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Beef cattle , including steers as well as heifers, were treated in large numbers, especially in the USA and the UK, with DES or hexoestrol, administered orally, until the use of these compounds was restricted.

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Reported side effects of hormone treatment for growth stimulation are few and generally concern the use of oestrogens in steers. Changes in body conformation such as feminization and raised tail-heads were described as early as Similarly, bulling has occurred with increased frequency 57, , , although in most animals it is limited to the first few days after implantation However, it has been reported from Kansas that 2.

In a study of the effect of reimplantation of oestrogens in steers, all animals were given a 30 mg DES implant at a live weight of kg, and then reimplanted 91 days later, with either 30 mg DES or Synovex S. Following the second implant, the frequency of the steer-buller syndrome was 1. The steer-buller syndrome is a special problem in feedlots. No reliable explanation of how the growth-promoting hormones act has yet been furnished.

Some observations indicate an indirect influence through changes in the balance of endogenous hormones. However, others have found no such effect 49, 60, 67, 77, Recent experiments indicate that DES reduces the rate of muscle catabolism in steers As regards the anabolic androgens, evidence exists indicating competition with glucocorticoids for receptor sites on the muscle cell membrane. Since glucocorticoids have a catabolic effect on tissues, their displacement from muscle cells would reduce catabolism.

The significance of these findings is not yet clear. For a fuller discussion of possible mechanisms of action of the hormones, see references 2, 27 and Any discussion of possible health hazards connected with the use of hormones in animal production must take into account the normal occurrence of hormones and their metabolites in body fluids and tissues, and the fact that the levels of these hormones vary greatly, according to the physiological state of the animal.

Thus, oestrogen levels in the blood of female farm animals may vary from a few pg up to 5—6 pg per ml plasma 6. As to males, the plasma of stallions and entire male pigs contains high levels of oestrogens, although mainly in the conjugated form. More recently, reliable data have also become available concerning concentrations in edible tissues; some of these are presented in Table 2.

For the sake of comparison, levels of oestrogen activity normally present in products of plant origin widely used in human nutrition are included. The general patterns of metabolism and elimination of endogenous hormones in farm animals have been outlined Progesterone is partially converted to androgens before excretion. The faecal route of elimination dominates in ruminants, while in the pig urinary excretion is more important. After repeated injections of progesterone to cows and steers over 2 to 3 weeks followed by 14 C-progesterone for 2 to 5 days, the animals were slaughtered 2 to 3 hours after the last injections.

Activity levels were 2 to 7 times higher in the fat, 3 times higher in the kidneys, and 13 times higher in the liver than in the muscle. Cooking or frozen storage did not affect the nature or quantity of metabolites In fat the levels were 3 to 5 times higher. Oestradiol was completely conjugated during absorption and its first passage through the liver.

Some conversion to oestrone took place The metabolism of DES in food-producing animals has been reviewed recently The substance seems to be eliminated to a large extent in unaltered form.

After oral administration of 14 C-DES to beef cattle, Free radioactivity was almost completely associated with unchanged DES. At the time of slaughter, levels were less than 0. In a study in steers implanted with 14 C-DES, on day after implantation radioactivity in muscle was not distinguishable from background.

It was above background in spleen, lung, adrenal glands and kidney, but less than levels corresponding to 0. In a similar study on steers, days after implantation, levels in liver, kidney, lungs and salivary glands were in the range of 0.

In a recent study of DES metabolism in rhesus monkeys and chimpanzees, most of the substance was excreted with the urine. Extracts in the organic and aqueous phase mostly contained unchanged DES in the free and conjugated form respectively Current evidence indicates that the oxidative metabolism of DES leads to at least three compounds that may have cytotoxic or mutagenic activity , but these have not been identified as DES metabolites in ruminants, but in the mouse.

Using a gas chromatographic method with a sensitivity limit of 20 ppb, no residues of zeranol could be detected in edible tissue from cattle slaughtered 65 days following implantation of 36 mg, or from lambs 40 days following implantation of 12 mg In another study, tritiated zeranol was implanted in cattle as part of mg doses.

Skeletal muscle obtained 10, 30 and 50 days following implantation contained no detectable residual activity This confirms previous results based on the use of 14 C-labelled zeranol Another metabolite of TBA in cattle is the keto compound, analogous to oestrone; quantitatively it appears to be of very little importance.

Other metabolites occurred in extremely small quantities in cattle , Similar findings have been reported in studies based on the use of implants cit. The major route of excretion is by faeces. Much work has been devoted to the development of sensitive methods of detecting hormone residues in meat from hormone-treated animals. As regards compounds given orally, it should in principle be possible to realize claims of zero-tolerance residue levels, by selecting the proper withdrawal time.

During recent years, the use of implants has, however, gained in importance. While removable implants have been tested in steers, with no decrease in performance when withdrawn 32 and 39 days before slaughter , the wide use of non-removable implants makes residue studies important.

Determination of normal levels of endogenously produced natural hormones is also important, to enable risk evaluation to be carried out in realistic terms.

Several residue studies have been made of synthetic as well as natural compounds, mainly in cattle. In the latter case there was almost complete overlap between values for untreated and treated steers after days Zeranol implants have so far not left detectable residues in edible tissue 99, , Most studies of androgens have concentrated on TBA.

