Feed Additives: Do they work and how do they work?- A Practical Review

There is no universal definition for livestock feed additives, but as their name suggests, in general, they are substances of a low-inclusive nature that are added to stockfeed for a specific purpose. The European Commission describes feed additives as “products used in animal nutrition for purposes of improving the quality of the feed … or to improve the animal’s performance and health…” (EC, 2014). In poultry, feed additives have been extensively used for many decades and their popularity seems to always be on the rise. The most popular applications for additives is, the supplementation of compounds or substances that are generally not found or found at sub-optimal levels in standard stockfeed rations, such as vitamins, minerals, pigments, amino acids and enzymes (Bell and Weaver, 2002; Poultryhub, 2014; AVPMA, 2014) Other applications for feed additives is the removal of anti-nutritional factors found in raw ingredients and the treatment or prevention of health disorders that may arise in different production systems. In commercial poultry rations, the presence of feed additives in feedstuffs may be required for maximising the genetic potential, the production requirements of modern poultry breeds and meeting market needs for commercial operations. Coccidiostats and antibiotic growth promoters also fall under the feed additive category but nowadays and in practical terms they are mainly referred to as “medications or pharmaceuticals” rather than feed additives. Therefore, the latter two will not be extensively covered in this paper. This paper will discuss and summarise information regarding some of the most popular feed additives in the current Australian and worldwide poultry markets. Since some uncertainty exists regarding the efficiency and modes of action of certain feed additives, the paper will touch on two main questions: Do feed additives work? And if so, how do they work? 

In the modern poultry industry, new additives and applications are constantly emerging, together with new scientific findings, genetic advances, scarcity of raw materials and the evolution of environmental and social pressures. Amongst other drivers for the development and emergence of novel feed additives are: the ever changing challenges of producers, and the pressures and necessities of feed additive companies to remain profitable and stay in business. It is not uncommon to find that a surprising number of research and scientific works in the poultry industry are being driven by feed additive companies to fulfil the need to innovate, solve production challenges and provide better insights into the modes of action for their specific products. The “by-product” of these scientific findings is a competitive advantage over other companies, as well better advertisement and sales materials for specific products.

It must be mentioned that current the market is flooded with many additives, some of which vary in quality and price, with some of them being sold with plenty or very little evidence for their modes of action. Also there are many “like-to-like” products which may or may not have the same manufacturing and processing qualities. The current phenomena presents a big challenge for producers/end-users, as they generally would expect that the feed additive they purchase, will indeed perform the specified mode of action that is sold under. In commercial terms, it is important that the feed additive will actually assist in improving production parameters or solving certain challenges.

Similarly to many other countries, in Australia a series of regulations exist for the selling, distribution, application and inclusion rates for additives that are used in poultry feedstuffs. A deep discussion on the current regulations and implications involving feed additives is not within the scope of this review. In brief, there are a series of categories in which additives fall under, depending on their inclusion rate, modes of action and for what purpose they are sold for. For particular actives, such as antibiotics or molecules with specific modes of action, the feed additives need to be registered and approved by the AVPMA. In other instances and depending on their category their use may also require a veterinary script. For these types of additives the regulations are very strict, requiring a long and thorough registration process. On the other hand, other additives that do not contain any claims or simply do not fall into the veterinary use category or are under the exceptions list, the regulations are less strict; requiring certain paper work; import permits and being cleared by Australian Quarantine Inspection Services (AQIS).

It is worth mentioning that technically speaking many feed additives that are currently sold and used in Australia should be registered by the AVPMA. Some companies have learnt to get around the legislations by not making direct claims or not declaring specific activities that may modify physiology or performance or animals, or declare that a specific active ingredient has been manufactured for a specific purpose. In response to this trend the AVPMA is currently reviewing its regulations and drastic changes are expected to be implemented by January 2015. Detailed information on the code and use of specific ingredients, as well as a list of approved registered products can be found on the AVPMA website (www.avpma.gov.au).

In practical terms and for the interest of end-users, feed additives that do not fall under the AVPMA regulatory umbrella: do not contain any claims or instructions for their use or any information regarding their possible modes of action. Therefore, the ability of specific additives to alleviate production challenges may need to be tested and assessed by the end users; sometimes making it challenging to distinguish fact from fiction. 

As previously mentioned, there are several categories in which feed additives fall under. With most AVPMA registered products for which mode of action and recommended use is clearly specified on the label; it is relatively easy for the end user to select and use products for a specific function. A good example for this is the selection of phytase enzyme preparations for the digestion of phytic acid in grains and legumes. However, for additives with no specific claims, the selection of a specific additive to perform a specific function such as; alleviating production issues, improve performance or prevent a recurring disorder or conditions can be a little more challenging. The selection of the right additive may require the implementation of a series of steps and procedures to assess their efficacy and economic return. For some organisations or farms, additive assessments may be costly, timely and require extra labour. Other operations may use dedicated facilities where trials can be executed. In some instances the end user may not have the will or the time to assess the additive or the data presented by the company selling it and may just rely on a given sales pitch or word of mouth.

