What is the True Supply of Amino Acids for a Dairy Cow?

Abstract

Improving the prediction of milk protein yield relies on knowledge of both protein supply and requirement. Definition of protein/amino acid supply in ruminants is a challenging task, due to feedstuff variety and variability and to the remodeling of nutrient intake by the rumen microflora. The questions arise, therefore, how and where should we measure the real supply of AA in the dairy cow? This review will follow the downstream flow of AA from duodenum to peripheral tissue delivery, with a glance at the efficiency of transfer into milk protein. Duodenal AA flow comprises rumen undegradable feed, microbial protein, and endogenous secretions. Most attention has been directed toward definition of the first two contributions but the latter fraction can represent as much as 20% of duodenal flow. More information is needed on what factors affect its magnitude and overall impact. Once digested, AA are absorbed into the portal vein. The ratio of portal absorption to small intestinal apparent digestion varies among essential AA, from 0.43 (threonine) to 0.76 (phenylalanine), due to the contributions of preduodenal endogenous secretions to the digestive flow, non-reabsorption of endogenous secretions and gut oxidation of AA. Few data are available on these phenomena in dairy cows but the evidence indicates that they alter the profile of AA available for anabolic purposes. Recent comparisons of estimated duodenal flux and measured portal flux have prompted a revisit of the NRC (2001) approach to estimate AA flows at the duodenum. Changes to the model are proposed that yield predictions that better fit the current knowledge of AA metabolism across the gut. After absorption, AA flow first to the liver where substantial and differential net removal occurs, varying from zero for the branched-chain AA to 50% of portal absorption for phenylalanine. This process alters the pattern of net supply to the mammary gland. Overall, intermediary metabolism of AA between the duodenum and the mammary gland biologically explains the decreased efficiency of the transfer of absorbed AA into milk protein as maximal yield is approached. Therefore, variable, rather than fixed, factors for transfer efficiencies must be incorporated into future predictive models.

Strategies for optimizing nitrogen use by ruminants

“…In sheep, the net PDV release of EAA represented about 65% of the net release across the MDV, reflecting the uptake of EAA from arterial blood by the reticulorumen and other tissues not drained by the cranial mesenteric vein. As discussed by Lapierre et al (2006), this uptake of EAA from arterial blood by the reticulorumen in part reflects the use of EAA for synthesis of endogenous proteins, which contribute to the EAA absorbed from the small intestine. The utilization of AA from the arterial blood supply can be measured by using labelled AA, while the 'first pass' or absorptive use of AA can be measured by differentially labelling the supply of AA from the lumen of the small intestine.…”

“…Microbial protein also includes non-protein N recycled from the blood as urea, but these are not 'endogenous' proteins per se. Measurement of the flow of these endogenous proteins is technically challenging, but available data suggest that they can represent from 8% to 16% of total protein flow to the small intestine (Reynolds, 2005;Lapierre et al, 2006). Therefore, measurements of total flow overestimate the true supply provided by the diet and fermentation.…”

“…Digestion of AA in the small intestine can be measured as the disappearance between the small intestine and ileum using cannulas. The interpretation of these measurements must also consider the contributions of endogenous secretions, as well as the digestibility of endogenous proteins entering the small intestine, in order to estimate the true supply of AA from the diet (for a detailed discussion, see Lapierre et al, 2006). On a net basis, the disappearance of total AA from the small intestine of lactating dairy cows averaged 70% (s.d.…”

“…There have been numerous recent reviews of the metabolism of AA by splanchnic tissues and their role in determining AA supply to the mammary gland for milk protein synthesis (Reynolds, 2006a;Lapierre et al, 2006). The AA absorbed are potentially available for synthesis of constitutive or secreted proteins, transamination or oxidation before reaching the mesenteric blood.…”

“…Measurements of AA appearance in the portal vein can be obtained using venous-arterial concentration differences and blood flow (Reynolds, 2006a;Lapierre et al, 2006). These measurements represent the net supply of AA after metabolism by the PDV during absorption and from arterial blood.…”