Double muscling or muscular hypertrophy is an inherited condition in cattle, characterised by hyperplasia (increase in number) and, to a lesser extent, hypertrophy (enlargement) of muscle fibres. Double-muscled animals are characterized by an increase in muscle mass of about 20 %, due to general skeletal muscle hyperplasia and, to a lesser extent, hypertrophy. This relative increase in the number of muscle fibres (hyperplasia) occurs during intra-uterine development, such that double-muscled cattle possess nearly twice the number of muscle fibres at birth as do normal cattle. The muscles of double-muscled cattle also have a significantly reduced amount of connective tissue (collagen). Not only is collagen reduced in amount, but it is structurally different to normal collagen in that it has a lower proportion of stable, non-reducible, cross-links. Muscular hypertrophy and hyperplasia is not uniform throughout the beast, being minimal around the neck and increasing as one moves to the hindquarters where it is maximal. This distribution results in the carcasses of double-muscled animals having a higher proportion of “expensive” cuts of meat relative to carcasses of normal cattle.
The bone mass of double-muscled cattle tends to be around 10 % less than that of normal cattle. This is primarily due to their long bones being shorter, slenderer, and of lower density. This reduced bone mass results in a significantly higher muscle: bone ratio in double-muscled cattle. Double-muscled cattle exhibit hypo-development of their fatty tissues. This is due to a reduction in the volume of fat cells rather than to a reduction in their numbers. Not only is the total fat content reduced, but its composition is different, with double-muscled animals having a much higher percentage of polyunsaturated fats (11% compared with 5% in normal cattle). During forced exercise, double-muscled cattle show signs of fatigue faster than normal cattle. This is thought to be due to a reduced capacity for aerobic metabolic activity by the exercising muscles. Double-muscled cattle tend to have a reduced tolerance for heat stress. This is thought to be due to the increased heat production associated with their increased muscle mass.
The syndrome of double muscling is associated with a number of reproductive problems. In the case of those animals where the syndrome is fully expressed there may be delays in puberty, reduced fertility due to an increased incidence of mortality in double-muscled embryos, increased incidence of dystocia, reduced milk production and increased calf mortality.
Most double-muscled calves tend to have higher birth weights and higher pre-weaning growth rates than their normal contemporaries. Post-weaning, however, their growth rate tends to fall behind that of their normal contemporaries; this appears to be due to a lower feed intake. If muscle weight gain per unit energy intake is taken into account, double-muscled cattle have better feed efficiency than normal cattle.
When compared with normals, the carcases of double-muscled cattle have many desirable characteristics. They have higher dressing percentage. The carcases of double-muscled cattle dress out at between 65 and 70 percent. This is due to a combination of increased muscle mass, reduced body fat, reduced bone mass and smaller internal organs. Double-muscle cattle have higher proportion of “expensive” cuts of meat. This is due to the non-uniform distribution of the muscular hypertrophy and hyperplasia which is found in double-muscled cattle. They also have better meat quality. Meat from double-muscled cattle is significantly more tender than that from normal cattle. Much of this is thought to be due to its lower collagen content and to the fact that what collagen is present is not as tough due to its lower proportion of stable, non-reducible, cross-links. The significance for producers is that double-muscled animals produce a higher proportion of desirable cuts of lean meat with greater efficiency than do comparable, conventional cattle. For consumers, this meat is more tender and, being lean and having a higher polyunsaturated fat content, conforms more closely with current nutritional guidelines than meat from normal animals.