In preparation of this document, several producers or end-users were interviewed and their opinions regarding the use of feed additives are anonymously summarised below.

In many cases, end users report that the main way to assess the efficiency of additives is to run a “field-trial” on one or two sheds and historically compare the results with previous flocks or current untreated flocks. This again presents a challenge as in reality, field trials can be hard to assess as it is hard to set up positive and negative controls or maintain all variables constant. Some of these variables may include equipment malfunctions, change of staff, raw material availability or inadequate data collection. All of these circumstances may leave the end user in a grey area, wondering if the additive actually works or not. In some instances producers report to be on a “roller-coaster” of feed additive trials where it is difficult to trace back or identify what has or has not work. On the other hand some end-users report to be hesitant to try new technologies as they truly believe that what they are already using has no substitute. In extreme circumstances, end-users reported to not be interested in using many of the current feed additives at all as they believe them to be inefficient and lack solid evidence that they actually work. Some end-users quoted that aside from vitamins, minerals, amino acids and some enzymes, they consider most additives to be “snake-oils” and would rather spend their money on increasing the protein or energy levels in the diet.

Perhaps a good approach for the adequate selection of a specific feed additive would be to categorise the purposes for their inclusion and ask the following questions:

Once the use category is identified, perhaps a further step would be to identify the frequency the additive is used in other poultry diets, for example, a methionine or phytase enzyme would be found in most poultry diets around the world, where as an essential oil extract may only be found in a small percentage of poultry diets (Kim et al, 2008). For frequently used additives the selection criteria might be easier, as the end-user may only require purchasing it from a reputable supplier, the feed mill or additive company which he/she already has a good business relationship with. The end-user may also call on resources like veterinarians, nutritionist consultants and other farmers to get advice from. For selection of less frequently used additives the end-user may have to not only look at the supplier’s data and credentials but also take the time to look through and analyse any literature and data from any research trials available. In addition, in commercial operations if the use of a specific additive is driven by an underlying performance issue or the necessity to improve certain production parameters on farm; it may be important to consider its economic viability. For example, the economic returns arising from the addition greater than the cost of the additive per se. In other instances, non-economic reasons may need to be considered, for example, the addition of certain additives for environmental, occupational health and safety and marketing reasons. Table 1, below summarises some points that may assist in assessing the feasibility of a feed additive and its use. 

Table 1. Summarised feed additive assessment criteria for the selection of the right additive to alleviate production challenges or enhance production parameters.

To attempt to answer the question on which is the right additive, the end-user would normally have to sort through a plethora of available information. This information may come through internet search engines, websites, magazines, forums and industry meetings or information directly supplied by the feed additive sales or technical agent (Table 1). Perhaps a good approach to assist in the selection process is to gain a good understanding of the scientific principles behind the nature of the feed additive and, if available, the results obtained in research or commercial trials. The latter is an important factor as there are several variables that the end-user needs to assess regarding the interpretation of research or commercial trials. For example:

Experts have reported that obtaining positive results followed the addition of a certain additives; can be influenced by many parameters (Bell and Weaver, 2002; Saif, 2008; McMullin, 2004). Amongst some of these are: management, vaccination programs, bio-security, nutrition, water quality, rearing conditions and climate. Many management books and workshops suggest that improving a flock’s production parameter should not be reliant on a “silver-bullet approach” but rather on taking a holistic one; for example the utilisation of benchmarks, statistical data analysis, historical compilation of events or changes to the farm’s operation and revision of feeding and vaccination programs. Experts stress that a thorough analysis of the current management practices for the flock or flocks in question is also crucial. Once a good understanding of the farm’s situation has been obtained, it is the job of the responsible person to identify the point by which most improvement(s) can be made to the operation. It is also imperative that the main improvement factor/driver is identified (i.e. husbandry practise, deficiencies in the diet, hygiene or other health conditions). If part of the main source of the issue can be alleviated, corrected or improved with the use of an additive; the selection of the right additive will be a pivotal component for achieving a good result. In other words an additive will not be successful in improving a given production parameter, if its mode of action is not capable of directly or indirectly alleviating the issue. Achieving a good result will therefore involve a solid understanding the mode(s) of action for the additive(s) in question.

The following section of the paper focuses on providing a brief summary on the mode of actions and uses of the different types of feed additives on the current Australian market. 

