THE USE OF HOU4ONES IN ANIMAL PRODUCTION

THE USE OF HOU4ONES IN ANIMAL PRODUCTION

1. INTRODUCTION

Hormone-dependent sex differences in growth rate have been known for a long time. it has also been known that growth rata 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 1O—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 par 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 10w, 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 hare on research since that time.

3. HORMONE PREPARATIONS USED IN ANIMAL PRODUCTION

HORMONESOF ENDOGENOUS ORIGIN

These comprise the “classical” steroid sex hormones, oestradiol-173, testosterone and progesterone. The two former era 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.

2.2 HORMONES OF EXOGENOUS ORIGIN

Of the oestrogens, the stilbene derivatives diethylstilboestrol (DES) and hexoestrol possess high biological activity and have been used most widely. They ere active orally as well as by implantation. Other orally active oestrogens include ethynyl-oastradiol, a more slowly metabolized derivative of the true hormone, with higher activity. An oestrogen with en entirely different structure 15 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 (TEA) 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 lo 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.

RANGE OF APPLICATION

In cattle the use of hormones is limited to veal calves and beef cattle. Veal calves are produced mainly o continental Europe, to an extent of about 8 million par year. Research has demonstrated that hormone treatment improves growth rata, 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 phytooestrogen zeranol.

1. Hormonally-active substances used in animal production

In sheep, especially in whether lambs, some increase in gain has been reported (11), but results are somewhat ambiguous

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