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Northern Territory Government
5 How is the hormone released? There are two main ways in which HGPs are designed as compressed pellets or silicone rubber implants. These two systems differ in the way the hormone is released, that is the rate of hormone release and the length of time the product will act. Compressed pellet formulations The dose rate of compressed pellets is controlled by the number of pellets in the implant (the greater the number the higher the dosage rate). They have a functional life (the period of time that they will promote growth) from 60120 days. Most compressed pellet products state that the minimum implant period is 70 days suggesting this is their functional life. Typically, release rates are high in the first few days of implantation when large quantities of hormone are released and then quickly deplete. Silicone rubber implants In comparison, silicone rubber HGP implants can have a much longer functional life than compressed pellets. The dose rate is controlled by the thickness of the silicone rubber, with the thicker the rubber, the slower the release rate. Products using this delivery system can release hormones at slower rates over much longer periods of time (e.g. up to 400 days) than compressed pellets, and therefore can have longer functional lives. Further, they deliver an initial high burst of hormone over the first 28 days which is much longer than compressed pellet formulations. Figure 2 illustrates the difference in theoretical release rates for compressed pellet and silicone rubber implants. There are a number of products on the market now suitable for northern Australia. These differ in their functional life, the gender that they are targeted at and the growth rates that they are formulated for. There are a number of key principles to consider when choosing HGP products suitable for your production system. Look at the hormones and delivery system (compressed pellets vs silicone rubber) of each product. For animals on native pastures oestrogen based or combined oestrogen and androgen products will be most suitable Growth rates of over 0.8kg/day are required for androgen only products Compressed pellet HGPs will act for 60120 days and then will require re-implantation for continued liveweight gain advantages over longer periods of time Silicone rubber products can have functional lives of up to 400 days hence are more suitable for animals which receive minimal handling throughout the year Do not use HGPs in breeding females The next article in this series will consider different implantation strategies for stations based on practical constraints and the latest research findings. Silicone rubber coat impregnated with crystal of oestradiol 17 Silicone rubber coat impregnated with crystal of oestradiol 17 500 microns Compudose 400 Inert rubber core 250 microns Compudose 200 Figure 1. Silicone rubber thickness in products with different release rates. O es tr ad io l r el ea se Compressed tablet (theoretical) Silicone rubber (theoretical) Minimum dose for maximum anabolic response Figure 2. Theoretical release rates for compressed pellet and rubber implants. Figure source R.A. Hunter (2009) Hormonal Growth Promotant use in the Australian Beef Industry, Final Report. Compressed tablet (theoretical) Silicone rubber (theoretical) Minimum dose for maximum anabolic response Silicone rubber coat impregnated with crystal of oestradiol 17
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