Department of Physiology
Veterinary College of Norway
Hormone-dependent sex differences in growth rate have been known for a long time. It has also been known that growth rate and FCE (feed conversion efficiency) are higher in intact males than in castrates. It was natural, then, that the availability of hormones and other natural or synthetic substances displaying hormonal activity led to experiments aiming at their use to increase production. Beginning in the mid-1950s, DES (diethylstilboestrol) and hexoestrol were administered to cattle increasingly in the US and the UK respectively, either as feed additives or as implants, and other types of substances also gradually became available. In general, such treatment has resulted in 10–15% increases in daily gains, similar improvements in FCE and improvement of carcass quality (increased lean/fat ratio). Thus there has been a substantial reduction in the amount of energy required per unit weight of protein produced (1,2), and the economic implications of this have been great.
While the use of hormonally active substances in animal production rose, opposition to their use also increased, because of the theoretical possibility that residues in edible tissues might endanger consumers. The factors leading to the ban on DES in the US, first imposed in 1973, have been described (3). Several reports confirm that DES endangers the health of animals and man, when repeatedly used in large doses (4,5). However, as regards risks due to the presence of residues in meat produced according to regulations, no documented deleterious effects have ever been reported in man, either from DES or any other substance with hormonal activity.
A distinction should be made between the hormones as such, for which the metabolism in the body is relatively well known, and synthetic or other substances for whose metabolic inactivation the body may not possess the enzymes necessary. When natural hormones are used in animal production, claims of zero-tolerance residue levels are not meaningful, since these compounds occur in detectable and highly variable concentrations in body fluids as well as in the tissues of all animals, treated or not (6,7). For other substances with hormonal activity the situation is different. However, when residue levels are extremely low, it seems reasonable to weigh the potential risks against the undisputed positive effects some of these compounds have in animal protein production.
This paper will discuss types of substances with hormonal activity currently in use or under investigation, their effects, mechanism of action, metabolism/elimination, tissue levels, risks to the consumer and their economic importance. Finally, other avenues to increased animal production as alternatives to use of hormones will be briefly envisaged. For the sake of simplicity the term hormone will be used, even if incorrectly, to cover all substances with hormonal activity, whether natural or synthetic. Since much information on the question collected before 1975 has been reviewed previously (8), the main emphasis will be placed here on research since that time.
These comprise the “classical” steroid sex hormones, oestradiol-17β, testosterone and progesterone. The two former are used either in the free form or as esters, mainly those of propionic or benzoic acid. Esterification generally causes prolongation of the half-life of the compounds in the body by 40 to 50%. The natural hormones having low bioavailability when administered orally, owing to rapid conjugation and metabolic transformation in the liver, they are therefore administered by subcutaneous implantation.
Of the oestrogens, the stilbene derivatives diethylstilboestrol (DES) and hexoestrol possess high biological activity and have been used most widely. They are active orally as well as by implantation. Other orally active oestrogens include ethynyl-oestradiol, a more slowly metabolized derivative of the true hormone, with higher activity. An oestrogen with an entirely different structure is zeranol, a derivative of a resorcylic acid lactone occurring in the fungus Giberella zeae.
The synthetic androgens comprise a large number of substances, most of which are steroids. Of these, trenbolone acetate (TBA) possesses strong anabolic properties and has received much attention during recent years, used alone or in combination with an oestrogen. Another anabolic steroid is methyl-testosterone.
Of synthetic gestagens, only one will be mentioned here: melengestrol acetate, which stimulates growth in heifers but not in steers, and which can also be used for the suppression of oestrus. Numerous other gestagens also exist, but at present few other than progesterone and melengestrol acetate are used to stimulate growth.
In addition to these substances, numerous others exist, and some of them are used more or less frequently in clinical veterinary medicine. However, clinical applications of hormones are not considered to be of consequence to the consumer, since such treatment is much less frequent than the use of hormones to promote growth.
Hormone preparationsin current use as growth stimulants are listed in Table 1, which also shows modes of application, dosages, etc. It will be noted that almost all preparations currently in use are based on implantation, the site usually being the base of the ear, or less frequently, the dewlap.
