The concept of balancing rations for swine and poultry with pure amino acids has been a part of commercial feed formulation programmes for many years. This has led to lower nitrogen levels of diets, improved efficiencies of nitrogen utilization and, in general, lower ration costs. However the use of pure amino acids in ruminant rations has lagged far behind.
There are a number of reasons that commercial application of amino acid nutritional concepts to ruminant ration formulation has not reached the level attained in the swine and poultry industries. The most obvious reason is that the action of rumen microbes sharply changes the amino acid profile of the ration consumed to create a different one that reaches the intestinal absorptive site. This has made prediction of metabolizable amino acid delivery to the intestine highly inaccurate and, for practical purposes, values cannot be validated due to procedural difficulties with the only in vivo systems available to do so. In addition, there has been a lack of research on metabolizable amino acid requirements of dairy cattle, due equally to the high cost of the research and variability of the data generated. This led NRC (2001) to conclude that insufficient data exists to predict metabolizable amino acid requirements for any amino acids, save lysine and methionine for which relatively large data sets exist from which empirical relationships could be developed. There can be no reasonable expectation that this lack of information will be addressed in the near future, at least partly due to worldwide degradation of the applicable research infrastructure.
Other reasons for a failure to balance dairy rations for metabolizable amino acids are more mundane. In most parts of the developed world, where high producing dairy cattle are concentrated, the costs of proteins have been near historical lows for many years thereby removing the economic incentive for dairy nutritionists to reduce nitrogen levels of rations. In addition, only dairy producers in a very few areas of the world (e.g. The Netherlands) have been forced through government action to pay, in some form, for the environmental impact of nitrogen excretion from their dairies. Under these circumstances, it makes economic sense for the ruminant nutritionist to elevate the protein concentration of the ration by 5-10% over calculated requirements with high UDP sources, known to be rich in potentially limiting amino acids, thereby virtually guaranteeing that metabolizable amino acid supplies will not limit animal performance.
Finally, even if a dairy nutritionist wished to balance dairy rations based on metabolizable amino acids, the tools available are limited. Only one amino acid, methionine, is commercially available in a rumen protected (RP) form and its cost is very high per gram of metabolizable amino acid. There is little evidence from published research to support enhanced performance, and(or) improved nitrogen efficiency, with RP methionine supplementation of dairy cattle rations (Robinson, 1996), which tends to be primarily associated with modest increases in the milk protein proportion. The low market penetration of the available RP methionine products, developed at very high expense by corporate groups, has not encouraged these, or other, corporate groups to invest in development of other RP amino acids, particularly in the absence of research results showing repeatable commercially substantive benefits to dietary addition of these amino acids. Although a few studies have reported some benefits to dietary supplementation with isoleucine (Robinson et al., 1999) and histidine (Vanhatalo et al., 1999), this literature base is very limited in nature and volume.
However, the commercial situation is changing in two key dairy areas, and this may force commercial dairies in many parts of the developed world to change their nutrient management practices. The environmental impact of dairies in many parts of Europe and America is being scrutinized as never before. In America, the Environmental Protection Agency is under increasing pressure from public interest groups to both enforce current regulations, and introduce more restrictive regulations. The focus seems to be inexorably moving, albeit slowly, to site-specific criteria that will evaluate dairies individually for nutrient, particularly nitrogen and phosphorus, efficiency and penalize those that do not meet defined criteria/goals. These penalties could take the form of fines and(or) taxes, as well as limits on nutrient excretion that could force individual dairies to reduce animal numbers. The other critical recent change has been the elimination of the entire class of animal and marine source high UDP feedstuffs in Europe, and most animal feedstuffs in America, that have played a critical role in allowing dairies to economically overfeed dietary protein, relative to predicted requirements, in order to assure that metabolizable protein and amino acids do not limit animal performance. The elimination of this class of feedstuffs has had a major negative impact on the ability of dairy nutritionists to formulate rations designed to deliver high calculated levels of specific amino acids, most notably lysine, to the intestinal absorptive site.
In combination, the increasing severity of site specific environmental regulations, particularly in America, and the elimination of all (Europe) or many (America) animal and marine source protein feedstuffs has increased commercial production, and use by commercial dairies, of protein-rich feedstuffs treated to resist rumen degradation. It is likely that the future will see renewed interest in both producing cost-effective RP amino acids, quite likely beyond methionine and lysine, as well as completing research into better defining metabolizable protein and amino acid requirements of daiiy cattle. Together these events, if they occur, will allow more predictable formation of rations designed to optimize both animal productivity and nitrogen efficiency.
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