led to an overall reduction in fatness in beef carcasses. However, despite apparent preferences for leaner meat in the 1980s and 1990s there was little or no evidence of reduced fat in UK carcasses up to 1985 except for pigs. Between about 1975 and 1985, while the separable fat content of the average pig carcass fell from 27.4% to 22.8%, the composition of beef and lamb carcasses did not change appreciably (Kempster et al., 1986). In 1985 the average beef carcass contained 22.2% separable fat (equivalent to 20.2% chemical fat) and the average lamb carcass 25.6% (equivalent to 23.5% chemical fat).
The corresponding chemical fat in the pig carcass was 19.4%.
stilboestrol, hexoestrol) which were banned in the EU in 1981 and also North America, because of concerns, possibly unfounded, that they could cause cancer in people eating meat.
##-Adrenergic agonists
A class of pharmacological compounds that has shown enormous potential to alter the muscularity and ratio of lean to fat in meat animals is the #-adrenergic agonists. These have chemical structures similar to the naturally occurring catecholamines, adrenaline (epinephrine) and noradrenaline (norepinephrine). Adrenaline and noradrenaline are hormones that are secreted in response to stressful situations (see Chapter 10). Noradrenaline is also the neurotransmitter found in the sympathetic nervous system (equivalent to the acetylcholine of the parasympathetic system). Examples of #-adrenergic agonists are clenbuterol, cimaterol, salbutamol and ractopamine. Figure 2.7 shows how their chemical structures relate to that of adrenaline.
#-Adrenergic agonists are so called because they act on cells via
#-receptors on the cell membrane. The #-receptors can be subdivided into two types. #1receptors are characteristic of cardiac and intestinal muscle; #2receptors are characteristic of bronchial and uterine smooth muscle. However, both #1 and #2 receptors occur in many tissues including skeletal muscle and fat. The #-adrenergic agonists of interest in animal production mainly affect #2receptors but the classification into #1and #2is not clear-cut and some #1activity is also evident.
The potential value of #-adrenergic agonists in meat animals lies in their so-called repartitioning effects. They reduce the amount of fat in the body and increase protein accretion so promoting muscular development. They appear to do this by both reducing the production of fat (lipogenesis) and increasing fat breakdown (lipolysis; Cardoso and Stock, 1998). Their action on protein accretion seems to be through reducing breakdown, so favouring the synthesis component of the normal protein turnover. Some muscles, particularly those with a high proportion of type II, glycolytic, fast contracting fibres (see Chapter 3), are more affected than others. Although they influence circulating Table 2.10. Examples of products containing anabolic sex hormones.
Product name Hormone Manufacturer
Finaplix® Trenbolone acetate Hoechst
Ralgro® Zeranol Crown Chemicals
Compudose® Oestradiol Elanco
Synovex S® Oestradiol + progesterone Syntex Synovex H® Testosterone + oestradiol Syntex
levels of insulin, growth hormone and the thyroid hormones, it is not thought that the effects of #-adrenergic agonists are mediated through these.
Unlike anabolic steroid hormones, #-adrenergic agonists are effective in all sexes to the same extent. They are orally active and so can be administered in the feed, usually at levels of between 1 and 10 ppm.
They are therefore effective at very low doses. Although they may show small effects on growth rate this is not a consistent effect and is generally more apparent in ruminants than in non-ruminants. Carcass yield is improved by around 1–2% in pigs and poultry and by up to 5–6% in cattle and sheep. There is evidence that this is due to both an increase in carcass weight and a decrease in the size of the viscera. The carcasses have better conformation. The increases in muscle development are accompanied by reductions in subcutaneous, intermuscular and intra- muscular fat. The effects of administration of a #-adrenergic agonist to pigs are illustrated in Table 2.11. A comparison of the effects of treatment with #-adrenergic agonists on pigs and sheep is shown in Table 2.12.
These figures illustrate the potential gains to be had from administration of #-adrenergic agonists. However, the gains are offset by potentially poorer meat quality. Some #-adrenergic agonists may produce meat that
Fig. 2.7. The chemical structures of adrenaline and four #-adrenergic agonists.
is darker and duller in colour, and tougher after cooking. The darker colour is caused by reduced glycogen levels in the muscles at slaughter leading to a higher ultimate pH in the muscles. Toughness may result from a lower activity of proteolytic enzyme systems post mortem (see Chapter 5). By reducing intramuscular fat, #-adrenergic agonists may also reduce other eating quality characteristics.
Porcine and bovine somatotrophin (PST and BST)
Giving frequent injections of growth hormone (somatotrophin) to pigs reduces carcass fat and increases muscle development. Some studies have shown increases in lean tissue growth rate and reductions of fat deposition of up to 20%. Food conversion is also more efficient. It is unclear whether there are any major deleterious effects on muscle quality but marbling fat tends to be reduced in line with overall carcass fat and this might influence tenderness. Similar effects on fat and muscle development have been seen with administration of BST to Table 2.11. The effect of a #-adrenergic agonist on carcass composition in pigs.
Pigs treated Change in Control with #-adrenergic relation to
animals agonist control group
Killing-out percentage 77.6 78.8 ↑
Backfat thickness (mm) 12.6 11.0 ↓
Cross-sectional area of the
m. longissimus dorsi (cm2) 50.4 55.9 ↑
Percentage muscle in
foreloin joint 50.3 55.0 ↑
Percentage fat in foreloin joint 35.7 31.6 ↓
Percentage bone in foreloin joint 14.1 13.4 ↓
Based on data published in Warriss et al.(1990a, b).
Table 2.12. Comparative effects of #-adrenergic agonists in pigs and sheep (data for pigs as in Table 2.11; data for sheep from Warriss et al., 1989c).
Percentage change over control animals
Pigs Sheep
Killing-out percentage +1.5 +7.6
Backfat thickness !13 !36
Cross sectional area of m. longissimus dorsi +11 +28
Proportion of muscle in foreloin +9 +15
Proportion of fat in foreloin !11 !10
Proportion of bone in foreloin !5 !26
ruminants. Somatotrophins can be produced in relatively large quantities by bacteria using recombinant DNA techniques to insert the appropriate genes. However, because they also occur naturally, they may be more acceptable to consumers than #-adrenergic agonists even though not currently permitted in the EU.