*Dr. Charles Sniffen is with Fencrest LLC. Dr. Essi Evans is with Technical Advisory Services Inc. Dr. Elliot Block is research fellow, animal nutrition, at Arm & Hammer Animal Nutrition. This is part 1 of two articles. The second article in the series will address how to formulate dairy diets for amino acids.
FORMULATING diets for amino acids in the swine and poultry industries has been a standard practice for a considerable period of time.
Swine and poultry nutritionists found that by formulating for amino acids, they could reduce protein intake and increase productive efficiency compared to formulating for crude protein alone.
In contrast, the dairy industry has been tepidly formulating rations for amino acids for about 20 years.
Amino acid balancing has gained more traction in the past five to six years due to increased concerns about the environment, volatile ingredient prices and the continued development of more precise nutritional models.
The dairy industry gained considerable formulation flexibility with the recent introduction of rumen-protected lysine sources to complement the availability of rumen-protected methionine, which has been on the market for a relatively long time.
Beyond the ideal
Initially, researchers did not clearly understand the mechanisms involved in amino acid uptake by tissue, nor did they understand the differing needs of each for maintenance as opposed to the various productive functions of an animal.
A profile, believed to be perfect, was developed along with a requirement for one amino acid, usually lysine. All other amino acids were estimated as a percentage of, or ratio to, lysine.
As amino acid nutrition became more sophisticated, methods were developed to determine the digestibility or availability of amino acids from ingredients. Then, models were developed to be more quantitative in defining the amino acid requirements of an animal. By knowing the weight of the animal and the losses in nitrogen, maintenance could be estimated. Knowledge of the amino acid composition of weight gain and milk further allowed for the calculation of productive requirements on a net basis.
The ongoing challenges are to more fully understand the efficiencies for each amino acid to meet the net needs and to clearly understand the metabolic priorities that occur for stages of growth, lactation and pregnancy, as well as their interactions.
This should not discourage researchers and nutritionists from using current models as they are much closer to reality than not. However, it is no wonder that, for years, monogastric nutritionists have continued to use the ideal protein approach.
The rumen provides an additional level of complexity in the assessment of amino acid utilization. Proteins from feeds enter and exit the rumen, with the amino acid profile changing along the way. Proteins become degraded by rumen microbes, and microbial protein is synthesized.
Which assays have been used to measure the rumen-degrading characteristics of proteins in feedstuffs?
* Researchers initially used soluble protein to give a rough measurement of the protein that rapidly degrades in the rumen (Wohlt et al., 1973). This needed an improvement after it was learned that insoluble protein is partially degraded over time.
* Next was an enzymatic method to mimic the rumen protein breakdown (Krishnamoorthy et al., 1983) involving Streptomyces griseus. The S. griseus enzymatic procedure is still available in several of the major forage analysis laboratories and continues to see some use. The method would benefit from a clearer understanding of the best enzyme-to-substrate ratio.
* This method was followed by placing feeds into nylon bags and exposing them to microbial digestion in the rumen — an adaptation of the method developed earlier by Orskov and McDonald (1979). This technique was used as the basis for measuring protein degradability by the National Research Council (NRC; 2001).
The primary drawback to this system is the cost involved with the lengthy procedure. There are also problems with small particles leaving the bags as well as microbial contamination of the residue. (NRC addressed this through the use of a microbial correction.)
* A procedure developed with the CNCPS system (Sniffen et al., 1992) was based on the soluble protein method and then further measured protein fractions that were buffer insoluble using the detergent system. This resulted in separating the rapidly degradable, slowly degradable and unavailable protein fractions.
The issue with this procedure — to this day — is assessing the rates of digestion of each of these fractions. Ironically, the rates assigned to each of the pools generated by this approach were initially estimated using the S. griseus procedure.
In the NRC model (2001), the rates were determined from available in situ data. It's even more of a challenge to measure the amino acids of each of these fractions so a better understanding could be obtained of the amino acids available in the rumen and those escaping the rumen.
