Antibiotics and chemical antimicrobial agents are currently banned or under severe scrutiny by authorities in many countries. 

In replacing antibiotics, either enforced by the law or as a preparatory measure, a number of alternative additives can be employed, along with other intervention measures including farm biosecurity protocols and feed formulation techniques.

Feed additives

When it comes to replacing in-feed antibiotics in poultry and pig diets, the first step should be to ensure animals do not need to rely entirely on such additives. Thus, not only overall farm biosecurity protocols must be enhanced, but feed must be reformulated to contain less protein, less iron, less anti-nutritional factors, and less contaminants, either pathogens or toxins. 

Then, as a final step, a range of additives must be selected to complete the effort in replacing long-standing in-feed antibiotics used for their growth promoting properties. The list below offers a limited selection of such additives based on scientific evidence and practical experiences.

Zinc 
Zinc oxide is perhaps the most potent antibiotic replacement agent as it has clear bacteriostatic and bacteriocidic properties, and it also helps in the healing of injured tissues (such as in the gut of newly weaned pigs after a period of low feed intake). Unfortunately, zinc oxide is prohibited in most countries where antibiotics are banned.

Copper 
This is, in general, equally effective to zinc, especially in diets for young pigs, but not as potent for the immediate period after weaning. Nevertheless, it has a very strong record against diarrhea in young animals, and when it is used as part of a comprehensive program, it can offer certain advantages at very low cost.

Organic acids 
Certain organic acids have a very strong record against pathogenic bacteria, but currently recommended dosages (1-2 kg) are almost always insufficient for effective control of gut pathogenic bacteria. More likely, 5-10 kg per metric ton are needed, at least according to the original research work that proved their efficacy as antibiotic replacements. 

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Extracts 
Certain plant-derived compounds, such as extracts from spices and herbs, have strong (in-vitro) antibacterial and (or) antiviral properties, but results under field conditions remain variable and very brand-specific. Apparently, this technology is still progressing, but for all practical purposes, plant extracts perform the best when used in conjunction with organic acids as they appear to exhibit a strong synergistic effect.

Carbohydrases 
These enzymes can reduce the level of non-starch polysaccharides found in cereals like wheat, barley, oats and rye. These anti-nutritional factors when left unchecked tend to increase digesta viscosity, which in turns favors the growth of bacteria in the gut.

Bacteria 
This is a direct way to enhance the numbers of “good” bacteria at the expense of the pathogenic ones, assuming the in-feed bacteria (either live or sporulated) find favorable conditions to colonize and compete successfully with the pathogenic ones. Again, this technology gives quite variable and brand-specific results.

Fiber 
Also, called prebiotics, these specific fibers (for example, inulin) serve as “food” to the beneficial flora aiming to indirectly boost their numbers at the expense of the pathogenic ones. Sometimes, functional fibers are used in conjunction with probiotics.

Immunoglobulins 
When designed against specific pathogens, they act by tagging and eliminating such bacteria, in a very efficient and powerful mode, as they resemble the natural immunoglobulins secreted in the gut by the infected animal.