMF% rather than marble score. Genetic correlations of IMF% with growth, retail beef yield (RBY%), P8 fat, residual feed intake (RFI) and tenderness are now available to underpin selection indices. A favourable allele for marbling (TG5) on chromosome 14 has been identified by CSIRO/MLA as a direct gene marker for the trait. This is now being marketed as GeneSTAR marbling. Other favourable chromosomal regions are under investigation by the CRC. Nutritional manipulation of marbling remains problematic. It is accepted that high-energy grain diets achieve higher marbling than pasture diets. Within grain-based feedlot diets higher marbling is achieved with maize than barley, while barley diets in turn are better than sorghum. Steam flaking produces higher marbling than dry rolled grain and this effect is more marked with sorghum than maize. Beyond these establishments there are many uncertainties: experiments have examined the effects of diets with high protein; low protein; protected lipid; protected protein; added oil with and without calcium; vitamin A deficiency. None of these manipulations gave consistent improvement in marble score or IMF%. Commercial feedlots supplying Japanese B3/B4 markets may have successful dietary manipulations to enhance marbling but because of its proprietary nature the information is not normally available for scientific scrutiny. Japan is the only market for Australian beef where marbling is an important component of the market specification. There can be no doubt that marbling meets a special consumer preference in that niche market. In other markets scientific evidence for a link between marbling and beef tenderness or eating quality has been difficult to define (marbling is a key component of the USA grading scheme for primal cuts but Australia is not a big supplier to that market). In the domestic Meat Standards Australia market there is a trend for marbling to become more important as a consumer issue in 5-star products where higher order sensory attributes of beef come into play. Early meat science investigations concluded that beef flavour elements were water-soluble. This would exclude marbling fat as having a notable influence on flavour. Marbling remains the major determinant of carcass value in Australia's most valuable beef market. Research should continue to assist Australian producers to meet the specifications of that market with increased precision and reduced costs.