The production systems commonly adopted by Australia’s retail meat producers means there is little difference in marbling between grassfed and grainfed cattle.
Marbling refers to the fat found inside the muscle (intramuscular fat) and increases as animals get fatter, irrespective of feed type. It’s a simple equation: the longer cattle stay on a nutrient-rich diet, the fatter they become and the higher their marbling.
Since the nutritional content of grain-based feeding regimes is prescribed and food is more easily available, growth tends to be faster in grainfed than grassfed animals, so you might expect grainfed animals to have higher marbling. However as grain feeding in Australia is usually of short duration, it has little impact on marbling.
An Australian study comparing meat from grassfed and 80-day grainfed cattle found very similar total amounts of fat (as fatty acids) in the fully trimmed muscle. However, when animals were fed grain for a longer time period (150-200 days), total fat content increased.
With limited marbling, the major determinant of fat content in Australian cattle is therefore separable fat, which is found either on the outside of meat (selvedge fat) or between muscles (intermuscular fat). When trimmed of all separable fat, fat content can be as low as 2% to less than 6% for raw beef, and 3% to less than 7% for raw lamb, depending on the cut.