Antibiotics have been widely used in animal production for decades, first primarily to control disease, and more recently as antimicrobial growth performance promoters (AGPs) to improve growth rate and feed conversion efficiency. Globally, AGPs have made access to animal protein easier; however, its use has been banned in many countries and has been severely limited, or will be eliminated, in many others in the near future via regulations or by initiatives driven by consumer concern. Everything points to, if not a widespread removal of AGPs, a strong reduction in their use.
This removal of AGPs is leading to animal performance problems and a rise in the incidence of certain animal diseases, causing reduced digestibility and performance at the end. The effect is more ostensible in animals housed in unhygienic and suboptimal conditions. Consequently, together with the increased performance, nutrient immunomodulation and maintenance of normal gut flora in animals are important considerations to take into account in the search of alternatives to AGPs.
Seeking AGP alternatives
AGP alternatives are being sought to improve health, immune status and performance in animal agriculture. While the industry is focused on developing solutions that work similarly, AGPs exact mode of action is not yet fully understood. Different potential mechanisms have been proposed to explain AGP-mediated growth enhancement; the most accepted mechanism would be through modulation of the gut microbiota, which plays a critical role in maintaining the host health and can spend up to 6 percent of the energy of the feed.
Most AGP alternatives try to prevent proliferation of pathogenic bacteria and modulate indigenous bacteria so that the health, immune status and performance are improved. It may well be necessary to adopt changes in the management of livestock and also introduce different feed components in order to compensate for the antibiotics effects and maximize production. Therefore, some of the alternatives described below may be part of the solution even though they do not reproduce all the beneficial effects of antibiotics.
Organic acids and their salts
Organic acids and their salts are not antibiotics but, if used correctly along with nutritional, management and biosecurity measures, they can be a powerful tool in maintaining the health of the GI tract of poultry, thus improving their zootechnical performances.
Organic acids in poultry might be a meaningful tool of controlling enteric bacteria, both pathogenic and non-pathogenic. Inhibition of intestinal bacteria may lead to a reduced bacterial competition with the host for available nutrients and diminution in the level of toxic bacterial metabolites. As a result of the decreased bacterial fermentation, protein and energy digestibility might be improved and, thereby, ameliorate the performance of bird.
For example, sorbic acid supplemented broilers had lower coliform counts in the duodenum and lower yeast and mold counts, but higher bacteroides counts in the caeca. Also, studies report that the inclusion of an antibiotic and an organic acid mixture (lactic, fumaric, propionic, citric and formic acids), separately or in combination, reduced the Enterobacteriaceae count in the ileum of broilers.
Several authors demonstrated how the addition of sodium butyrate to the diet decreased the counts of Salmonella and bacterial populations in different organs of the animal. Sodium butyrate also has demonstrated to increase villus height in the small intestines, which increases the absorptive intestinal surface and might facilitate the nutrient absorption and growth performance.
Regarding performance, several authors have reported that certain organic acids alone, or in combination with others, improved performance of broilers even in absence of antibiotic. The question at this point is, Do these organic acids improve performance as much as AGP’s? The answer is that they may, but it needs to be further tested.
The variability of phytogenic products makes this a controversial group. The challenge is to identify and quantify the multitude actions and claims of improved feed utilization, animal physiology and health status. The mechanism of the action of these additives is not completely clear.
Some plant extracts influence digestion and secretion of digestive enzymes and, besides, they exhibit antibacterial, antiviral and antioxidant activities. Because of possible synergy between constituents, it remains unclear which of the many bioactive chemical components may stimulate endogenous digestive enzymes production; act as antioxidants; antimicrobial agents; or immunomodulators.
There are examples where these supplements act similarly to AGPs. The studies suggest the effect may not be only on improved body weight, average daily gain, and the feed:gain ratio, but also a stronger immune response and control of bacterial populations in the gastrointestinal tract.
