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Update on amino acid nutrition for broilers

Nutritionists must pay attention to current research when it comes to the amino acid requirements of modern broilers.

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David Tadevosian |

Modern developments require attention to current research

Modern commercial nutritionists already formulate broiler feeds based on metabolizable energy (AMEn) or even net energy (NE) and amino acid standardized ileal digestibility (SID). So, the need to further convince these professionals to literally “upgrade” their feed formulation systems is no longer an issue. In contrast, it is the plethora of suggestions and recommendations that arise from different sources with diverse interest that causes most headaches, and this will be the focus on this brief discussion.

Lysine levels

Having decided on an energy level of a particular broiler formula, which can be the focus of an entirely different story, the commercial nutritionist must decide on the digestible (SID) lysine level. Lysine is chosen above other amino acids because it is used exclusively for protein synthesis, making its determination less problematic. The level of lysine depends on many factors, and many commercial nutritionists follow the guidelines of the breeding company providing the broiler stock.

However, unless we face extreme genetic lines, most modern broilers have similar growth characteristics. In fact, very recent research has even questioned the need to follow the old recommendation of having a fixed energy-to-lysine ratio for each broiler feed (starter-grower-finisher). The exact level differs whether the feed is going to be sold as a generic commercial feed or fed to in-house broilers (integrators). At the end, it is advisable to be open to testing (challenging) birds with graded levels of lysine (plus or minus 5-10% steps are recommended), according to goals and economic conditions.

Ideal protein

Ideal or balanced protein is the ratio that must exist between lysine and all other amino acids. These ratios are better established for essential amino acids (those that must be provided by the feed) than those for non-essential amino acids (those that can be synthesized by the bird). This distinction becomes complicated as very-low protein diets deprive the animal of “excess” non-essential amino acids for internal interconversion.

There are three sources of ideal protein ratios: universities, broiler breeders and feed-grade amino acid suppliers. As is to be expected, breeders are a bit more generous than universities as they want to ensure maximal broiler performance, regardless of profitability. In the same line of thought, very low-protein diets benefit the sales of feed-grade amino acids, but we have reached a limit where we have started to observe negative effects on animal performance.

As ideal protein is composed of two parts (maintenance and growth), it is to be expected that, as the bird grows, this ratio will change. Indeed, this was the case when the birds were marketed at 56-plus days, but today it is questionable whether we need a different ideal protein for a broiler that is marketed at 35 days. Instead, feather cover and health condition should come into consideration for the ideal protein for each bird flock.

Nevertheless, the older ideal ratios proposed by the University of Illinois and recently confirmed by Texas A&M University appear to serve well the modern broiler. Small differences in values among those three sources, however, are of no great commercial importance as ingredients are neither well defined in their composition nor is it possible to segregate batches of ingredients of different composition – at least in most commercial cases.

Amino acid equations

A good part of commercial nutritionists still needs to implement amino acid equations that properly predict the level of amino acids as protein levels change. For example, a 10% increase in crude protein in corn or soybean meal does not automatically translate into 10% increase in lysine or any other amino acid. In essence, the changes are not linear. Several sets of equations are used, and it is recommended to use one that is constantly updated and supported by a large batch of data. Some large broiler integrators, using near-infrared (NIR) technology, even developed their own amino acid equations. In my opinion, this is an area most commercial feed mills need to consider more closely.

Poultry Amino Acid ProfileLow-protein diets

As mentioned already, there is a trend to reduce crude protein levels in order to protect the environment (from excreted excess nitrogen) and improve feed conversion rate (from improved nitrogen utilization). However, very low crude protein diets become limiting in non-essential amino acids. Also, recent research has shown that traditional protein sources (such as soybean meal) are not just protein but offer a variety of other phytogenic molecules with functional properties.

As we cannot go back to high-protein diets, there is need for a balance between using straight ingredients and feed-grade amino acids. The old recommendation that diets without any feed-grade amino acids should not be reduced by more than 2-4 percentage points by the addition of said amino acids still holds true, especially when a highly experienced nutritionist is not supervising animal performance.

Non-essential amino acids

These amino acids are essential by the bird, but they are not needed in specific proportions in the feed, like essential amino acids, such as lysine or methionine. Today, there is much discussion about the ratio of essential to non-essential amino acids, and many experts point to a 50:50 ratio, especially in very low-protein diets. To supplement this part of non-essential amino acids in feed-grade form, it has been recommended to add glycine from which other non-essential amino acids can be converted internally.

Glycine and serine

Following from the above, a new term has been introduced to ensure that two non-essential amino acids glycine and serine are used correctly into formulating broiler diets. Glycine equivalence (Gly-equ) takes into account that these two amino acids are freely interchangeable but have different molecular values. The following equation is used, from Kleyn and Chrystal (2020) Broiler Nutrition Masterclass, Context.:

Gly-equ = glycine (g/kg) + [serine (g/kg) x 0.7143]

However, establishing a correct and meaningful Gly-equ requirement is not as easy as calculating this value for each natural ingredient. This is because the intermediate metabolism of many amino acids – and even choline, a vitamin – affects the requirement for this new value. But, this is a new and interesting area of research.


The advent of corn DDGS has brought into attention the importance of having a correct balance among the branch-chain amino acids: leucine, valine and isoleucine. Today, it has been demonstrated that an excess of leucine (which abounds in corn and corn DDGS) can increase the requirements for valine and isoleucine and even that of tryptophan (a most expensive commercial feed-grade amino acid). Luckily, those who use soybean meal instead of high levels of DDGS do not face such problems.

Modern developments for modern broilers

Modern broilers are marketed at such young age that having multiple sets of values for various indexes of protein quality appears to be questionable. The increased use of corn DDGS, the use of very-low protein diets in association with the introduction of even more feed-grade amino acids has created unforeseen issues that draws the attention of current research and deserve the attention of commercial feed nutritionists. Having a qualified nutritionist on staff is a must for keeping up with modern developments.

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