Mature Size

Body composition is a primary determinant of the value of agricultural animals, because composition determines nutrient requirements as well as the value of meat. There is wide variation among breeds within species in body composition at specific weights. Most of this variation is due to differences in mature size. The amounts of carcass muscle, fat and bone for a hypothetically large-mature-size and a hypothetically small-mature-size animal are plotted against carcass weight in Fig. 1. The underlying allometry square[1] shows the different patterns of tissue growth. The mature carcasses of both large-mature-size and small-mature-size animals contain 50% muscle, 35% fat, and 15% bone,[2] but the mature carcass of the small-mature-size animal is 75% of the weight of the mature carcass of the large-mature-size animal.

As an animal grows, bone and muscle are gained rapidly at first. The rate of bone and muscle growth slows as the animal approaches its genetically predetermined maximum skeletal size and muscle weight. Well-fed animals can continue gaining weight after reaching maximum bone and muscle size, but the gain consists mostly of fat. The percentage of fat in mature animals varies with stage of reproduction and lactation. Although there is variation of mature fat concentration among breeds and among animals within breeds, it is not clear what proportion of this variation is independent of mature size.

Dietary and environmental effects on composition in time-limited growth studies are primarily a function of differences in stage of growth at slaughter because animals that grow faster reach heavier weights than animals that grow slower.

The effect of mature size on growth has been modeled into genetic size scaling rules[3] and has important implications for speed and efficiency of growth and for carcass composition. Although about 75 kJ of metaboliz-able energy is required to gain a gram of either protein or lipid,[4] the water content of muscle is about 70%, whereas that of fat is only about 20%. Thus, muscle is accumulated much more efficiently than fat. At comparable immature weights, large-mature-size animals will have gained more muscle and less fat than small-mature-size animals (Fig. 1), so they have leaner carcasses, their maintenance requirements will have been similar, and they will have required less feed per unit of gain than small-mature-size animals. Because the large-mature-size animals will have consumed at least as much feed per day as small-mature-size animals, they will have grown faster.

An example of the effects of stage of growth and mature size on composition, efficiency, and growth of lambs[5] is given in Table 1. As expected based upon the relationships shown in Fig. 1, lighter lambs were leaner and accumulated more weight per unit of feed than lambs slaughtered at heavier weights. Also as expected, lambs from large-mature-size rams and ewes were leaner and accumulated more weight per unit of feed than lambs from small-mature-size rams and ewes slaughtered at the same weight.

Identifying genetic variation of body composition independently of mature weight could be important for improving the value of agricultural animals. As an example, chemical composition of small-mature-size Dorset rams was compared with that of large-mature-size Suffolk rams slaughtered across a range of ages from 1 to 600 days.[6] Because the rams were continuously fed a high-energy diet, the oldest and heaviest rams contained almost 50% lipid. This provides an example of how choosing mature weight is difficult, because mature animals gain and lose lipid in response to nutrient demands and availability. In place of the mature-body-weight estimates for the rams, four lipid-free estimates of breed mature-size were used: 1) mature amount of water;

Bone

Fig. 1 Growth of the carcass to mature proportions of 50% muscle, 35% fat, and 15% bone.'2-1 Solid lines represent data for a large mature size animal. Dashed lines represent data for a small mature size animal. Note that muscle, fat, and bone weights must add to carcass weight. Inset: The underlying genetically standardized allometry square'1- with q values of 1.8, 0.1, and 2.0 for muscle, fat, and bone, respectively.

Bone

Carcass weight

Fig. 1 Growth of the carcass to mature proportions of 50% muscle, 35% fat, and 15% bone.'2-1 Solid lines represent data for a large mature size animal. Dashed lines represent data for a small mature size animal. Note that muscle, fat, and bone weights must add to carcass weight. Inset: The underlying genetically standardized allometry square'1- with q values of 1.8, 0.1, and 2.0 for muscle, fat, and bone, respectively.

2) mature amount of the combination of water, protein, and ash; 3) metacarpal bone length; and 4) metacarpal bone weight. When the chemical composition of the Dorset and Suffolk rams was described in genetically standardized allometry squares, the same conclusion was reached using any of the lipid-free estimates of breed mature-size: Water, protein, and ash had about the same standardized growth, whereas lipid accumulated faster than expected in Suffolk rams at advanced stages of growth.

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