Relationships between circulating plasma concentrations and duodenal flows of essential amino acids in lactating dairy cows

The objective of this study was to better define essential AA (EAA) requirements in lactating dairy cows through examination of the relationship between plasma essential AA concentration (p[EAA]) and predicted duodenal flow of essential AA (EAAduo). Our hypothesis was that at a given level of milk protein output, p[EAA] would remain steady in response to increasing EAAduo until the EAA requirement was met, at which point p[EAA] would increase rapidly in response to greater duodenal flow of EAA until p[EAA] reached a plateau as other body processes degraded excess EAA to avoid toxicity. Thus, the requirement of each EAA would be fulfilled when p[EAA] increased rapidly. To investigate this hypothesis, we compiled a literature database that included 102 studies with 420 treatment means that reported p[EAA], dietary nutrient content, body weight, and milk production. A second database was produced to validate relationships developed in the first database and included 32 studies with 98 treatment means. All relationships were evaluated as regression equations with study as a random factor. Breed, days in milk, body weight, and milk protein production had no effect on the plasma concentration of any EAA. Other than metabolizable protein supply, nutritional content of the rations did not affect p[EAA]. Only p[Arg] was affected by parity, with primiparous cows having higher concentrations of Arg than older cows. No break points in the relationship between p[EAA] versus EAAduo were detected as either steep increases or plateaus. Plasma Arg, Ile, Lys, Thr, and Val concentrations were best associated with their respective EAAduo as quadratic equations, whereas His, Leu, Met, and Phe were associated only linearly. Adding a quadratic term improved the adjusted R² or decreased the root mean square error marginally (<2.0%). Thus, we conclude that the main effect of EAAduo on p[EAA] is linear. Abomasal or duodenal infusions of Met, Lys, His, Lys+Met, and casein revealed that Met or Lys infused alone increased the plasma concentration of the infused EAA and lowered the concentration of other EAA, particularly His. Infusion of Lys+Met or His alone was associated with increases in concentrations of these EAA without affecting others. We conclude that over a wide range of protein intakes in lactating cows, plasma levels of EAA increase linearly with duodenal flow. No evidence was found that EAA requirements are reflected in blood plasma concentrations.