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Sex hormone production

INTRODUCTION Hormone-dependent sex differences in growth rate have been known for a long time. 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. While the sex hormone production 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.

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. Finally, other avenues to increased animal production as alternatives to use of hormones will be briefly envisaged. HORMONE PREPARATIONS USED IN ANIMAL PRODUCTION 2. The two former are used either in the free form or as esters, mainly those of propionic or benzoic acid. 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.

The synthetic androgens comprise a large number of substances, most of which are steroids. 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. RANGE OF APPLICATION 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. Poultry generally do not appear to respond to oestrogens by increased gain but by changes in lipid deposition. MODES OF APPLICATION 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.

Animals may be implanted at live weights from 270 to 450 kg. 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. 1 Veal calves In veal calves, hormone treatment may begin at a body weight of about 65 kg, the animals being slaughtered at about 170 kg. 2 Steers 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. Oestrogen implants have included DES, hexoestrol, oestradiol-17β and zeranol. In a series of 8 trials in the UK, the average additional daily gain amounted to 0. 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. Hormone preparations have also been tested in combination with substances such as monensin, which increase FCE by promoting propionic acid formation in the rumen. 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. 3 Bulls 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. 4 Heifers 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. 6 Swine and poultry 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. In poultry, redistribution of fat in the body is a known effect of oestrogens. 7 Undesirable side effects in treated animals Reported side effects of hormone treatment for growth stimulation are few and generally concern the use of oestrogens in steers.