In cattle the use of hormones is limited to veal calves and beef cattle. Veal calves are produced mainly in continental Europe, to an extent of about 8 million per year. Research has demonstrated that hormone treatment improves growth rate, nitrogen retention and FCE during the five- to six-week period before slaughter (9,10). 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. During the last several years, practice has changed dramatically in the direction of increased use of implants of natural steroids, synthetic anabolic steroids and the phyto-oestrogen zeranol.
Table 1. Hormonally-active substances used in animal production
|Substances||Dose levels||Form||Main use - Animals||Trade name|
|DES||10–20 mg/day||feed additive||steers, heifers|
|DES||oil solution||veal calves|
|Hexoestrol||12–60 mg||implant||steers, sheep, calves, poultry|
|Zeranol||12–36 mg||implant||steers, sheep||Ralgro|
|Melengestrol acetate||0.25–0.50 mg/day||heifers|
|TBA||300 mg||implant||heifers, culled cows||Finaplix|
|DES + Methyl-testosterone||feed additive||swine||Maxymin|
|Oestradiol-17β benzoate +|
|Oestradiol-17β benzoate +|
In sheep, especially in wether lambs, some increase in gain has been reported (11), but results are somewhat ambiguous.
In swine, hormone treatment may increase growth rate, FCE and lean/fat ratio of the carcass in male castrates.
Poultry generally do not appear to respond to oestrogens by increased gain but by changes in lipid deposition. In male and female turkeys, androgens have recently been reported to increase growth rate as well as FCE (13).
When DES was used as a feed additive, a usual procedure was to start treatment of steers at a body weight of 360 kg and continue administration for 120 to 170 days. Since restrictions on its use were imposed, most preparations have been administered as implants, whose effect is usually limited to 80 to 100 days. Practice varies with management systems. Animals may be implanted at live weights from 270 to 450 kg. Depending upon the age and weight at the time of implantation, the animals are either slaughtered at the end of this first period, or fed for an additional period, either without further treatment or after a second implant to act for another 80 to 100 days. Most types of implants in use are not removable, but removable types have recently been tested and their effects described (114). When tested in steers, no reduction in performance was recorded when the implants were withdrawn 32 and 39 days before slaughter.
Implantation is subcutaneous, usually at the base of the ear, thus eliminating the risk that residues of the implantation site will be present in edible tissue.
In veal calves, hormone treatment may begin at a body weight of about 65 kg, the animals being slaughtered at about 170 kg. Implants of 20 mg oestradiol-17β + 200 mg progesterone in males and 20 mg oestradiol-17β + 200 mg testosterone in females resulted in a 20% increase in daily gain and 21% higher nitrogen retention in the period studied (14). In other studies, improvements of 10 to 12% in gain and 10% in FCE have been reported (15, 16, 17, 18). Nitrogen retention is about 70% in the very young veal calf, but decreases gradually to below 40% at the age of about 15 weeks. For ages of 10 to 15 weeks, the average conversion of feed protein to body protein is about 40%; this rate can be increased to about 60% by hormone treatment. The effective preparations were DES, oestradiol-17β, and the combination of TBA + oestradiol-17β (9). More recently, positive effects have been reported (19, 99) for zeranol alone (36 mg) and for zeranol (36 mg) + TBA (140 mg), with increases in nitrogen retention of the same order as for DES and E2 + TBA. When zeranol + TBA was implanted at the age of 56 days, the growth rate up to day 106 increased by 18% (19).
The most extensive studies of the effects of hormones on growth and FCE have been carried out on steers, under strictly controlled conditions as well as in the field. Since 1975, most studies have involved implants of oestrogens alone, androgens alone, or combined oestrogen/androgen preparations, although many trials have also been based on oestrogen/progesterone combinations during recent years.
