The Relationship Between Marbling and Intramuscular Fat

Livestock Update, December 2002

Scott P. Greiner, Extension Animal Scientist, Beef, VA Tech

The beef industry's focus on consumer satisfaction has resulted in a renewed interest in carcass value determinants in recent years. Cow-calf operators, seedstock producers, feeders, and packers share the common goal of satisfying the consumer. Grid marketing, alliances, and consumer-friendly specification products have provided the economic incentives to place genetic selection pressure on carcass merit. Both carcass quality and cutability have been emphasized. However, premiums associated with quality grade have generally been higher and more certain than those for red meat yield as a result of the large number of alliances and branded beef programs that emphasize quality grade as a primary specification. Consequently, genetics that are capable of achieving premium quality grades have been widely sought after.

Both marbling score and intramuscular fat are closely associated with carcass quality grade. However, each term has taken on its own definition in recent years within the industry. The relationship between marbling score and intramuscular fat and the role of each in selection programs for carcass quality are frequently misunderstood and even debated within the industry.

Marbling score is a component of the USDA beef quality grading system, and refers to visible fat found between muscle fiber bundles within the ribeye muscle. Marbling score is assessed visually by a USDA grader during the process of assigning a USDA Quality Grade, and is therefore a subjective measure. In addition to the quantity, the distribution and texture of visible fat flecks within the ribeye are considered during assessment of marbling score. USDA Quality Grade (Select/Choice/Prime) is a function of both marbling score and carcass maturity, although maturity has little or no influence on final QG for young steers and heifers harvested at less than 24 months of age. Therefore, quality grade is primarily a function of marbling score for most of the U.S. fed cattle population.

Intramuscular fat also quantifies the fat found between muscle fiber bundles within the ribeye muscle. Intramuscular fat is determined by chemical extraction of lipid from a thin facing of the exposed ribeye muscle. Therefore, percent intramuscular fat is an objective measurement that quantifies the total fat content within the ribeye muscle.

Research studies have found a strong relationship between marbling score and percent intramuscular fat, with correlation values ranging from .70 to .90. Since both marbling score and intramuscular fat quantify lipid with in ribeye muscle, why is this relationship not perfect (correlation 1.0)? With one measure being subjective and one objective, a perfect relationship would not be expected. Additionally, marbling score considers amount, distribution, and texture of fat whereas intramuscular fat simply quantifies the amount (percent) of fat within the ribeye. The following table outlines the relationship between intramuscular fat percentage, marbling score, and USDA quality grade. Note that within a quality grade, a range for both marbling score and percent intramuscular fat exist.

Relationship Between Chemical % Intramuscular Fat, Marbling Score, and USDA Quality Grades

% Intramuscular Fat	USDA QG	Degree of Marbling/Marbling Score
< 2.30	Standard	Traces/ 3.0-3.9
2.30-3.00 3.10-3.99	Select- Select+	Slight / 4.0-4.9
4.00-5.79 5.80-7.69 7.70-9.89	Choice- Choice0 Choice+	Small/ 5.0-5.9 Modest/ 6.0-6.9 Moderate/ 7.0-7.9
9.90-12.10 >12.10	Prime- Prime0	Sl. Abundant/ 8.0-8.9 Mod. Abundant/ 9.0-9.9

Both marbling score and intramuscular fat records are useful for genetic evaluation programs. Marbling score data generated through structured sire evaluation progeny testing programs designed by beef breed associations is utilized to generate Marbling EPDs. This process involves a planned mating structure utilizing several bulls, and the subsequent collection of marbling score data at harvest on sire-identified slaughter progeny that have been maintained in a defined contemporary group. Consequently, this process is time consuming, expensive, and requires extensive cooperation by various segments of the industry.

The development of software that predicts percent intramuscular fat using real-time ultrasound has significantly expanded the number of bulls with carcass merit EPDs. Ultrasound predicted intramuscular fat models have demonstrated the ability to explain 80-85% of the actual variation in chemically determined intramuscular fat. Ultrasound records are generated on yearling seedstock (both bulls and heifers) and % Intramuscular Fat EPDs estimated. This process shortens the time interval required for evaluation of sires, and generates a carcass record on the animal itself. Additionally, ultrasound scanning of yearling seedstock allows animals to more readily be evaluated at a proper endpoint and within a proper contemporary group.

Research has also demonstrated a strong relationship between Marbling and % Intramuscular Fat EPDs. Bulls that have both Marbling EPDs and % Intramuscluar Fat EPDs tend to rank similarly for both traits. In other words, bulls that rank high for Marbling EPD as a general rule also rank high for % Intramuscular Fat EPD. This relationship is especially strong for high accuracy bulls that have a significant number of progeny utilized to calculate each EPD. This fact provides evidence that data generated on yearling seedstock can accurately be used to predict slaughter progeny carcass performance.

In summary, marbling score and percent intramuscular fat are very similar traits. Both are strongly related to carcass quality. Either Marbling or % Intramuscular Fat EPDs are the most useful tool to make genetic improvement in carcass quality.

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