Raju Kushwaha*, Vinod Kumar, Muneendra Kumar, Ram Dev Yadav, Dhapse Sanket Rajkumar
Department of Animal Nutrition, College of Veterinary Science and Animal husbandry,
DUVASU, Mathura-281001
*Email: rajuvet15@gmail.com
Feed proteins are complex amino acid polymers which are broken down in the gut into amino acids. These amino acids are absorbed and assembled into body proteins which are used in the construction of body tissue e.g. muscles, nerves, skin and feathers. Dietary crude protein levels do not indicate the quality of the proteins in a feed, protein quality is based on the presence and balance of essential amino acids in the feed ingredients.
Dietary crude protein (CP) requirements are somewhat of a misnomer as the requirement is based on the amino acids content of the protein. Once digested and absorbed, amino acids are used as the building blocks of structural proteins (muscle, skin, and ligaments), metabolic proteins, enzymes, and precursors of several body components. Because body proteins are constantly being synthesized and degraded, an adequate amino acid supply is critical to support growth or egg production. In poultry, 22 amino acids are needed to form body protein, some of which can be synthesized by the bird (non-essential), whereas others can not be made at all or in sufficient quantities to meet metabolic needs (essential). Essential amino acids must be supplied by the diet, and a sufficient amount of non-essential amino acids must also be supplied to prevent the conversion of essential amino acids into non-essential amino acid.
Additionally, if the amino acids supplied are not in the proper, or ideal, ratio in relation to the needs of the animal, then amino acids in excess of the least limiting amino acid will be deaminated and likely used as a source of energy rather than towards body protein synthesis. This breakdown of amino acids will also result in higher nitrogenous excretions.
Ideal amino acid profile
It is important to supply the broiler with an appropriate balance of digestible amino acids. As an aid to achieving this objective, an Ideal Amino Acid Profile can be used. This is a system where the requirement of the main amino acids that may be limiting in broiler feeds are calculated and then lysine is used as the reference amino acid to which ratios are set for other amino acids.
The best way to reduce N in poultry excreta is to lower the amount of CP that is fed by supplementing diets with amino acids. Reductions in the non-essential amino acid pool, coupled with supplying a more “ideal” amino acid profile in the diet can substantially increase the efficacy of overall N retention by the bird. On a practical basis, however, bird performance can be hindered by these lower CP diets due to a number of factors that tend to be associated with dietary CP and amino acid reductions. Formulation based on bird amino acid requirements rather than CP can minimize N excretion by simply reducing total dietary N intake. Reducing the amount of CP and excess amino acids being fed is the most obvious method to curb N excretion and the amount of NH3 that can be formed and volatilized. However, the extent to which N reduction can be accomplished is largely limited due to meeting the most limiting amino acid after threonine and through economic decisions on ingredient selection.
Unfortunately, there is a wide-spread belief that whenever CP concentrations are lowered, performance is negatively affected. Burnham (2005) speculates that lowered CP concentrations beyond practical formulation and then did not supplement back with sufficient amounts of limiting amino acids other than methionine (Met) and lysine (Lys). Reductions in the non-essential amino acid pool, coupled with supplying a more “ideal” amino acid profile in the diet can substantially increase the efficacy of overall N retention by the bird. On a practical basis, however, bird performance can be hindered by excessively lowering CP in diets due to a number of factors other than the reduction of CP itself. These factors can include: reduced potassium levels, altered ionic balance, lack of nonessential amino acids, imbalances among certain amino acids (e.g. branched chain amino acids), and/or potential toxic concentrations of certain amino acids.
Amino acids which are said to be essential cannot be synthesized by the bird. These essential amino acids must therefore be fed in order to supply the building blocks needed in the synthesis of body proteins thereby supporting growth. When supply of a single amino acid does not meet the bird’s requirement, it is considered to be “limiting”. At any given physiological stage of growth or age, a specific amino acid profile is needed to support optimal growth, with no limiting amino acids or surpluses. This profile has been termed an “ideal” ratio, or “ideal protein”. Therefore, to minimize N excretion, the “ideal” combination of essential and non-essential amino acids are needed to meet growth and/or egg production by the bird. However, due to available feedstuffs and a limited number of supplemental amino acids it is difficult to provide this optimal ratio to the bird. For the past couple of decades, the broiler industry has utilized feeding strategies in phases that are shorter as to more closely meet the nutrient needs of the developing bird.
