Muscle Distribution

Given that mature size is the primary trait affecting the amount of muscle at a particular weight, the other possible effect on carcass value could be differences in muscle distribution. In their classic book on cattle growth, Berg and Butterfield[7] summarized several studies of dissec-tible muscle distribution of breeds with widely different conformations. Data from one of those studies are summarized in Table 2. Similar data were summarized for sheep by Butterfield.[2]

The breeds in Table 2 represent widely different shapes and purposes. For example, Hereford cattle have been selected over centuries for beef, whereas Holstein and Jersey cattle have been selected for milk production and against the supposedly more meaty shape associated with Herefords and Shorthorns. And yet, the proportion of muscle weight in the high-priced cuts has not been changed through selection. The small differences among breeds of cattle and sheep and the necessity for anatomical dissection of progeny to detect genetic differences of muscle-weight distribution of breeding stock make the payoff unlikely to be profitable. Selection for animals with bulgier and shorter muscles (better conformation) may be important in situations where shape can be used as a marketing tool, as in the case of lambs produced for rib chops. Some beef cattle slaughtered at heavy weights, however, contain such bulgy muscles that they produce steaks unaccept-ably large for most consumers.

In addition to increased breast muscle size in selected turkeys and chickens, there are two examples of increased proportions of carcass muscle in cattle and sheep. Generalized muscle hypertrophy in double-muscled cattle

Table 1 Effect of mature size on growth and body composition of sheep

Item

Ewe size: Lambs: Slaughter weight kg:

Small

Large

Single

Twins

Single

Twins

54 kg ram

118 kg ram

Number of ewes Ewe weight, kg Total feed per ewe, kg Lamb slaughter weight, kg Total feed per lamb, kg Lamb weight/feed Fat trim per lamb, kg

Number of ewes Ewe weight, kg Total feed per ewe, kg Lamb slaughter weight, kg Total feed per lamb, kg Lamb weight/feed Fat trim per lamb, kg

47 448

10 49 450

44.0 53.9 118 350

2 44 510

38.2 55.6 118 537

49 513

65 67

538 609

130 365 83 513

49 68 72

574 622

78 279 73 421

results from mutations in the myostatin gene.[8] The increased muscle of purebred animals is associated with so much calving difficulty that delivery by cesarean section is common in countries, such as France, with a high proportion of breeds carrying one of the mutations. An ideal increase in muscle hypertrophy occurs in heterozygous lambs inheriting the Callipyge mutation from their sires.[9] Differentially faster muscle growth in the high-priced cuts of the loin and leg in Callipyge lambs begins when the lambs are about three weeks old. Unfortunately, this ideal mutation comes at the expense of tougher meat,[10] and such lambs are discriminated against commercially.

Table 2 Distribution of muscle weight in breed groups of bulls and steers

Item

Hereford

Shorthorn cross

Hybrid and other crosses

Holstein

Jersey

Bulls:

Number of animals Days of age Live weight, kg Expensive muscles, Hind quarter, % Fore quarter, %

13 461 465

53.3

Item

Hereford

Shorthorn cross

Hybrid and other crosses

Holstein

Brown Swiss cross

Steers:

Number of animals Days of age Live weight, kg Expensive muscles, % Hind quarter, % Fore quarter, %

32 434 461

54.4

480 466

54.4

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