Vitamins are organic chemical compounds that are necessary for maintenance, growth, and egg production (Bell and Weaver, 2002). They are required in small amounts, and when they are deficient or absent from the diet, characteristic manifestations may result in performance hindrance or health disorders. Many of which are enzyme-associated deficiencies. On average there are about 13 added vitamins that are usually listed as necessary for the optimal performance of commercial poultry (Leeson and Summers, 2005). In general, the recommended level of addition is provided in the breed standards. However, it must be recognised that these levels are only a guide as there are several factors that may influence the actual addition requirements of specific vitamins. Among these factors are: the intrinsic vitamin levels in the different feedstuffs, the manufacturing process of the finish feed and the health status of the flock. It has been extensively documented that factors such as heat stressed or onset of disease increase the requirement for certain vitamins.

Besides proteins, carbohydrates, fats and certain vitamins many minerals form a part of the bird’s nutritional requirements. In most cases, the quantity needed for each is small and often infinitesimal. In some instances, like with calcium in laying hens, relatively large amounts are necessary. Although, most of these elements are currently added to the diet in their mineral inorganic form, there seems to be a tendency to supplement with chelated minerals, often referred as “organic minerals” (Leeson and Summers, 2005). Their name was originally derived from binding a specific mineral to an organic substance, a carbohydrate or amino acid. Minerals presented in this form have higher bio-availabilities and can be readily absorbed through the intestinal wall and do not appear to form chelates with other minerals in the digestive tract like their inorganic counterparts. Chelated mineral research has found that the deposition of minerals in the manure was significantly lowered when minerals were given in the organic form. Some research also demonstrated a numerical advantage in feed conversion ratios for the birds fed the chelated mineral complexes (Leeson and Summers, 2005).

A number of factors have been shown to affect the requirements of different minerals. Phytate levels in feed not only keeps phosphorous from being bio-available but also form complexes in the digestive tract with calcium, zinc and iron and other minerals. Research has indicated that phytate in feed may also interfere with the bio-availability of zinc, copper and manganese. Dietary fat has been shown to form insoluble soaps with inorganic minerals such as calcium, decreasing their absorption. Certain diseases such as coccidiosis may also decrease the absorption of minerals by damaging the villi and microvilli in the intestine and thus affecting their absorption capacity. 

Due to the physiological needs of modern poultry breeds and the availability of raw materials; modern poultry diets require the supplementation of certain amino acids. The type and quantity of added amino acids will vary depending on the type of diet, the age and strain of the bird, as well as the performance objectives and budget of the commercial operation. In general the most commonly added amino acids are L-Lysine, DL-Methionine or hydroxy-analog, L-Threonine and L-Tryptophan. The adverse effects of feeding amino acid deficient diets on egg production, feather cover, cannibalism and other disorders have been well documented and will not be reviewed on this paper (Novak et al., 2004; Kidd et al., 2004). 

Enzymes have been added to poultry diets ever since workers at Washington State University showed improvement in digestibility of barley and rye-based diets when various enzymes were used (Leong et al, 1962). Enzymes are now being manufactured specifically for feed use, and can be broadly categorized as carbohydrases (also termed NSP enzymes), phytases, proteinases and lipases. Research has focused on increasing the digestibility of the carbohydrate content and the release of phytate-bound phosphorus of various cereals and plant proteins. There is growing interest in the potential for improving protein digestibility of both plant and animal proteins and other by-products.

NSPs are now frequently used to describe what in the past has been referred to as fibre. Birds have very limited ability to digest fibre because they lack the enzyme necessary to cleave or break up these large and complex molecules. Improving digestions of the complex carbohydrates by the use of enzymes not only increases energy utilization but also decreases the negative impact that certain fibres have on increasing the viscosity and digestibility of feedstuffs. To the poultry producer, the most notable effect of diets with high NSP content with the inadequate use of enzymes will be wetter, more sticky and viscous faeces. 

On a worldwide basis, the most widely used feed enzyme is phytase. Phytase cleaves the phytic acid or phytate, in grains and legumes to release phosphorous and calcium. Phytate is a complex structure that tightly binds phosphorous, and is the main storage source of phosphorous in plant material. Few animals possess the phytase enzyme necessary to cleave the molecule and so phytate acid is largely undigested. Interest in phytase enzymes increased because phosphorous has become an expensive nutrient, as well as the fact that undigested phytic acid adds greatly to manure loading of phosphorous. Phytase also binds calcium and other trace minerals and may bind with proteins and carbohydrates. Hence, when applying phytase to diets a theoretical value is given to the contribution of calcium, phosphorus, and sometimes energy and certain amino acids.

