Sustained Growth Promotion of Pasture Fed Steers

Summary

This MLA project Sustained growth promotion of pasture fed steers was conducted in conjunction with a complementary project on sustained growth promotion within the Cooperative Research Centre for the Cattle and Beef Industry (Meat Quality). For the sake of completeness, results from both components of the study are used in the discussion to define the principles contributing to the extent of the anabolic response in growth, carcass composition and meat quality when sustained growth promotion strategies are used with grazing steers in northern Australia.

Likewise data from both components were used to evaluate the economic benefits of sustained growth promotion in representative production systems in northern Australia. Hormonal growth promotants (HGPs) are commonly used in the northern beef production system to increase growth rates of cattle in the final 3-5 months before slaughter. The increased gain is typically 10-20 kg or about an additional 0.1kg/d gain. This short period of implantation suits production systems in the northern hemisphere where cattle are usually fed high energy diets and slaughtered at a young age. In that environment the few months of the implant period is a significant proportion of the animals post weaning life.

In contrast, pasturefinished steers in northern Australia often take at least 3 years post weaning to reach heavy slaughter weights suitable for the Japanese market. Sustained growth promotion throughout the post- weaning period is necessary if the age at which animals are ready for market is to be reduced substantially. An additional 0.1kg/d for 18 months translates to an additional 55kg over an animal's lifetime. This would reduce the age at slaughter substantially, with flow-on benefit for the whole herd. The prevailing perception at the start of this project was that with repeat implantation with HGP's the growth response attenuated and consequently sustained growth promotion was not possible.

The project objectives were:

1. To demonstrate that sustained growth promotion between weaning and slaughter could be achieved.

2. To determine the effect of sustained growth promotion on commercial carcass value.

3. To quantify the magnitude of the response in weight gain and carcass yield so the economic value of sustained growth promotion can be assessed.

The first approach taken was to demonstrate that sustained growth promotion could be achieved over extended periods of time. This was accomplished using the very aggressive strategy of treatment with an HGP every 100 days. The next challenge was to understand the factors governing the magnitude of the improvement in liveweight gain so implantation strategies suitable for the northern Australian beef production system could be developed. Briefly, it was found that an additional 40 kg of liveweight gain could be achieved annually using an HGP every 100 days, provided steers were kept in positive energy balance. Lower annual liveweight responses resulted when steers experienced periods of weight stasis or weight loss during the northern dry season. The series of experiments conducted in this MLA project and the companion Meat Quality CRC project has allowed elucidation of a number of scientific principles which govern the liveweight response to treatment with anabolic steroids and the impact of that treatment on some measures of product quality. These are listed below.

They provide guidelines that beef producers can use to develop commercial implant strategies specific to particular geographic regions, particular animal management systems and particular market requirements. The more frequently steers are reboosted with a new implant the greater the response in liveweight gain. Sustained growth promotion can be achieved by repeat implantation with oestrogenic hormones or by alternate treatment with an oestrogen followed by treatment with a combined implant containing both an oestrogen and an androgen. Sustained growth promotion at a reduced rate of response can also be achieved by use of a long acting implant.

Once an implantation program has been commenced it should be continued through to slaughter. Under the pastoral conditions of northern Australia, steers with a single implant containing only an androgen are unlikely to exhibit accelerated growth. Greater responses in liveweight gain are achieved when steers are continually in positive energy balance than when they experience periods of weight stasis or weight loss. Frequent (every 100 days) treatment with oestrogenic hormones or a twice yearly strategy involving alternate use of a combined implant (oestrogen plus androgen) followed by an oestrogen does not alter carcass composition of steers, provided comparisons are made at the same carcass weight. Repeat implantation with implants containing an androgen may lead to a reduction in carcass fatness. The more aggressive the implant strategy the higher the likelihood of an increase in meat toughness.

An independent consultant was commissioned to conduct a financial analysis of sustained growth promotion in the northern Australian beef industry. He chose a strategy of three implantations per year, two implants of 100 days duration during the wet season and a 200 day implant for the remainder of the year. The economic benefits of finishing steers off pasture for the domestic, Korean and Japanese markets were assessed. The net increase in return per head, excluding mustering costs, varied between $22 and $33. The strategy would be profitable for many beef enterprises in northern Australia. This project achieved its objectives. Accelerated growth rates between weaning and slaughter were demonstrated.

The effects of repeat implantation on carcass composition and meat quality were defined. An independent financial analysis described the commercial situations where adoption of the technology would be profitable. In addition, the scientific principles that govern the magnitude of the increase in growth rate were established. This understanding means that reasonably accurate predictions can be made about the growth rate, carcass and meat quality responses to treatment in any beef production system where some of the production variables are known. Thus, application of the findings of this project are not confined to environment allocations similar to those under which the project was conducted.