Feed Additives

A feed additive is any compound used in a ration that is not normally found in those feeds and does not directly meet a known nutritional requirement. Feed additives may enhance the effectiveness of nutrients in order to increases feed efficiency, weight gain and muscle. They may also work to decrease disease, improve handling and storage and acceptability. This post will outline the major additives that are included in the feed rations of animals. 

Preservatives

Preservatives work to protect the nutrients from oxidative or microbial damage (this includes bacteria, moulds and yeasts). Antioxidants are often included in feeds to prevent oxidation. Natural antioxidants include tocopherols, ascorbic acid (vitamin C) and rosemary extract. Synthetic antioxidants may also be used.

Antibiotics

Antibiotics kill microbes in several different ways. They may inhibit cell wall synthesis, bacterial protein or DNA synthesis and may even interfere with bacterial electrolyte balance. 

Antibiotics are used for three different purposes in animal production systems:

1. Therapeutic Use: to treat infection in sick animals
2. Prophylactic Use: for the prevention of infection when animals are at risk of disease
3. Growth Promotant Use: this is at sub-therapeutic levels to improve weight gain and feed efficiency and to reduce the incidence of disease.
• This works because the amount of harmful bacterial metabolites and pathogenic bacteria is reduced. The number of microorganisms that compete for nutrients is also decreased. This leads to an increase in the number of bacteria that are useful to the animal and causes structural changes in the gastrointestinal tract to increase nutrient uptake. This leads to a large increase in the growth performance of the animal. 

However, the effect of antibiotic growth-promoters depends on several factors, including:

• the species of the animal and its housing system: they are usually used in the pig and poultry industries. 
• age of the animal: the increase in growth performance is greatest in younger animals
• diet: the response is generally greatest in animals fed plant proteins.
• general herd health: the growth response is less in healthy herds. 

There are some problems associated with the use of antibiotics in animal production systems that limits their use. It is thought that the widespread use of sub-therapeutic levels of antibiotics increases the risk of drug-resistant bacteria developing. This makes the treatment of diseases with therapeutic levels of antibiotics more difficult. People are also concerned that this drug-resistance may be transferred to pathogenic human bacteria which has lead to bans on the use of some antibiotics as growth promotants.  

Ionophores

Ionophores are a type of antibiotic that depress or inhibit the growth of certain microorganisms that are present in the rumen. This alters the fermentation in the rumen in five ways:

1. They increase the efficiency of energy metabolism by increasing the amount of proprionate and less acetate being produced. This increases the efficiency of energy metabolism in addition to cutting down on methane production. The animal performs better because more energy is retained during fermentation in the rumen. 
2. Ionophores also decrease the breakdown of feed protein in the rumen which increases the amount of undigestable protein (UDP) present. This means more protein is digested in the small intestine which is good for high-performing animals (see this post for why).
3. The incidence of acidosis, grain bloat and coccidiosis is also reduced. 
4. The number of protozoa present in the rumen is also decreased. These protozoa compete with rumen bacteria so this increases the amount of bacterial protein that is passed from the rumen into the omasum. 
5. Ionophores also increase the retention of selenium, zinc and copper.

When used with cattle, ionophores generally decrease feed intake (this is because the animal can retain more energy per unit of feed). Feed conversion is also improved and daily gain is maintained or improved all without affecting the composition of the carcass. 

Ionophore toxicity may occur and this depends on the species and the drug used. 

Other Additives

Other compounds that are added to feed rations include:

• Probiotics: these are cultures of live microorganisms that provide benefits to the animal by improving the properties of the indigenous microflora. (ie. they increase the amount of good bacteria :) ). 
• Exogenous Feed Enzymes: these can be used to promote the breakdown of non-starch polysaccharides in the gut which improves digestibility.
• Organic Acids: these are used to inhibit the growth of gut pathogens
• Prebiotics: these are non-digestible feed ingredients that stimulate the growth and/or activity of beneficial bacteria at the expense of harmful bacteria. This may improve animal production. 
• Repartitioning agents: these are substances that affect body composition. For example, beta-agonists may increase muscle synthesis and fat breakdown which results in a bigger, leaner carcass as well as increased growth rate and feed conversion. 

That's all for now, see you next time :)