For example, broilers fed a blend of capsicum, cinnamaldehyde and carvacrol had significantly greater live weight, better weight gains and feed efficiency than broilers fed a control diet with avilamycin. In addition, the supplemented broilers showed lower caecal counts for E. coli and C. prefringens.
One study indicates that regardless of the type of plant extract supplemented colonization of non-invasive bacteria such as Lactobacilli in the ileum and caeca are stimulated compared to a negative control and a positive control with zinc bacitracin.
The combination of organic acids and certain phytogenic additives may reduce the incidence of certain sicknesses, as sodium butyrate, combined with ginger and capsicum, reduce the incidence of necrotic enteritis in broilers, for example.
Prebiotics are defined as non-digestible food components that have positive effect on the host in their selective growth and/or activation of certain number of bacterial strains present in intestines.
The most significant compounds that belong to the group of prebiotics are oligosaccharides: fructo-oligosaccharides, gluco-oligosaccharides, galacto-oligosaccharides and mannan-oligosaccharides. They work steering bacterial populations, acting as substrate for desired micro-organisms as Bifidobacteria (inulin, FOS), or eliminating pathogenic bacteria as E. coli and Salmonella spp (MOS).
A study found live weight, weight gain, feed intake, feed conversion ratio, mortality rate and hot carcass yield of broilers were not affected by mannan-oligosaccharide supplementation when compared to avylamicyn supplemented diets. Several authors did see differences in microbial populations and improvements of performance similar to those achieved by AGP’s (compared with enramycin and avilamycin).
Probiotics and direct-fed microbials
Probiotics are individual microorganisms or groups of microorganisms that have a favorable effect on hosts by improving the characteristics of intestinal micro-flora and, thus, health and growth. Unlike antibiotics, probiotics introduce live beneficial bacteria into the intestinal tracts.
Several researchers have assessed the efficacy of probiotics as growth promoters. Most concluded that when results are averaged over several trials, there is an improvement in growth rate and in the efficacy of feed utilization. In general, probiotics can improve conversion, decrease mortality, stimulate the immune response and protect against enteric pathogens.
Probiotics perform these activities with a higher or lower intensity depending on the environment, similarly to AGP’s (whose effect is stronger, the worst the scenario is) to a point. The probiotic bacteria produce enzymes; the host is benefited by these enzymes; probiotic bacteria stimulate animal’s immune system; and finally, probiotics have a strong positive influence on intestinal metabolic activities, such as increased production of vitamin B12, bacteriocins, organic acids and other compounds that have antagonistic action towards pathogen bacteria.
Other mechanisms have been proposed but remain to be confirmed.
There are many works that show how probiotics improve growth, FCR and survival rates, but are they comparable to AGPs? Several authors have reported better intestinal development than flavomycin, similar or sometimes better growth than AGPs (Zinc Bacitracin) and better feed digestibility.
Enzymes are routinely added to poultry feeds in order to facilitate the breaking down of certain components of the feed, such as β-glucans, proteins and phytates, that the animals may have problems digesting.
Enzymes are very effective at maximizing feed conversion efficiency already with AGPs, but their role is even more important when the AGPs are not used.
Additives offer alternative solutions
Consumer demands and legislative pressure on the withdrawal of antibiotics from poultry foods have created the need for alternative solutions which would influence improvement of health and production traits of broiler chickens.
To be effective, alternatives for AGPs should generate similar benefits as the antibiotics currently used as growth promoters. Enzymes, acids, pre- and probiotics and phytogenic additives are some examples of product classes that are used as alternatives for AGPs. However, while some products clearly have potential, the efficacy of others requires additional research.
It may be necessary to combine two or more alternative feed ingredients or to combine a new feed supplement with a change in husbandry practices to achieve the best effects. Further research is also needed to find the mechanisms of the action of these compounds and their interaction with others factors of production to set and meet standards for AGPs alternatives for broilers.
References available upon request.