Source: Alberta Agriculture and Forestry, http://www.agric.gov.ab.ca/
Some breeds of cattle are prone to double muscling. These animals have enlarged muscles, giving them the appearance of being the weight lifters of the cattle world! Double muscling in cattle is the result of a natural mutation of the myostatin gene. Normally this gene stops muscle development, but the timing is off because of the mutation of the gene.
There are a number of breeds that are prone to carrying the gene for double muscling, with two of these being the Piedmontese and the Parthenais. Both breeds have been in existence for a long time with the first official herdbook for the Piedmontese established in Italy in 1897, and for the Parthenais in France in 1893. Both breeds are raised in Alberta. One of the attractions of double-muscled cattle is the leanness of their carcasses. Backfat is generally found to be less in double-muscled cattle than in cattle with normal muscling. Whether or not this affects the amount of marbling fat in the muscle is open to dispute. Some studies have found reduced marbling in double-muscled cattle while others have found no effect of double muscling on carcass marbling.
As part of a large study to determine growth performance and carcass characteristics of cattle with varying degrees of marbling genetics, we included Piedmontese and Parthenais steers. The objectives of this portion of the study were to compare backfat depths and marbling of double-muscled and non double-muscled steers, and to determine if double-muscled steers have altered plasma hormone profiles that might explain how the gene for extra muscle growth is being expressed.
We compared the data from 10 Piedmontese and 8 Parthenais double-muscled steers with data obtained from 38 non double-muscled (control) steers). The control group had 19 Angus, 10 Hereford, 3 Holstein, 3 Hereford x Charolais and 3 Hereford x Simmental calves in it. Calves began the trial at weaning. During the first 2 weeks we put the calves on a roughage diet. We then adapted the calves over a 4-week period to a diet of 80% barley, 15% barley silage and 5% pelleted supplement, which they received until slaughter.
We weighed calves at weaning and every 28 day until slaughter. We also measured ultrasonic backfat depth when we weighed them. We assigned the control group for slaughter when their backfat depths approached 12 mm. However, we assigned the double-muscled steers for slaughter at 500 kg liveweight, instead of at 12 mm of backfat, since they were slow to deposit backfat. We had the calves slaughtered at the Lacombe Research Centre, where blue tag data was collected by certified AAFC beef graders. Carcass marbling was scored on an inverse 10-point scale where a score of ‘1′ is maximum marbling and a score of ‘10′ is zero marbling.
Average live weight at slaughter was slightly higher in double-muscled steers compared to control steers (506 vs. 488 kg). As we expected, double-muscled steers had much less backfat than control steers (5.1 vs. 12.1 mm) at slaughter. Despite this, carcass marbling was similar for both the double-muscled and control steers (8.6 vs. 8.6 marbling score), supporting the view that while double muscling results in less external carcass fat, it does not adversely affect marbling. This is important since marbling is believed to have a role in determining the palatability of beef.
We collected blood samples from the steers three times during this study for the measurement of several hormones known to be involved in the partitioning of energy into either muscle or fat. Plasma insulin concentrations were similar in the double- muscled and control steers (1.2 vs. 1.3 ng/mL). The concentration of plasma triiodothyronine, a thyroid hormone, was slightly lower in double-muscled compared to control steers (1.9 vs. 2.1 ng/mL). Plasma thyroxine, another thyroid hormone, was also lower in double-muscled compared to control steers (7.3 vs. 9.3 µg/dL). We also found that plasma cortisol, an adrenal hormone was substantially higher in the double-muscled steers (12.3 vs.6.9 µg/dL).
Our study indicates that Piedmontese and Parthenais steers put on much less backfat without reducing the amount of muscle marbling. These breeds have adrenal and thyroid hormone concentrations that are different from those of normally muscled cattle, an indication that the mutated myostatin gene may be expressing itself through these hormonal systems to alter muscular development and fat deposition in these cattle.
Dr. G.J. Mears, Agriculture and Agri-Food Canada, Lethbridge, AB
Dr. P.S. Mir, Agriculture and Agri-Food Canada, Lethbridge, AB
Dr. D.R.C. Bailey, Agriculture and Agri-Food Canada, Lacombe, AB