Antibiotic-free chicken: Finding digestive balance

In antibiotic-free broiler production systems, nutritionists can support the gut ecosystem with the judicious use of highly digestible diets for the young bird.

 

Varying degrees of antibioticfree production (ABF) of chicken are now imposed by government agencies or major retailers or are being voluntarily undertaken by integrated broiler companies.

 

It seems highly unlikely that broilers can always be grown without recourse to using antibiotics, but the obvious trend is away from routine use of so-called growth promoters, to be replaced by judicious targeted treatment of specifi c bacterial challenges as they arise.

 

When growth promoters were fi rst introduced some 50 years ago, their mode of action was unclear, and it was not until their removal from broiler diets in Europe that it was discovered that their major role was in controlling subclinical bacterial challenges in the digestive tract.

 

Today, ABF can apply to scenarios ranging from mere removal of growth promoters to the extreme of removing all additives, including ionophores and chemical anticoccidials. The most challenging production systems today involve feeding diets without growth promoters or any anticoccodial.

 

Bacterial and parasitic infections of the gastrointestinal tract are an everpresent threat to broilers grown on litter, and these pathogens affect the balance between digestion and indigestion and, ultimately, bird health.

 

Today, the microbial status of the tract has been kept in balance by using anticoccidials in conjunction with growth promoters.

 

The poultry industry is greatly hampered in the study of gut health by not knowing, with any great precision, the normal microfl ora present in healthy birds. It has been suggested that, at best, conventional culture techniques are isolating 60% of the bacterial species present in the gut. Newer techniques involving DNA fi ngerprinting of microbes may give a better understanding of the complexity of the microfl ora and, in particular, how they change in response to various dietary treatments.

 

The chick hatches with a gut virtually devoid of microbes, so early colonizers tend to predominate quite quickly.

 

Early on, any undigested nutrients will be available to fuel microbial growth in the lower intestine and ceca, so if these happen to include pathogens, then the chick will be disadvantaged.

 

The “normal” gut microfl ora develops quite quickly, so microbial numbers and species present on the hatching tray, during delivery and during the fi rst few days at the farm will likely dictate early colonization.

 

The Nurmi concept of manipulating gut microbial species relies on early introduction of non-pathogenic microbes. Ideally, these microbes will help prevent subsequent pathogenic colonization, and bacterial competitive exclusion is undoubtedly destined to be one of the management tools routinely used in broiler production.

 

There has been a signifi cant increase in the incidence of necrotic enteritis (NE) in Europe and elsewhere following removal of growth-promoting antibiotics and ionophore anticoccidials.

 

Unfortunately, broiler diets in Europe are often based on wheat as the major cereal grain, and it is well documented that clostridia multiply and colonize more quickly when the diet contains much more than 20% wheat. An interesting observation in Europe is that clostridia are now colonizing the upper digestive tract as well as the normal site of adhesion in the lower small intestine of broilers.

 

Coupled with the increased incidence of NE, “dysbacteriosis” is now common in European broiler operations and represents abnormal microbial overgrowth in the absence of antibiotic growth promoters. This latter condition does not seem to be related to diet composition or ingredient selection.

 

NE is also more common if the diet contains appreciable quantities of pectin or other non-structural polysaccharides (NSPs), indicating that digesta viscosity and associated indigestion affect bacteria proliferation.

 

Although many products claim to be antibiotic alternatives, it is obvious that early nutrition per se can have a profound effect on the potential for indigestion in young broilers.

 

 

The young broiler chick does not produce an adult complex of digestive enzymes, so digestibility is somewhat limited. This situation is further complicated by the change in nutrient substrate from the readily available yolk and albumen for 21 days during incubation to quite complex carbohydrates, proteins and lipids in conventional broiler starter diets.

 

Even though chicks grow fairly rapidly in the fi rst few days of life, often reaching 180 g at seven days of age, early growth and development can be further enhanced by use of specialized pre-starter diets, and with judicious ingredient selection, these diets can help support gut health. Such pre-starters either precondition the chick such that it can digest complex substrates more readily, and/or they provide more digestible substrates until the chick’s enzyme production has “matured”.

 

In this regard, “early” means about the fi rst 10 days of age. Specifi c activity of individual enzymes actually declines over the fi rst week of life, although this is compensated for by a rapid increase in secretory cell numbers. Early villi cell damage, especially in the duodenum, will greatly impair digestion.

 

Rapid early development of the intestinal epithelium is another prerequisite for normal digestion. The villi and microvilli grow rapidly in the fi rst few days, and any delay in this process will limit nutrient uptake. The presence of pathogens, mycotoxins and plant toxins will also delay microvilli development. Selection of highly digestible ingredients (animal proteins) devoid of natural toxins is, therefore, important for rapid early gut development. Butyric acid, either in the feed or as a consequence of NSP fermentation, is also important for villi development.

 

As the epithelium develops within the microvilli, mucus is secreted, and this acts as an important barrier against pathogenic colonization and also auto-digestion from the bird’s own digestive enzymes. Some bacteria are able to colonize because they are able to break down this protective mucus layer. Heliobacter pylori, the bacterium that causes gastric ulcers in humans, secretes urease enzyme that destroys the protective mucus coating, thereby making the stomach wall susceptible to degradation by hydrochloric acid and pepsin. It would be interesting to study the gut microfl ora of birds fed highurease soybean meal.

 

While corn/soybean meal diets are the standard reference for broilers, there is 0evidence that digestibility is suboptimal for the young chick. Compared to expectations, nitrogen-corrected apparent metabolizable energy and amino acid digestion may be reduced by as much as 10% in chicks fewer than 7-10 days of age. In the critical two to fi ve days of age period, digestion may, in fact, be 15% less than expected. Within ABF systems, it is the fate of these undigested nutrients that potentially fuels microbial overgrowth.