Oestrogen implants have included DES, hexoestrol, oestradiol-17β and zeranol. DES implants have, as in previous studies, resulted in an increase of about 12% in gain and in improvement in FCE of the order of 10% (20, 21, 22). Hexoestrol implants, usually in doses of 30 to 60 mg, have been shown in numerous experiments to lead to considerable improvement in growth rate and FCE (23, 24, 25, 26, 27, 28, 29). In 19 trials carried out over the years on experimental husbandry farms in the UK, the overall average increase in gain produced by 45 or 60 mg hexoestrol implants was 0.16 kg a day, and in only 2 of the trials was it less than half that figure (2). Oestradiol - 17β implants alone (30 mg) have resulted in a 24% increase in gain and a 13% improvement in FCE (30). Zeranol implants, usually at 36 mg, have consistently improved gain as well as FCE (20, 23, 24, 29, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41). In a series of 21 UK trials over several years, the average response to zeranol implants alone was an increase in daily gain of 0.15 kg. Only in one trial was there no response (23). Similar results have been obtained in Ireland (cit. 2). Positive effects on gain in steers have been observed under a variety of experimental conditions, under controlled feeding, on ad lib feeding of standardized rations, and on pasture.
TBA implants administered alone at a dose of 300 mg have also had positive effects on growth (23, 24, 25, 26, 27, 29, 37, 40, 41), even if combination with an oestrogen has yielded better responses (vide infra). In a series of 8 trials in the UK, the average additional daily gain amounted to 0.09 kg, with considerable variation among trials (23). Similar results have been reported from Ireland (cit.2).
Combined preparations. A number of trials have been carried out with implants containing two hormones. The combination of an oestrogen with an anabolic steroid, or with progesterone, has met with the greatest responses. Synovex-S has consistently increased gain as well as FCE, with responses averaging about 20% and 17% respectively (20, 21, 22, 34, 35, 36, 37, 42, 43, 44, 45, 46, 47, 48). Hexoestrol + TBA (usually 30 or 45 mg hexoestrol + 300 mg TBA) has resulted in marked increases in gain (24, 25, 26, 29, 49, 50, 51, 52, 53), of the order of 30% and in FCE (25, 49, 50, 51, 53) of the order of 20%. Oestradiol-17β + TBA (20/140 mg) has given similar results (27, 28, 37, 54, 55), as has Zeranol + TBA (36/300 mg), also recently tested (27, 37, 38, 39, 40, 41, 56).
Hormone preparations have also been tested in combination with substances such as monensin, which increase FCE by promoting propionic acid formation in the rumen. Results have varied from no effect (38, 52) to marked additional gain (43).
Reimplantation, tested under various forms of management with varying results (25, 26, 32, 36, 52, 56, 57, 58), has not gained general acceptance. Lamming (45) has stated that “repeat implantation of hormone is not likely to produce the benefits obtained from its initial use, since a second dramatic change in the endocrine balance of the animal is not likely to occur. In addition, double implantation increases the possibility of exceeding the optimum dose rate and the chance of deleterious side effects occurring.”
The evidence for highly significant positive effects on the growth rate and FCE of steers is thus beyond dispute, the most marked effects being provoked by implants combining an oestrogen with an androgen of high anabolic activity.
Since the entire male animal produces its own anabolic androgen, testosterone, an effect of additional hormones similar to that for steers is not to be expected. The number of trials with bulls is also limited. Positive effects on gain have been reported using DES alone (2, 58, 59) and combined oestrogen/TBA implants (60); in other studies, no effect on gain has been recorded (49, 61), while a certain increase in the deposition of fat in the carcass has been observed (61).
Recent trials with beef-producing heifers have mostly been based on the use of an androgen, although oestrogens have been tested, alone or in combination. Thus, zeranol has been reported to increase gain (62, 63), while in other trials no response has been observed (37, 62). TBA administered alone (300 mg) has led to increases in weight gain and FCE of the order of 36% and 25% respectively (24, 27, 37, 64, 65, 66, 67, 68, 69, 70). In other trials, combinations of an oestrogen with TBA (68) or testosterone (62) have yielded significant growth responses. In general, it appears that the effect of TBA alone in heifers corresponds closely to the effect of combined oestrogen/TBA implants in steers.
Trials have mainly concerned wether lambs, and positive effects of hormonal treatment have been reported using DES (69), hexoestrol (71) and zeranol (72, 73), although other reports have indicated that zeranol yields no significant effects (74, 75). Wether lambs implanted with TBA + oestradiol-17β have shown increases in gain, carcass weight and FCE (11, 69). In general, however, the results obtained in sheep thus far do not warrant the same clear-cut conclusions as for steers and heifers.
