Net absorption and ruminal concentrations of metabolites in nonpregnant dry Holstein cows before and after intraruminal acetic acid infusion

Effect of composition of ruminally-infused short-chain fatty acids on net fluxes of nutrients across portal-drained viscera in underfed ewes

Four ewes, each fitted with a rumen cannula and with catheters in the mesenteric artery and portal and mesenteric veins, received continuous intrarumen infusions of water or of short-chain fatty acids (SCFA). SCFA infusions were isoenergetic (83 kJ/h) and provided rumen molar proportions (acetate : propionate : butyrate) of 70 : 20 : 10, 50 : 40 : 10 or 50 : 20 : 30. The rumen SCFA production rate with the basal diet was 90·0, 23·1 and 8·8 mmol/h for acetate, propionate and butyrate respectively. Portal net fluxes indicated that 74, 67 and 22–30 % of infused acetate, propionate and butyrate respectively, reached the portal vein. Portal net release of β-hydroxybutyrate increased with SCFA infusions, irrespective of the amount of butyrate infused. Portal net release of lactate decreased with high-butyrate infusion. Portal net uptake of glucose increased with the SCFA infusions. In ewes infused with water, a portal net uptake of total amino acids (AA) was observed. SCFA infusions decreased the uptake of nonessential AA (glutamate, glycine, but not glutamine) and increased the net release of tyrosine and essential AA (isoleucine, leucine). Portal net fluxes of AA were similar with both high-acetate and high-propionate infusions. Lower net uptake of glutamine and net release of most essential AA and some nonessential AA were observed with the high-butyrate infusion. Energetic summation of portal net release was not significantly different between the three SCFA infusions, although it tended to be lower with high-butyrate infusion. This may be related to the higher trophic effect of butyrate on the digestive mucosa.

Physiology, regulation and multifunctional activity of the gut wall: a rationale for multicompartmental modelling

A rationale is given for a modelling approach to identify the mechanisms involved in the functioning and metabolic activity of tissues in the wall of the gastrointestinal tract. Maintenance and productive functions are discussed and related to the distinct compartments of the gastrointestinal tract and the metabolic costs involved. Functions identified are: tissue turnover; tissue proliferation; ion transport; nutrient transport; secretions of digestive enzymes, mucus and immunoglobulins; production of immune cells. The major nutrients involved include glucose, amino acids and volatile fatty acids.In vivomeasurements of net portal fluxes of these nutrients in pigs and ruminants are evaluated to illustrate the complexity of physiology and metabolic activity of the gastrointestinal tract. Experimental evidence indicates that high, but variable and specific, nutrient costs are involved in the functioning of the gastrointestinal tract.

Comparison of techniques to determine the clearance of ruminal volatile fatty acids

The objective of this experiment was to compare measurements of fractional clearance rates obtained by using an unlabeled valerate-CoEDTA technique with measurements obtained by using a (13)C-labeled volatile fatty acids (VFA) technique. The exponential decay rate of the (13)C/(12)C ratio after pulse-dosing (13)C-acetate, (13)C-propionate, or (13)C-butyrate into the rumen was compared with the decay rate of rumen valerate concentration following a simultaneous pulse dose. The unlabeled valerate, CoEDTA, and each labeled VFA, one at a time, were concurrently mixed with the evacuated ruminal content of 6 lactating cows in two 3 x 3 Latin squares. The clearance of VFA by passage to the omasum was assumed to be equivalent to the decay in ruminal Co concentration and was around 50% of the total clearance. Acetate, propionate, and butyrate had similar fractional clearance rates (31.2, 33.4, 30.4%/h, respectively), but propionate had a higher absorption rate (19.2%/h) than butyrate (14.2%/h). Linear regression determination coefficients using the valerate clearance rate as an estimator for acetate, propionate, and butyrate rumen clearance were 0.51, 0.56, and 0.99, respectively. In a second experiment, the (13)C-valerate fractional clearance rate estimate (33.7%/h) was similar to the estimate obtained with unlabeled valerate (35.0%/h) by the valerate-Co technique. No (13)C enrichment of rumen microbes was noted 4 h after the intraruminal infusion of (13)C-valerate. Fractional VFA absorption rate estimates obtained in both techniques were similar, although both were lower than estimates reported in the literature by other methods.