 

Although the amino acid requirements of clostridia bacteria are not known with any great precision, it seems as though culture in vitro increases in the presence of elevated levels of lysine and serine. Clostridia are anaerobic proteolytic bacteria, so it stands to reason that any protein or even excess nitrogen reaching the ceca is going to support their growth. The idea when formulating specialized pre-starter diets is to correct any such defi ciency in digestion, limit the supply of nutrients — especially protein/amino acids — that evades digestion/absorption and hopefully increase the early growth rate and/or improve the uniformity of such early growth.

 

Two types of pre-starter diets are often used for broiler chickens. The fi rst option is to use greater-than-normal levels of all nutrients, while the alternate approach is to use more highly digestible ingredients with little change in nutrient specifi cations. A potential problem with the former approach is that with conventional ingredients, nutrients still will not be optimally digested, and as such, undigested nutrients will fuel microbial overgrowth.

 

In this situation, increasing the nutrient profi le of the diet often defeats the effort and makes matters worse. If using conventional ingredients, it may, in fact, be better to reduce the nutrient content, especially the levels of protein and amino acids.

 

An alternative approach is to use more highly digestible ingredients, with little change in the level of nutrients. Such pre-starter diets are expensive, since alternative ingredients are more expensive per unit of nutrients supplied than corn and soybean meal are. In one such study, my team has shown male broilers to be 34% heavier than standard at seven days of age when a highly digestible pre-starter was offered for just the fi rst four days. Because ingredients such as blood plasma are used in these formulations, such diets can be twice as expensive as a conventional corn/soybean meal broiler starter diet.

 

Economically, one has to balance the additional cost of some 100 g of highly digestible feed for each bird versus the effect of increased growth rate and the intangible effect of early gut health and how this affects subsequent growth within an ABF production system.

 

The most critical feature of specialized pre-starters is to replace less-digestible vegetable proteins with more-digestible animal proteins. Blood plasma and fi sh meal are routinely used in diets for early-weaned pigs and, perhaps, offer an interesting alternative to soybean meal in the broilers’ fi rst few days of life.

 

For baby pigs, the almost universal use of dietary blood plasma is due to its inherent supply of digestible amino acids, coupled with functional proteins that directly affect the animal’s immune system.

 

In North America, one often hears that clostridia infection is always more problematic when meat meal is used in broiler starter diets. This will undoubtedly be true if the meat meal is inadequately processed, yet good-quality meat meal and poultry byproduct meal should fi nd a place in these pre-starter diets.

 

An interesting feature of so-called antibiotic alternatives is that individual additives rarely work under all possible production circumstances. The poultry industry became complacent in the use of growth promoters, since they were usually efficacious regardless of environment or diet and their response was predictable. Much more work needs to be done to select alternative products. Their efficacy invariably is predicated on a willingness to make necessary changes to many facets of production so as to support the mode of action of these various products.

 

The use of highly digestible prestarters is but one example of a change in conventional philosophy necessary to support other aspects of any ABF system.

 

Another critical aspect to long-term ABF broiler production is the inevitable acceptance of cocci vaccines. Since clostridia and cocci infection are so interrelated, it seems likely that any long-term ABF strategy will involve a willingness to make cocci vaccinations work, and again, nutrition can play a supporting role.

 

Any time a change is made in the diet, be it nutrients, ingredients of electrolyte balance or even ingredient particle size, the microbial population will be affected. The well-known adverse effect of changing from starter to grower feeds in broilers is due, in part, to the change in pellet size and also due to transient changes in the microfl ora. When using growth promoters, the impact of a change in the microfl ora due to any dietary change will be minimal, although within ABF systems, the impact on this delicate ecosystem is not well documented.

 

It stands to reason that in ABF systems, diet changes should never be made within the critical window of potential clostridial multiplication, namely 15-20 days of age. Also, where possible, more effort needs to be made in tempering any dietary changes in terms of ingredients, nutrients and texture because they affect the gut microfl ora. Perhaps consider additives such as betaine that can positively affect gut digesta hydration and, thus, indirectly support the microflora.

 

Probiotics possibly represent the best potential for long-term control of the gut ecosystem. The concept of their use is well-known and understood, yet there aren’t any products that work universally under all farm and feeding scenarios. To some extent, this may be due to the inappropriate application of such live organisms and, in particular, the time at which they are introduced to the bird. In some instances, the chick’s microfl ora may be established by the time it gets to the farm, so the probiotic faces more of a challenge to establish itself as the predominant species.

 

Exogenous enzymes should also be considered in an attempt to maximize digestion and limit the fl ow of nutrients to the large intestine and ceca. The new protease enzymes perhaps have the most potential in this regard, since they allow nutritionists to reduce dietary crude protein and hopefully reduce the supply of nitrogen that fuels proteolytic clostridia bacteria in the large intestine and ceca.

 

In ABF broiler production systems, nutritionists can support the bird’s gut ecosystem with the judicious use of highly digestible diets for the young bird and by ensuring that minimal protein and amino acids reach the ceca.

 

Additives such as butyric acid — added as a direct supplement or indirectly via fermentation of judicious levels of soluble fi ber — encourage optimal gut villi development. Control of cocci via vaccination and selection and screening of antibiotic alternatives are a necessary step imposed by this new paradigm.

 

Successful ABF systems necessarily involve change, and as such, the future lies in the close cooperation of the nutritionist, health care workers and environmental engineers.

 

The poultry industry has the genetics for phenomenal broiler performance; it just needs the willingness to make new paradigms work.