Dairy producers and their feed suppliers have greater incentives today to use the cow's diet to manipulate milk components, such as protein, and reduce pollution, including nitrogen wastes. Practicing nutritionists, therefore, are beginning to recognize the need to define not just the limiting amino acids (AAs), but the general requirement for all AAs for growth and milk production.
However, accurately predicting the AA requirements of dairy cows and other ruminants presents a number of challenges. Current National Research Council recommendations do not explicitly allow for diets to be balanced for essential amino acids (EAA) or limiting AA contents, except to recommend a ratio of 3:1 lysine-to-methionine in metabolizable protein (NRC 2000, 2001). Thus, feeding schemes subdivide EAA requirements into net requirements first for maintenance functions, then for growth, lactation, and reproduction. The metabolizable protein requirement reflects only the roles AAs serve as precursors for synthesis of proteins, such as muscle, milk caseins, and products of conception from fertilization to birth. This approach works because a major part of the metabolizable protein prediction derives from the AA composition of endogenous protein losses, and that is needed for net synthesis of muscle, milk, and fetal tissue proteins.
However, there is much more to be learned beyond the conventional essential AA approach. In particular, what are the conversion or efficiency factors that partition AAs in the metabolizable protein for the cow's various needs? A better understanding of the inefficiency' of these conversion factors may offer greater potential for improving predictions about nutrient needs and subsequent production.
Information on what comprises the AA requirements of producing dairy cows and other ruminants remains limited. But what is available is intriguing. This brief account cannot provide much detail, but we can point out a few aspects of recent work from which we have a better realization of the significant ways in which EAAs are involved in a number of metabolic pathways. The key metabolic pathways serve important functions in protein and energy metabolism, gluconeogenesis, lipid-fatty acid metabolism, and in mammary synthesis of milk protein and lactose. The next generation of practical diets for dairy cattle, goats, and other species is likely to depend upon the definition and quantification of these other requirements, which take us well beyond the current emphasis on identification of limiting AA.
More accurate prediction of AA requirements
More accurate prediction of AA requirements can help enable balancing of dietary nutrients to increase milk protein yield. One place to start is to focus on AAs in the context of the dairy cow's metabolism. For producing animals, the composition and amounts of the proteins accreted (such as skeletal proteins in growing animals) or secreted (such as milk proteins in lactating animals) determine the dietary requirement for EAAs. The availability of intermediates of AA metabolism within tissues bears importantly on the rate of synthesis of other AAs, glucose, and milk components. From a metabolic standpoint, the only truly non-EAAs are glutamate and serine (Reeds, 2000) (see figure Primary pathways'). The cow can synthesize these non-EAAs from non-amino nitrogen (ammonium ions) and appropriate carbon skeletons derived from intermediates of glycolysis (3-phosphoglycerate) and the Krebs cycle (-ketoglutarate). The diet still requires glutamate as long-standing research demonstrates growth depression without it, suggesting that under normal growing conditions other factors may limit the rate of glutamate synthesis. All other non-EAAs derive their amino group or carbon skeleton from other AAs. Glutamate and serine are the primary precursors for non-EAA synthesis.
In ruminants, the essentiality of AAs relates to the supply of AAs leaving the rumen, mainly in the form of microbial proteins with variable contributions from undigested feed proteins. In the core group of EAAs, the cow synthesizes the branched chain AAs, methionine and phenylalanine from their corresponding keto-acids. However, there is no new or net synthesis, unless the corresponding keto-acid is provided in the diet. Thus, racemic mixtures of D and L-isomers of AAs can provide an effective source of supplemental AAs to balance rations. Supplemental dietary keto-acids can yield the corresponding L-isomer. The keto-acids of all AAs except for lysine and threonine, which are not transaminatedcan be converted to the corresponding AA upon transamination.
Methionine for milk protein
This perspective sheds light on attempts to increase milk protein yield by supplementing rumen-protected methionine, which can yield mixed results. The cow can synthesize methionine via remethylation of homocysteine, but this also does not represent new or net synthesis of methionine because the only source of homocysteine in the body derives from methionine catabolism. By contrast, the synthetic hydroxyl-analogue of methionine4- thiomethyl-2-hydroxybutanoic acid (HMtBA) functions as a source of methionine in pig, poultry, and dairy cow diets. HMtBA is not toxic, in contrast to free methionine, and the analogue does not appear to be removed by the gut and liver tissues to the same extent as methionine.
Furthermore, HMtBA has no known mammalian transporters, so it readily diffuses into tissues for conversion to methionine. Sheep convert HMtBA to ketomethionine and via transamination to L-methionine (Wester et al, 2000), with the greatest contributions to tissue methionine arising from the kidneys (22%), followed by the liver (14%), and the gastrointestinal tract (5-12%). The mammary gland is also a significant site of conversion of HMtBA to methionine with 20% of milk protein methionine derived from the supplemented HMtBA given to dairy cows (Lobley and Lapierre, 2001; Lapierre et al, 2002).
AA cost of higher milk protein
In any case, achieving higher milk protein yield by post-ruminal protein supplementation comes at a cost, with poor efficiency of utilization of the additional absorbed AA (Guinard and Rulquin, 1994). In comparing the EAA profile of microbial protein or typical rumen bypass protein sources (such as corn gluten, soybean meal, and fish meals) to the profile required for tissue gain or milk protein secretion, the patterns are similar with the exceptions of leucine and histidine, and possibly also methionine (NRC 2000, 2001).
Studies in dairy cows where AA (in the pattern of casein) was infused into either the duodenum, the blood supply prior to the liver, or the peripheral blood, suggests that AA supply to the udder is probably modified as it passes through the body (Bequette et al, 2003). Responses in milk protein output and recovery of EAA were lowest when AA was provided to the gut. Moreover, the profile of AA delivered to the mammary gland for milk protein synthesis did not reflect the AA profile leaving rumen, available for absorption.
Therefore, if our goal is to define exactly the pattern of AA required to maximize production and efficiency, we need to consider the potential transformations of the AA supply beginning at the level of the small intestines, traversing through the gut tissues and liver (in series with the gut), and how these transformations are divided between the mammary gland and other tissues. Also, we need to consider mammary metabolism of AA, and the factors and mechanisms that regulate or limit transport and incorporation of AA into milk protein.
A paper presented earlier this year at the Florida Ruminant Nutrition Symposium in Gainesville provides additional detail on how growing and lactating ruminants partition AAs in the metabolizable protein for their various needs. Integral to these considerations is the realization of the intimate connection of AAs to many central pathways of metabolism, particularly the Krebs cycle. Pathways for glucose, lactose, fatty acid, glycerol, and non-EAA synthesis link to the Krebs cycle. The balanced synthesis of non-EAAs ensures optimal functioning of these pathways and the cell, organ, and animal. Moreover, these functions must be balanced against the Krebs cycle's requirement to generate energy. Thus, the role of AAs appears to be vital and increasingly important to practicing nutritionists as they seek to enhance milk protein output or reduce nitrogen excretion.