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Hormones may be used to promote efficient growth. Estradiol, progesterone, and testosterone (three natural hormones), and zeranol and trenbolone acetate (two synthetic hormones) may be used as an implant on the animal's ear. The hormone is time released and is effective for 90 to 120 days. In addition, melengesterol acetate, which can be used to suppress estrus, or improve weight gain and feed efficiency, is approved for use as a feed additive. Not all combinations of hormones are approved for use in all classes of cattle. Hormones are approved for specific classes of animals, and cannot be used in non-approved classes.
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5.1 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. 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).
Its important to also understand that Hormones do in fact naturally occur in meats and that is going to vary..but its what makes it hard to determine whats naturally and whats man made response..