To date, none of the procedures used provide a perfect estimate of the feed amino acids available in the rumen, or in the small intestine, but better estimates are provided as gaps in knowledge and understanding are filled.
The nylon bag method would seem intuitively better than the use of soluble protein or an enzyme. The premise is that material escaping from the bag is soluble in rumen fluid and would mostly be broken down in the rumen. The extent of degradability is controlled, in part, by the nature of the proteins being fed and the passage rate of the feeds from the rumen.
Proteins that go into solution will pass at the rate that the rumen fluids leave the rumen. The escape of the feed proteins that do not go into solution is a partial function of intake relative to body size, particle size and particle density. This approach depends on measuring, in a quantitative manner, the rate of degradation of protein by using multiple time point analyses combined with mathematical curve peeling to estimate rates.
It also means that the industry needs to predict the flow of both the liquids and the solids out of the rumen. This is a daunting task.
Researchers have relied on the in vivo data from dairy experiments in which rumen and duodenally cannulated animals have been used in combination with indigestible metal markers that will flow with either liquids or solids. The data are, for the most part, limited to lactating and non-pregnant dry cows, with essentially no data for replacements.
In many of the nutrition models in use today, only the solids' rate of passage is predicted with relatively simple equations. More complex models have been proposed but not implemented.
In the latest CNCPS system, both solid and liquid equations have been developed and will undoubtedly serve as a framework to apply newer information.
Some newer in vitro methods resulted in reducing costs versus in situ methods, allowed rates to be assessed (for example, Broderick et al., 2004) and are being slowly adopted.
Such procedures also highlighted the fact that soluble and degraded proteins are not the same. Soluble peptides can exit the rumen with the liquid outflow.
Colombini et al. (2011) developed a method to estimate the amount of soluble protein that is released as free amino acids and the amount that is released as peptides. As a result, some proteins that are highly soluble provide more escape protein than previously assumed. This method improves the accuracy of the assessment of rumen escape amino acids and can be applied in a laboratory within a reasonable period of time.
After addressing the amino acids derived from feed sources, estimates of the amino acids contributed by rumen microbes were needed. The microbial ecosystem depends on the carbohydrates fed to the animal. However, growth can be maximized only if the correct sources of nitrogen are supplied as well.
The proteins degraded in the rumen that provide peptides and ammonia are essential to the growth of the bacteria in the rumen.
In the CNCPS system, two major microbial niches in the rumen digest fiber and non-fiber carbohydrates. Fiber digestion optimization depends mainly on ammonia and the isoacids generated from the branched-chain amino acids from feed ingredients. The non-fiber bacteria are dependent on both ammonia and peptides for optimal growth.
The microbial mass that is generated in the rumen partially flows from the rumen as the particles are washed out of the rumen as well as in the liquids that leave the rumen.
Feed analysis methods are required to estimate the relative availability of non-protein nitrogen and peptide nitrogen to support microbial growth and to ensure that the mass exiting the rumen can be predicted with some degree of accuracy.
The research being conducted by the Forage Research Lab (Colombini et al., 2011) and The Swedish University of Agricultural Sciences (Hedqvist and Uden, 2006) is breaking new ground in that regard.
The high-quality protein in bacteria provides an excellent amino acid source to the cow. Nutritionists formulating rations attempt to optimize the microbial growth in order to provide a significant amount of amino acids required by the cow on a daily basis. However, in nutritional models, it is assumed, that the amino acid profile of the microbial true protein is constant, but this is not true.
First, a varying degree of protozoal protein flows from the rumen, which has a different amino acid profile. Also, solids and liquids associated with bacteria have been shown to have different amino acid profiles.
However, early researchers got over this hurdle by concluding that the differences were inconsequential and that average values could provide estimations of the profile of amino acids arising from the rumen (Bergen et al., 1968; Hvelund, 1985; Purser and Beuchler, 1966; Harrison et al., 1973).
Since then, researchers have developed a procedure to separately predict bacterial and protozoal flow (Shabi et al., 2000). This refinement may provide further accuracy to this prediction in future models.
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