Results based on radio-immunoassay of extracts or on radioactivity measurements 88, , , , , , have indicated levels in edible tissue of the order of 1 ppb or below. In a recent study using implants containing tritiated TBA in heifers, it was found that when slaughter took place 60 days after implantation, the major proportion of tritium-containing residues was not extractable with organic solvents.

This suggests that the major part of the residues after TBA implantation occurs in a non-extractable, possible covalently bound form in tissues Residue levels of gestagens have been also measured, in connection with their use as growth stimulants. Residues of melengestrol acetate used as a feed additive in daily doses of 0. Table 3, which provides data on normal levels of these hormones in certain dairy foods, shows that some foods represent hormone sources vastly richer than meat from hormonetreated animals.

Based on these values, and averages for consumption of various foods, the relative contribution of meat from hormone-treated animals to the total consumption of hormones has been calculated on the assumption of proper use of the hormones see Table 4.

It is clear that in most cases the contribution from meat of treated animals is insignificant when hormones have been properly used, and must be considered to be biologically without impact. This becomes even more evident when seen in relation to normal endogenous hormone production in man, as illustrated in Table 5.

It will be seen that even for oestrogens, the hormones considered the greatest risk, the maximal contribution from meat assuming proper use of the hormones is less than 0. Thus far the discussion has been limited to the natural hormones. For synthetic substances the situation may be different.

But again, considering the very low residue levels found when hormones have been properly used, the question may be raised whether the risk to the consumers is being grossly overestimated. Relative contribution of meat from hormone-treated steers to total hormone intake via food per cent. Contribution of hormones from hormone-treated steers relative to total daily hormone production in man 1 per cent. The improvement in FCE which usually accompanies the increase in gain adds to the economic benefits, and at the same time makes possible greater production of edible protein per unit energy used, and this in itself is of importance in a world lacking in protein supplies.

Few analyses of the economic advantages of using hormones as growth stimulants appear to have been made. For the UK, a recent calculation see Table 6 is based on the estimated increased return to producers for 1 cattle treated over a month period Assuming that 1 of these were steers and were heifers, and that the estimated daily gain was only 0.

Estimated increased return to producers from the use of hormones in animal production 12 months. From these figures are subtracted the cost of treatment. These calculations must be taken as an example only. Availability of the various feeds, variations in feed and product prices as well as in types of management from time to time and from place to place may play an important role. However, shortening the time required for producing a certain weight at slaughter will represent an economic advantage, especially under feedlot conditions, since non-feed costs also contribute significantly to the total cost of production 10 to 18 cents per head per day in the USA.

Growth rates are influenced by many factors, especially genetic constitution and feeding. Over time, selection as well as improvements in management systems, feed composition and feeding programmes have contributed much to increasing productivity in meat as well as milk.

Although it is difficult to evaluate the exact relative contributions of these factors, the overall improvements have been dramatic.

An example is the increase in milk yield per head in US dairy cattle. These gains represented a saving of about 23 billion kg of total digestible nitrogen per year, the volume of milk produced remaining relatively constant. The saving is equivalent to about 1. Data illustrating progress in beef production over the years are scarce, but increases in productivity similar to those for milk production are unlikely.

In addition to the use of hormones, many avenues are still open for increasing productivity in meat and milk production see , including breeding programmes, regulation of rumen fermentation, optimalization of the balance between the indirect and direct feeding of the ruminant organism proper, and disease control.

Systematic selection of high-quality sires, combined with an increase in the number of offspring from high-yielding females through embryo transfer, may bring about further improvements in beef and milk production.

In many countries, development along these lines has hardly begun. However, the establishment of effective breeding associations and the strict organization of programme planning and execution are prerequisites for realizing the potentials in this sector. The microbial systems in the rumen are extremely complex, and the balance between the various strains of bacteria is susceptible to changes brought about by many factors.

The recent introduction of substances such as monensin offers great promise in altering the fermentation pattern to the benefit of productivity by increasing FCE. Since the very extensive breakdown of carbohydrates and protein represents loss of much energy, research is currently being conducted in many laboratories in order to find new methods of increasing FCE.

To a large extent, feeding a ruminant means feeding the rumen microbes which then themselves serve as feed for the organism proper. This is indirect feeding, expensive in energy. On the other hand, the ruminant possesses, in the postruminal part of its digestive tract, all the enzymes necessary for utilizing all types of nutrients except cellulose.

The rumen microbes are necessary for the utilization of cellulose, which globally represents an enormous source of energy. However, it is possible to sustain an adequate microbial population in the rumen even when ruminal breakdown of part of the easily digestible nutrients is prevented.

Enabling nutrients to bypass the rumen will increase the utilization of feed for production, and also create a more adequate supply of amino acids.

Increased rumen bypass of nutrients can currently be brought about by several means, including formaldehyde and heat treatment of protein-rich feeds. A third method, aiming more at specific substances that may be rate-limiting for production e.

In the future, new methods of increasing rumen bypass will undoubtedly contribute significantly to increased productivity of ruminants. Whatever management system is adopted, effective disease control is essential for productivity.

In many areas of the world, infectious and parasitic diseases inflict heavy losses on animal production. A recent study has disclosed nearly a one-to-one relationship between investment in agricultural research and annual productivity of edible protein in ruminants. Investment in disease control is an important aspect of this work.

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