Breed Differences

The proportion of fat to muscle in animals continually changes during growth so that comparison of breeds must consider whether animals are being compared at constant weight or constant chronological age.[2,4,5] Comparison at constant weight is often used, because the weight chosen approximates the market weight for that species. This comparison has practical outcomes regarding production of lean meat. The difficulty with constant-weight comparisons is that different breeds have different mature size and growth rates and reach maturity of body composition at different ages. Thus, comparison at a given weight can represent comparison of animals at divergent stages of maturity. Because fat deposition is a predominant aspect of later-stage growth, earlier maturing animals will be fatter than later maturing animals at a given weight. For example, comparison of two breeds with mature body weights of 100 and 125 kg at 95-kg body weight would show one breed at 95% of mature composition (i.e., more fat) and the other breed at 76% of mature composition (i.e., less fat). This is the result of comparing early maturing cattle, e.g., Angus, with later maturing cattle, e.g., Simmental, at constant weight. Comparison at constant age contrasts breeds that may have different weights or maturity, i.e., stages of achieving mature body composition; these comparisons have important biological implications regarding growth.

Comparison of different breeds of cattle at constant age (Table 1) indicates considerable diversity in weight, fatness, and yield of retail product. If these animals are examined at later ages, they have more fat and less retail product.[6] The earlier maturing breeds, e.g., Angus, have

Table 1 Growth of cattlea

Breed

Carcass weight, kg

Fat thickness, mm

Fat trim, %

Retail product, %

Marbling score

Angus

277

14.6

21.5

66.97

13.3

Hereford

273

14.2

19.0

68.4

9.1

Gelbvieh cross

303

9.3

16.6

70.7

9.2

Chianina cross

304

8.0

12.3

73.8

8.0

aCarcass composition at constant age (473 days). (Adapted from Ref. 6.)

aCarcass composition at constant age (473 days). (Adapted from Ref. 6.)

a higher percentage of fat and a lower percentage of retail product than the later maturing breeds, e.g., Chianina, regardless of the time of comparison. An approximation of maturity can be made at constant fat-trim percentage;1-6-1 the assumption is that all breeds will eventually achieve the same percentage carcass fat. This assumption may not apply when extremes in mature body composition are compared, e.g., Angus vs. Chianina.

Comparison of breeds of pigs at constant weight (Table 2) indicates that the rate of gain is quite divergent, as is the rate of gain for lean, fat, and nonfat viscera. The larger and faster growing Landrace breed also has greater gain of lean mass than the Duroc breed; both Landrace and Duroc are considerably greater than the fat breed shown in Table 2. The lean gain/total gain is 43 and 42% for Landrace and Duroc, respectively, during growth from 85 to 105 kg. However, the fat gain is 26 and 32% of the total gain, respectively, for the two breeds during the same period of growth. Also, the gain in viscera during this period is 6.4% of the total gain in the Landrace breed, but 8.9% in the Duroc breed; this has implications on the efficiency of growth because the viscera require considerable energetic input.

The Callipyge lamb represents an animal with a single gene mutation that produces exceptional muscle growth. When compared at constant weight, the Callipyge carcass is slightly heavier than the normal carcass and has more muscle and less fat. The accretion rates (g/day) for protein are higher in Callipyge lambs than in normal lambs, whereas fat accretion rates are lower in Callipyge lambs than normal lambs (Table 3). The differences between Callipyge lambs and normal lambs were observed at constant weight, constant age, and constant 12th-rib fat depth. [7]

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