Proteases are enzymes that assist in the digestion or breakdown of proteins. Some protease activity can be found in some commercial NSP multi-enzyme preparations. However, in recent years, single strain proteases have gained popularity due to the high costs of protein rich feedstuffs. At present most commercial single strain proteases are costly and would only come into poultry rations where raw materials with low protein digestibility are used. In the current Australian market, most diets consist of high quality raw materials with good digestibility’s and thus the price of the enzyme does may not justify their addition to the diet. 

There is potential for adding lipase enzymes to feed or fats, to improve digestibility. Improvement in digestion of saturated fats for young birds seems to have to greatest potential. Commercial lipase enzymes are not readily available in the Australian market. 

Organic acids have been defined as organic substances (natural or chemically synthesised) with acidic properties that may lose or donate proton(s) (Dibner and Butin, 2002). By definition, an acid is a substance or compound that when added to water increases the concentration of hydrogen (H⁺) ions. In animal feed, acids are generally categorised by the length of carbon atoms in their chemical structure; short chain fatty acids, SCFAs (less than 6), medium chain fatty acids, MCFAs (6 to 12) or long chain fatty acids, LCFAs (more than 12). In brief, short chain fatty acids are generally used for acidification purposes whereas MCFAs are mainly used to target the reduction of specific pathogenic bacterial strains. Organic acid preparations are sold either individually or as a blend of various acids. Some acids are sold in straight forms, while others are treated in proprietary forms, put into a carrier or encapsulated. Authors have reported variability in the efficiencies of acids to control bacterial conditions or modulate the gut microflora (Dibner and Butin, 2002). There are several arguments for this phenomenon but in general certain properties of organic acid preparations need to be considered. Firstly, the ability of some acids to act on bacterial population is pH dependent. For example, some acids work better at pH 5 than others. Secondly, antibacterial activity of some acids is contact dependent, meaning the acid needs to be in the right concentration and in the right place; for example if an acid is absorbed in the foregut, it will have no or very little effect in the hindgut. Thirdly, not all acids affect bacteria on the same way. For example, sorbic acid is known to act better against on mould while lactic acid is known to act better against bacteria. Finally, some bacterial strains (including pathogens) are more resistant to acids than others. For example E.coli O157:H7 is widely known to possess more sophisticated acid resistance and tolerance mechanisms than other pathogenic and non-pathogenic E. coli (Leyer et al, 1995). 

Phytogenic additives include a variety of herbal, spices and plant derived extracts or products, also referred to as essential oils. Out of all the current additives discussed in this paper essential oils are perhaps the most complex and least understood in the scientific community (Kim et al., 2008). Their reported modes of action range in a variety of functions. Amongst these are: palatability enhancement, immune regulation and anti-oxidant, anti-inflammatory and antibacterial properties. Although, studies have reported and demonstrated the efficiency of essential oils to exert the abovementioned activities in vitro; in vivo evidence is still quite limited (Kim et al., 2008). Experiments suggest that phytogenics have effects on gut modulation by showing a reduction in bacterial counts and fewer ammonia and biogenic amines in the faecal matter. Commercial studies have reported considerable production improvements in feed conversion ratios, livability, egg production and end-product improvements for flocks treated with phytogenic products and compared to negative controls.

Prebiotics are classified as substances that may favour the growth of beneficial bacteria in the gut micro flora. Two modes of action have been proposed for this activity; 1) the probiotic provides a substrate that may assist or benefit the growing and proliferation of beneficial bacteria in the gut micro flora, 2) the probiotics provide substrates that inhibit directly or indirectly the growth and proliferation of pathogenic or energy taxing bacteria in the gut micro flora. In the literature, prebiotics have been show to alter the gut microflora, the immune system and reduce pathogen invasion such as Salmonella enteriditis and E.coli (Cummings and MacFarlane, 2002). Hajati and Rezaei (2010), summarised the advantages for probiotics in commercial poultry for providing improvements to:

Probiotics have been defined as “a culture of specific living micro-organisms which implants or proliferates in the gut micro flora and beneficially affects the hosts health or performance”. It is believed that probiotics act by competing against the non-beneficial bacteria or pathogens by competitive exclusion means (for attachment sites and nutrients), pH modification by the production of fermentation by-products (i.e. hydrogen ions, fatty acids) and or the production of specific substances (bacteriocins) with targeted antimicrobial power (Kabir, 2009).

It is believed that during optimal health/performance a balance exists between the host and the bacteria present in its gut flora, however, when a stressor is imposed (nutritional, husbandry, pathogenic) there is a reduction in number of beneficial organisms such as lactobacilli. Thus, allowing the proliferation of non-beneficial bacteria or pathogens which may have detrimental effects on feed utilisation and/or compromise bird’s health. It is therefore, believed that the addition of probiotics into poultry rations assists in the regulation of a beneficial microbial balance. 

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