There is little evidence that existing hormonal preparations influence the growth rate and FCE to an extent that would be interesting from a practical point of view. The lean/fat ratio in male castrate and female pig carcasses may be increased by the use of oestrogen/androgen combinations (76). In poultry, redistribution of fat in the body is a known effect of oestrogens. Recent research indicates improved growth rate and FCE using androgens in young male and female turkeys (13, cit. 27).
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 1958 (118). Similarly, bulling has occurred with increased frequency (57, 118, 119), although in most animals it is limited to the first few days after implantation (46). However, it has been reported from Kansas that 2.2% of all steers fed in pens have to be removed, at an estimated loss of $23 per head (119). 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 260 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 1.65% for the DES-DES group, and 3.36% for the DES-Synovex S group. The economic advantage of using DES + DES was estimated at $1.15 per head (57). 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. Thus there have been reports of DES and TBA increasing the levels of growth hormone and/or of insulin in plasma (51, 63); these hormones are known to stimulate amino acid transport across the cell membrane. However, others have found no such effect (49, 60, 67, 77, 82). Bulls fed DES (10 mg/day) over two years had significantly higher plasma testosterone levels than controls (78); those levels are positively correlated with growth (78, 79, 80). Recent experiments indicate that DES reduces the rate of muscle catabolism in steers (81).
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. TBA alone, and even more when combined with oestradiol-17β, causes a marked decrease in the concentration of total thyroxin in plasma of steers (82). In another study, combined oestradiol-17β-progesterone implants (20 + 200 mg) in steers caused a uniform but slight increase in thyroxine binding capacity (44). 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 83.
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 5–6 000 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. Milk also contains oestrogens in very high concentrations in the first drawings after parturition; in non-pregnant animals, levels in the range of 80–100 pg/ml have been reported (6, 84). 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.
Table 2. Concentrations of endogenous hormones in edible tissues of farm animals
2 500–5 500
|Wheat germ oil||4 000 pg/g DES equivalent|
|Soy-bean oil||2 000 000 pg/g DES equivalent|
Sources: 85, 86, 87, 88, 89.
The general patterns of metabolism and elimination of endogenous hormones in farm animals have been outlined (90). In ruminants, testosterone and oestradiol-17β are rapidly converted to their epimers, biologically much less active, epitestosterone and oestradiol-17α. Progesterone is partially converted to androgens before excretion. In the pig, epimerization of testosterone and oestradiol-17β does not appear to take place to a significant degree. 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 14C-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. Excretion of radioactivity amounted to 50% and 12% in faeces and 2.0% and 1.2% in urine in cows and steers respectively. About 50% of the activity in muscle and milk was associated with unchanged progesterone, most of the remaining activity being associated with a mono-hydroxy compound. Cooking or frozen storage did not affect the nature or quantity of metabolites (91).
Following daily injections of 1 mg oestradiol-17β or its benzoate to heifers and steers for 11 days, followed by the 14C-compounds on days 12, 13 and 14, the animals were slaughtered 3 hours after the last injections, when residual levels were maximal. In muscle extracts, oestradiol-17β represented the major fraction of extracted activity (38 to 71%), followed by oestrone (17 to 45%). Levels in muscle were 161 to 225 pg/g and 40 to 86 pg/g for oestradiol-17β and oestrone respectively. In fat the levels were 3 to 5 times higher. The authors conclude that residual levels are extremely low when these hormones are administered as growth stimulants to growing/finishing cattle (92). Glucosides of the 17β- and the 17α- epimers, and the glucoronide of the 17α- epimer are the major metabolites in cattle (125). When oestradiol-17β was administered orally to swine, plasma concentrations were very high 7 min after administration. Oestradiol was completely conjugated during absorption and its first passage through the liver. Some conversion to oestrone took place (93).
