Why have antibiotics been added to pig diets?

The initial use of antibiotics in diets arose from the discovery in the late 1940's, in the United States, that including the fermentation products of Streptomyces aureofaciens (a strain of bacteria) in the diets of simple-stomached animals, such as pigs and poultry, resulted in growth responses. In the next fifty years, the use of antibiotics as feed additives in pig and poultry production became virtually universal.

How do feed antibiotics promote growth in pigs and poultry?
The exact mechanism as to how feed antibiotics 'promote' growth is not entirely clear. It is probable that they allow animals to express their natural potential for growth, and that the 'growth promotion' is achieved by antibiotics exerting their effects through a direct influence on bacteria in the animal gut, since there is no response in germ-free animals. Antibiotics used as routine feed additives also appear to prevent some diseases, since their withdrawal can result in the emergence of endemic conditions particularly diarrhoea in the weaned piglet.

It is important to make a distinction between antibiotics used in the treatment and prevention of disease in farm animals (prescribed therapeutic and prophylactic use), which differs from their use as feed additives to enhance growth. As feed additives, antibiotics are used at low concentrations of 2.5 - 50ppm (depending on the compound used).

Why are we becoming concerned over feeding antibiotics to farm animals?

From the outset, there were worries that through over-use, the effectiveness of feed antibiotics might diminish and that strains of bacteria would arise which were resistant to their effect. Of greatest concern was the possibility that resistance generated on the farm could lead to a loss of effectiveness of key antibiotics in human medicine.

Through the years there have been a number of expert committees (e.g. The Netherthorpe Committee, 1962 and The Swann Committee, 1969) and enquiries into this aspect. Sweden considered the risks unacceptable to their public and placed heavy restrictions on feed antibiotics in 1986.

Recently the European Community reviewed the technical information and changing social attitudes to the use of additives in animal feed. As a result the EU has introduced legislation, which effectively bans most feed antibiotics from August 1999.

Are some antibiotics still legally available for use in animal feed in the EU?

Yes. Although the EU has banned several antibiotics since 1997, four antimicrobial additives, monensin, avilamycin, salinomycin and flavomycin, are still permitted for use in the European feed industry. However, several countries are taking a proactive stance and are pressing for a comprehensive ban.

In addition, in the UK and elsewhere, some large-scale farming enterprises are publicly declaring their own policy on antibiotic growth promoters and withdrawing all of these from livestock feeds.

In the EU it is probable that there will be a voluntary or legislative ban on the feeding of all antibiotics and some antimicrobial feed additives such as zinc oxide to farm animals. This would leave only prescribed antibiotics for the treatment of acute disease conditions.

What was the effect on pigs in Sweden following the withdrawal of feed antibiotics?

The general observation was that the Swedish farmers were badly prepared for the change and had assumed that the withdrawal of antibiotics would barely affect their enterprise. The main effect in the first year of the ban was that post-weaning scours (diarrhoea) were greatly increased (indicating poorer gut health status), post-weaning mortality increased by about 1.5 percentage units and days taken for piglets to reach 25kg live-weight were extended by about 5 days.

The strategies to cope with the new problems were somewhat disappointing. The demand for veterinary prescriptions of antibiotics for feeds was greatly increased so that the usage of the antimicrobial olaquindox increased to levels above those prevailing before the ban was imposed. (75% of the pig population in Sweden was being treated with antibiotics.) A wide range of alternative feed additives was tried with mixed success. It became clear that no single approach could adequately offset the effect of the withdrawal of the antibiotics. Major problems appeared to have been averted by an increase in the use of prescription only antibiotics, and the introduction of zinc oxide into weaner feeds as a non-antibiotic gut bacteria modifier.

The current position is far more encouraging. Over the years, producers and veterinarians have adopted farming methods, which promote health without reliance on antibiotics. These include delayed weaning, use of all-in-all-out housing systems and regular veterinary inspection of pig farms. In 1998, 85% of piglets were reared and finished without the use of antibiotics or zinc oxide. However, changes to systems after the withdrwal of feed antibiotics have increased the cost of production by between 4 and 10%.

What will be the impact on pigs from withdrawing feed antibiotics in Britain?

It is difficult to predict the precise effect of withdrawing antibiotics on the growth and health of pigs on British farms. The protracted economic crisis in the UK pig industry, the lack of investment available for upgrading housing and husbandry standards could result in greater losses through reduced growth, disease and mortality. An estimate of the economic impact of withdrawing feed antibiotics in the UK is between £1.50 and £3.00 per pig finished.

Are there any alternatives to antibiotics in pig diets that have similar effects on the efficiency of production?

Currently there is no single substance, which could replace the function of feed antibiotics. Moreover, any single substance (e.g. zinc oxide), which is intended to replace the role of antibiotics in farm animals, will be subject to the intense scrutiny that antibiotics have been subjected to over the past 40 years. Since the growth benefit found from feeding antibiotics is achieved through many different effects on the gut, the strategy for replacing them will depend on a combination of nutritional, housing and husbandry factors.

Over the years a number of alternative feed additives have been tried often in combination with existing antimicrobials, such as antibiotics, zinc oxide and copper sulphate. A review of the scientific literature suggests that, in general, alternative additives provide little consistent growth benefit. Among the more promising are organic acids, fermentable substrates and enzymes.

The efficacy of the many individual feed additives promoted as alternatives to antibiotics are summarised below, with a score to give an indication of their potential value.

The efficacy and potential for developing alternative additives and strategies to replace the role of antibiotic feed additives in pig production

Acknowledgements

This paper was prepared on behalf of BSAS by Dr Pinder Gill (Meat and Livestock Commission), with the assistance of Dr Vernon Fowler (University of Aberdeen) and Derek Armstrong (Meat and Livestock Commission). For further information contact the author.

For further copies please contact the BSAS office on 0131 445 4508.