Absorption and Delivery of Nutrients for Milk Protein Synthesis by Portal-Drained Viscera

Abstract

The predictability of diet effects on milk composition is limited by the lack of understanding of the metabolic transformations that absorbed nutrients undergo within the portal-drained viscera and liver of high yielding dairy cows. The mass of splanchnic tissues increases dramatically in early lactation, but little is known about the regulation of gut growth and adaptation in early lactation, and further research may provide strategies for optimizing gut adaptation. Glucose is critical for milk synthesis, but portal-drained visceral tissues normally use rather than absorb glucose on a net basis. Dietary starch of low ruminal digestibility increases postruminal starch digestion and decreases net use of glucose by portal-drained viscera slightly, but increases in glucose absorption by portal-drained viscera never account fully for increases in starch disappearance from the small intestine and occur at the expense of VFA absorption. For cows in positive energy balance, greater glucose availability increases tissue energy balance and glucose oxidation, but has little effect on milk or milk protein yield. Similarly, chronic increases in propionate absorption have little effect on milk or milk protein yield. In contrast, casein infusion into the small intestine consistently increases milk and milk protein yield, but the mechanisms responsible remain unclear. There are few data describing the absorption and metabolism of AA by splanchnic tissues of lactating dairy cows, but, as for glucose and VFA, utilization of many AA by portal-drained viscera is substantial. In addition, the contribution of peptides to AA absorption and transport is uncertain and must be clarified. Therefore, measurements of nutrient disappearance from the lumen of the gut cannot be equated with nutrient appearance in the portal vein. Data describing metabolism of nutrients by portal-drained viscera and liver of high yielding dairy cows are needed to improve feeding standards.

Post-ruminal or intravenous infusions of carbohydrates or amino acids to dairy cows 1. Early lactation

“…Our results indicate that there was a higher use of AA for glucose production leading to a higher milk lactose yield on the AB treatment than on the AP treatment. This agrees with the lower plasma concentrations of Ala, which is perhaps the most important glucogenic AA (Reynolds et al, 1994).…”

“…Therefore, the present experiment was designed to compare increased AA and glucose supplementation to cows in negative energy balance. The infusion periods lasted for 6 days and thus the duration should be suitable for achieving milk production responses since infusion responses largely occur within 1 to 3 days (Metcalf et al, 1996;Reynolds et al, 1994). Moreover, Knowlton et al (1998) showed by summarising literature data that short-term (,1 week) infusion of glucose increased milk yield, whereas longer infusion indicated no prolonged responses.…”

“…The abomasal infusions have to cross the splanchnic tissues and are first modified by the liver (Reynolds et al, 1994) before it goes further to the mammary gland or other tissues. Thus, abomasal infusion will probably reduce the AA available for milk production compared with intravenous infusions.…”

“…The highest consumption is found for Glu and Asp, which relates to their preferential use as energy substrates in intestinal tissues (Reynolds et al, 1994). However, the net flux of many AA is perhaps affected more by their uptake in the PDV from the blood (Reynolds, 2002).…”

“…Production and infusion experiments that have evaluated these factors have shown variable success in affecting both milk protein production and content. The low and variable response of metabolisable protein supply on milk protein production (Aikman et al, 2002;Raggio et al, 2004;Schei et al, 2005) may be due to the fact that some AA are utilised as an energy source, some AA are used as a glucogenic precursors and also, increased AA supplementation may affect hormones regulating lactogenic metabolism (Reynolds et al, 1994). Further improvement in prediction of dietary effects on milk protein production requires a better understanding of factors affecting intermediary metabolism of glucogenic substrates and AA.…”