The metabolism of DES in food-producing animals has been reviewed recently (94). The substance seems to be eliminated to a large extent in unaltered form. After oral administration of 14C-DES to beef cattle, 99.5% of the radioactivity was excreted within 5 days after withdrawal. In liver extracts, radioactivity associated with DES-conjugate and free DES was found to be 75% and 25% respectively. Higher than background levels of activity were observed after withdrawal in kidney, liver, bile and urine/faeces for up to 5, 7, 9 and 11 to 12 days respectively (95). The fate of 24-mg DES implants containing 14C-DES and implanted in the dewlap of calves was studied over 98 days. Free radioactivity was almost completely associated with unchanged DES. At the time of slaughter, levels were less than 0.1 ppb in muscle and fat, and 1 to 1.5 ppb in liver and kidney (96). In a study in steers implanted with 14C-DES, on day 120 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 0.5 ppb. In a similar study on steers, 120 days after implantation, levels in liver, kidney, lungs and salivary glands were in the range of 0.07 to 0.13 ppb of DES equivalent (98). 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 (121). Current evidence indicates that the oxidative metabolism of DES leads to at least three compounds that may have cytotoxic or mutagenic activity (121), 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 (101). In another study, tritiated zeranol was implanted in cattle as part of 36-mg doses. Skeletal muscle obtained 10, 30 and 50 days following implantation contained no detectable residual activity (99). This confirms previous results based on the use of 14C-labelled zeranol (100).
Trenbolone is a 17β-OH steroid esterified in the 17 position with acetic acid. Upon release in the organism the ester is rapidly hydrolyzed to the free compound TB-17β-OH and acetate. In cattle the 17β-OH compound is rapidly transformed to its 17α-OH epimer, in the same manner as oestradiol-17β in this species. The 17α epimer possesses only about 5% of the biological activity of the 17β epimer. Another metabolite of TBA in cattle is the 17-keto compound, analogous to oestrone; quantitatively it appears to be of very little importance. Following intravenous injection of TBA, levels of TB-17β-OH and TB-17α-OH of 0.05 and 0.005, 0.10 and 1.0, 0 and 191 ppb have been recorded for muscle, liver and bile respectively. Other metabolites occurred in extremely small quantities in cattle (102, 103). Similar findings have been reported in studies based on the use of implants (cit. 102). The major route of excretion is by faeces. Metabolism studies of TBA thus clearly show that the substance is rapidly subjected to biological inactivation in cattle, mainly by epimerization of the free steroid to the 17α-compound, and that the major route of excretion is via the bile.
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 (104), 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. When regulations governing dose, sites of implantation and timing in relation to slaughter are adhered to, residue levels of DES (88, 95, 96, 97, 98), hexoestrol (105) and oestradiol-17β (106, 107) in edible tissues have generally been in the lower ppb to the ppt range, i.e. from a few ng/g down to some hundred pg/g of tissue. In the latter case there was almost complete overlap between values for untreated and treated steers after 105 days (107). Zeranol implants have so far not left detectable residues in edible tissue (99, 100, 101).
Most studies of androgens have concentrated on TBA. The ester being rapidly hydrolyzed, measurements of residues have been limited to the free compound and/or its major metabolite. Results based on radio-immunoassay of extracts or on radioactivity measurements (88, 102, 103, 106, 108, 109, 122) 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. In muscle 95.5%, in liver 94.4%, in kidney 98.8% and in fat 59.1% of the radioactivity remained in the aqueous phase, not quantifiable by radio-immunoassay. This suggests that the major part of the residues after TBA implantation occurs in a non-extractable, possible covalently bound form in tissues (123).
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 0.25 to 0.50 mg per head have consistently been below the sensitivity levels of the methods used (i.e., below 10 ppb in fat, liver, muscle and kidney), whether or not the compound was withdrawn 48 hrs before slaughter (124).
According to the Agricultural Research Service, United States Department of Agriculture (ARS), the average per caput consumption of beef is 157 g per day in the US (110). Calculations show that 157 g of beef from an animal implanted 61 days before slaughter with a combined implant containing 20 mg oestradiol-17β + 200 mg progesterone or testosterone will contain 3.43 ng oestrogen and 19.5 ng progesterone or 16 ng testosterone. 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 0.01% in the prepubertal boy who represents the lowest endogenous oestrogen production.
|Milk, from non-pregnant cows||80||9.5|
|Milk, from pregnant cows||126|
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.
