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4 Body Building In The Cattle Industry Hormone Growth Promotants by Trisha Cowley Pastoral Production Officer, Pastoral Production Division Hormone growth promotants (HGPs) are used widely in the north, however often little is understood about the product and methods of use. This article is part of a series which will aim to provide some information about HGPs and their use in northern production systems. How do they work? HGPs do exactly what their name implies promote growth. When used properly they can be a cost effective way to reliably increase liveweight performance. HGPs work by improving feed conversion efficiency and thereby increasing growth rates and overall weight gains by 1030%. For example Compudose 200 and Ralgro gave an average additional weight gain of 0.1kg/day over 100 trials. They increase muscle growth, mature size and lean yield and tend to delay fat deposition. Beef CRC research has linked HGPs to reduced meat quality, however this isnt yet an issue in the northern live export market. There are a range of HGP products available which differ chiefly in the formulations of hormone compounds and their release mechanisms. Different products are targeted at different growth rates and sexes. Each hormone type and delivery system has its advantages and tradeoffs. These will be considered in this article. What are they made of? HGPs are made up of compounds which mimic the actions of hormones that influence cattle growth. These are released into the blood stream where each hormone operates differently to influence growth. There are several common chemical compounds used in HGP products registered in Australia: Oestrogens (oestradiol 17-, oestradiol benzoate, zeranol (non-steroidal compound)) Androgens (testosterone propionate, trenbolone acetate (synthetic androgen)) progesterone In Australian HGP products, the major hormones used are Oestrogen (female hormone) or a combination of Oestrogen and Androgen. Pure androgen (male hormone) appears to require animals to be achieving higher weight gains, of around 0.8kg/day, before it has an effective growth promoting action, and HGP made up of pure androgen are therefore not commonly sold on the North Australian market. The commonly used HGPs are therefore either oestrogens (for example Compudose) or Oestrogen/Androgen combinations (for example Revelor) both of which will be effective with the range of weight gains that we experience on pastures in our region. For example the average daily gain for weaner steers over one year post-weaning at Kidman Springs in the VRD is 0.3kg/day (-0.05kg/day over the dry and 0.52kg/day over the wet). With the use of HGPs we could expect 0.1kg/day additional growth over a 200 day growing season, resulting in an extra 1015kg of liveweight per steer at the end of the growing season. Gender specific products Hormones derived from one sex, when used in the opposite sex produce additional growth. Manufacturers will specify if a product is for use in steers, heifers or both and this is because of the compounds used. HGPs that contain both oestrogenic and androgenic products result in an additive effect as both products act independently. For best growth outcomes it is important to follow the manufacturers recommendations. This is particularly the case for breeding females as both androgenic and oestrogenic based HGPs can temporarily or permanently impair reproductive function.