Ingredient selection
Selection of feedstuffs with relatively high digestibility can help with overall reductions in amino acid formulation. Sources such as feather meal are not typically considered due to their amino acid profile, and their digestibility. Similarly, formulation for emission reduction should also consider the protein quality as exemplified in the range of apparent digestibility where processing temperatures could cause Maillard reactions as well as other conditions that would reduce amino acid availability. Formulation on a digestible amino acid basis can a) reduce the total amount of CP fed, and b) limit the excessive amount of non-essential amino acids fed particularly if higher digestible CP feedstuffs are available.
Formulation on a digestible amino acid basis
Digestible amino acid values are considered by many to be the best measure of the amino acid value of ingredients. Long term reductions in CP formulation with adoption of the digestible amino acid concept should greatly reduce feed cost and N emissions. Further benefits of formulating on a digestible amino acid basis include decreasing safety margins, increasing the accuracy of predicting performance, and increasing the uniformity of product after processing. Unfortunately, knowledge of what the causes of variation in amino acid digestibility within and between ingredients is not sufficient. Additionally, inconsistent methodologies make it difficult to make the switch to using digestible amino acid values, especially for non-traditional feed ingredients.
Determination of ingredient amino acid digestibility from feedstuffs has traditionally been done with either cecetomized roosters or collection of ileal digesta from birds fed only the test ingredient or a semi-purified diet with the feedstuff being analyzed as the sole source of protein and amino acids. These assays have an obvious down-side as they are expensive and have long turn-around times. Therefore, real time formulation on known amino acid digestibility for any feedstuff is unrealistic. Other approaches to improve the turn-around time include correlation of bird digestibility studies with near-infrared reflectance spectroscopy (NIR) or in vitro assays (Schasteen et al., 2007). Most of the grow out poultry studies focusing on use of digestible amino acid formulations have only focused on performance and economic considerations and not necessarily on N excretion or emission reduction (Dari et al., 2005). Formulation on a digestible basis can have large economic and environmental benefits, particularly when formulating with ingredients known to have lower digestibility.
Possible impact of crude protein reduction
Reducing CP content of broiler diets by less than 2% units resulted in decreased litter N content but no significant differences in NH3 concentration in the house (Ferguson et al., 1998). The 13.3% decrease in N intake did correspond to 18.2% reduction in litter N content. Pope et al. (2004) also has looked at the advantages to increasing the number of phases during the broiler growth cycle. By changing diets every other day to more closely meet the bird’s amino acids from 21 to 63 days of age, performance and meat yield did not change, but N excretion was reduced by 7 to 13%.
Conclusions
As a general guide, for each 1% reduction in dietary CP, estimated NH3 losses are reduced by 10% in poultry. As animals are fed closer to true N requirements, further reductions in dietary CP may result in less pronounced reduction in N excretion and NH3 losses. When poultry are fed closer to requirements and strategies are implemented to improve CP and amino acid digestibility, reductions in the amount of N excreted by the bird can be 10 to 20% depending on how much N is currently being fed. The poultry industry, however, currently utilizes substantial safety margins for formulation of N, due in large part to uncertainty of nutrient requirements and variability in ingredient amino acid content and digestibility. Reduction of N consumed, use of ingredients with complementary amino acid profiles, and use of ingredients with higher amino acid digestibility, therefore, can have dramatic impacts on the amount of N excreted.
Key Points
- Consider amino acid levels together with factors affecting feed intake (e.g. energy levels, feed intake control programs, feed form, feeder arrangement) when formulating broiler diets.
- Use high quality sources of protein, especially in circumstances when broilers are likely to suffer heat stress.
- Maintain updated ingredient amino acid and protein values on the formulation matrix.
- Balanced Protein approach produces benefits in broiler and economic performance.
- The AA+ broiler is particularly responsive to dietary amino acid levels. Feeding to AA+ recommended levels provides an economic advantage.