Increasing demand for cheap food and the advent of modern production methods resulted in the development and use of feed additives, including sub-therapeutic levels of antibiotic growth promoters (AGP). For the past half-century, low, prophylactic doses of antibiotics have been standard practice. Although the response to AGPs depends on a number of factors including farm management, exposure to pathogens, environmental stresses and diet their use is widely recognised to improve the rate and uniformity of broiler growth whilst increasing feed efficiency.

The main concern regarding the continued used of AGPs is the development of direct antibiotic resistance of pathogens in the species receiving the feed, as well as the indirect resistance to similar antibiotics used in human medicine as the result of food chain residues. Public concern and government regulations increasingly press for restricting the use of AGPs.

In the European Union (EU), the suspension of licences for several major antibiotics began in 1971. This process continued and since January 2006, licences for all sub-therapeutic levels for growth promotion have been withdrawn. Producers in several countries that export poultrymeat to the EU have to follow the same guidelines. There is also a demand for alternatives to AGPs in an increasingly health-conscious market. Some producers have imposed a ‘commercial' ban on use of AGPs in order to market meat and eggs guaranteed free of residues. Ahead of the EU ban on AGPs in 2006, Evialis began work on new products to replace them.

Table 1: Overall performance of broilers in a series of growth trials at the company's research centre 

Table 2: Example of an experiment involving a challenge 

Role of gut microflora?

Gastrointestinal microflora of chickens play a significant role in nutrition and growth. Competition from gut microflora for nutrients was managed in part through the use of low levels of antibiotics. This option is rapidly disappearing. There is a need to understand the role played by microflora so that their effects on nutrition, growth, health and disease can be managed by suitable replacements for antibiotics.

In chickens, the gut microflora play a minor role in digestion of feed ingredients but an important role in absorption of nutrients.

Gut microbes may be beneficial or harmful. Beneficial bacteria inhibit the growth of pathogenic bacteria and prevent some specific intestinal diseases such as Clostridium perfringens and E. coli. Any disruption in this balance may cause the proliferation of pathogenic or performance- hindering microflora.

What can we expect without AGPs?

The ban on in-feed antibiotics has left the industry at higher risk from increased mortality and greater variations in performance.

Clostridium perfringens is associated with necrotic enteritis (NE) and a general over-growth of pathogenic microflora in the intestine may lead to enteric disorders and wet litter. Wet litter is associated with reduced performance, respiratory and leg problems, increased down-grading at processing and greater use of therapeutic antibiotics.

Without AGPs, enteric disorders become more likely. Their degree and severity depend on chick quality and management such as brooding conditions but diet formulation plays a leading role. Among the dietary factors involved are:

  • levels of minerals (potassium, chloride, sodium).
  • excessive level or unbalanced protein.
  • fat source: different digestibility coefficients.
  • starch source: digested at different rates, which impacts the composition of the microflora.
  • presence of non-starch polysaccharides: these poorly digested fibre components found in wheat, barley and rye increase susceptibility to enteric disease.
  • particle size: coarser particles increase gizzard function, which improves digestibility and intestinal integrity.

With so many factors involved and the practical impossibility of controlling all of them, there is great interest in developing natural alternatives to AGPs in order to maintain both animal performance and health.

Graph 1: Growth and feed conversion in an experimental trial in Brazil 

In search of alternative natural solutions

A wide range of alternative products has been investigated to replace the AGPs following their complete ban in 2006.

Many, such as the feed enzymes, have been around for several years, and have enjoyed success in some poultry diets. Probiotics and plant extracts are already quite popular in human nutrition and therapy, and some have been developed for use in poultry feed. Acidifiers, prebiotics, probiotics and herbal products are already in use in some feeding programmes. They limit number of pathogenic bacteria, enhance the absorptive capacity of the small intestine and improve performance.


Lack of knowledge about the mode of action of AGPs has made difficult the search for the best alternatives. To select suitable candidates on a scientific basis, we have carried out several trials at our research centre in Saint-Nolff. Since 2000, different commercial products have been tested alone and in combination, and at different doses.

Each experiment included a negative control (basal diet) and 10ppm avilamycin. Avilamycin is no longer permitted in EU but it was used as a positive control to gain a better understanding of the results.

Limited responses at the research centre

The results vary widely from one experiment to the other. In half of the experiments, the performance of the positive control was no better than the negative one. Therefore, we have presented the results in two groups: group 1 includes trials where the negative control was better than positive control; group 2 includes those where the positive control was better than the negative control.

Under the good conditions of the research farm, improvements in performance due to AGPs were not important overall, and the individual results are not constant between flocks. The most important effect seems to be on growth. The alternatives fed alone or in combination also had inconsistent effects on performance.

Surprisingly, the average effects of the alternatives were opposite to those with avilamycin. Avilamycin is mostly active against gram-positive bacteria, so the observations could relate to differences in the digestive flora of the birds in the different experiments.

The data confirm that the selection of AGP substitutes from experimental results under good sanitary conditions need special consideration, and highlight the importance of using the right experimental protocol.

Table 3: Results of broiler performance in five field tests 

Results under more challenging conditions

The imposition of various challenges microbial infections, viscous feed, high stocking density can be used to make the conditions of an experimental station more like those of a commercial farm. In two trials, we tested our product, which is a natural mix of synthetic and natural clays, essential oils and plant extracts.

The first of these was conducted in Brazil, where the use of AGPs is still permitted. There were 8 pens (56 birds/pen) per treatment. A negative control had neither AGP nor challenge. All three other treatments involved a challenge in the form of re-used litter: an unsupplemented positive control, AGP (10ppm virginiamycin) and our own product.

The second experiment in the Netherlands compared a negative control without challenge to two treatments using highly viscous feed and microbial challenge: a negative control (without supplementation) and our product.

Both tests show how the challenges impaired broiler performance, and how some feed additives may allow a recovery in growth and feed conversion following an earlier set-back. Note also that such experiments provide additional information such as litter scoring an important parameter for growers. Trials that simulate field conditions can stimulate interest in the use of alternative growth promoters.

Testing under commercial conditions

Variations in husbandry, age of birds, disease patterns and management support the need for a series of well organised long-term field studies. Not only can they take into account the variations in commercial broiler growing but they may help to establish significant differences between treatments.

We carried out a series of field trials with a broiler integrator in Turkey. Despite repetition of the test at the same time of year and using the same broiler breed, the results showed considerable variation, with some tests showing improved growth and others demonstrating better feed conversion. This variability is typical of field conditions.

Our experience shows that potential natural growth promoters should be firstly screened at an experimental station, including under challenging conditions. This should be followed by a series of field tests in order to address the variation in situations found in the field and to review the cost-effectiveness of AGP alternatives under commercial conditions.