Table 4. Relative contribution of meat from hormone-treated
steers to total hormone intake via food
|Child under 1 year||0.22||0.014|
|Child 6 to 8 years||1.56||0.1|
Source: Condensed from 103.
Table 5. Contribution of hormones from hormone-treated steers
relative to total daily hormone production in man 1
1 The figures represent effective fractions (i.e. 10% of real fractions), to take into account the low bio-availability of the hormones absorbed orally.
In the production of meat for human consumption, a hormonally-induced increase in growth rate of the order of 10% evidently has major economic implications. 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. Some of the hormones that have become available recently appear on average to increase gain as well as FCE considerably beyond the 10% level, and in examining whether they should be approved for use in animal production, the risk/benefit analysis must take this fact into account.
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 1 350 000 cattle treated over a 12-month period (111). Assuming that 1 155 000 of these were steers and 195 000 were heifers, and that the estimated daily gain was only 0.06 to 0.11 kg for steers and 0.05 to 0.06 kg for heifers, depending on the preparation used, the overall gross increased return was calculated at ₤21 306 000, without taking into consideration improvements in FCE.
Table 6. Estimated increased return to producers from the use of hormones in animal production (12 months)
|1. Number of animals treated1|
|2. Average increase in daily gain (kg)2|
|3. Average increase in slaughter weight (kg)2|
|4. Estimated total increase in slaughter weight (carcase weight) (tonnes)||5 478|
|5. Estimated overall increased gross margin per head to the producer (₤)3|
|6. Estimated gross return (₤)|
14 580 000
15 360 000
4 920 000
1 026 000
5 946 000
|Total||21 306 000|
|7. Estimated price per dose (₤)||1.20||1.80|
|Total (₤)||900 000||1 080 000|
|8. Net return (₤)4||14 460 000||4 866 000|
|Total (₤)4||19 326 000|
1 Estimated from sales of the preparations during a year.
2 Based on results from Meat and Livestock Commission trials using yard finishing cattle receiving only one implant.
3 Based on 1978 data showing that 0.1 kg increase in daily gain gave an increase in gross margin of ₤13 per head, and that increase in slaughter weight averaged 85 p per head. From these figures are subtracted the cost of treatment.
4 Does not include costs of veterinary services, etc.
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. In the period 1944–1975, the number of dairy cows decreased by 33%, while the average yield per cow increased by 60%. 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 1.1 billion bushels of maize (112). 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 115), 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.g. certain amino acids), or of significance in treatment of diseases, is protection against rumen degradation by such means as incorporation into the ration of long-chain fatty acid mixtures in the form of small pellets (113, 114).
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. An increase of about 45% in scientist/man years and a corresponding increase in funding for research and development is considered sufficient to raise productivity in this sector by 50% (115). Investment in disease control is an important aspect of this work. Annual world mortality losses from disease exceed 50 million cattle and buffalo, and 100 million sheep and goats. Non-lethal diseases are believed to lead to an equivalent reduction in production (115). Thus, investment in disease control holds great promise for future augmentation of animal protein production.
In these perspectives, the significance of hormones in animal production may seem marginal, leading to the question of what priority to give to the various efforts to increase productivity and production. In the global context it is, however, at least at present, impossible to adopt one approach to the exclusion of others. As long as preparations exist that combine positive effects on yield and feed utilization with low or non-existing risk to the consumer, there will be a market for them. What is more, the use of hormonally-active substances in the future may not be limited to those currently available. Common to the present compounds, natural or synthetic, is that they are degraded in the body only to a limited extent. An entirely different situation exists for proteid hormones, which are broken down completely to amino acids, leaving no residues whatever. An example is the growth hormone which not only stimulates growth (116) but also milk secretion, even in high-yielding cows (117, 126). This anabolic hormone is currently available only in small quantities for research. However, a recent breakthrough in the use of recombinant DNA technique (see 127) has made large-scale microbial production of species-specific peptide hormones a realistic possibility. Combined with the development of miniaturized automatic delivery systems for subcutaneous use, a new era may be visualized as regards the use of hormones in animal production.
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