The influence of a fermentation-resistant glucose diet on the glucose concentration and other metabolites in portal and jugular blood in cows

The goal of this study was to determine the effect of fermentation-resistant glucose on the glucose concentration and other metabolites in portal and jugular blood in 15 non-lactating cows. In all cows, an indwelling catheter was placed in the left jugular vein and the portal vein for collection of blood samples. Five control cows were fed hay as a normal diet, five control cows were fed straw to induce an energy deficit and five cows were fed hay and they received additionally 2000 g of a fermentation-resistant D-glucose product. The glucose concentration in jugular and portal blood was not influenced by feeding. The concentration of urea and bile acids were significantly higher in portal blood than jugular blood. There was no difference between portal and jugular blood of glucose and total solids. Diet had a significant effect on the concentrations of ammonia, urea, free fatty acids and triglycerides. The concentrations of ammonia and urea were higher in blood of cows fed straw than in blood of cows fed either hay or a fermentation-resistant glucose product. The concentration of urea remained constant in cows fed hay, but increased in cows fed straw and decreased in cows fed a fermentation-resistant glucose product. The concentration of free fatty acids and triglycerides were significantly higher in cows fed a fermentation-resistant glucose product than in cows fed hay. In the present study, a single administration of 300 g of fermentation-resistant glucose did not affect the concentration of blood glucose. Therefore, despite ongoing promotion of such products, there is no indication at this time that administration of fermentation-resistant glucose to cows at the start of lactation results in an increase in blood glucose concentration.

Butyric acid stimulates rumen mucosa development in the calf mainly by a reduction of apoptosis

In ruminants the stimulation of papillar growth by butyric acid is well described but effects on mitosis and apoptosis are not known. To clarify the effect of short chain fatty acids three groups of three calves received a basic ration of 100 g hay per day for 6 weeks and additionally milk replacer. From these, two groups were fed with increasing amounts of the salts of either propionic acid (53 to 390 g) or butyric acid up to (54 to 326 g). The control group instead received an additional isocaloric amount of milk replacer. Mitosis was characterized by Ki67 immunoreactivity, apoptosis by a modified TUNEL assay and by electron microscopy. The feeding regimes led to significant differences of papillar length, increasing from 1.0 mm (controls) to 2.2 mm (propionic acid) and 4 mm (butyric acid). This enlargement was partly explained by an increased mitotic rate for the two fatty acid groups. The difference between the fatty acid groups was mainly explained by different apoptotic rates which were only one third for butyric acid compared to propionic acid (P < 0.001). In conclusion, butyric acid is a specific inhibitor of ruminal apoptosis in vivo.

Absorption and metabolism of short-chain fatty acids in ruminants

Short-chain fatty acids (SCFA), viz. acetate, propionate and butyrate are quantitatively important substrates in ruminant energy metabolism. In the reviewed literature, 16 44% of ME intake was recovered as portal appearance of SCFA. This is considerably lower than expected when related to the estimated intragastric flux of SCFA. The discrepancy is caused by portal drained viscera metabolism of arterially abundant metabolites e.g., acetate and the metabolism of acetate and butyrate to acetoacetate and D-3-hydroxybutyrate in the absorptive epithelia. Even though considerable variations between experiments on acetate and propionate appearance are found, there seems to be a great deal of evidence that the proportion of gastrointestinally produced acetate and propionate absorbed to the portal blood is 50-75%. The portal recovery of butyrate has been found to be between 10 and 36% dependent on intraruminal infusion rate. It is concluded that major parts of acetate and propionate are directly absorbed to the portal vein. The true absorption rate of acetate can only be estimated by taking the portal drained viscera metabolism of arterial acetate into account. Butyrate is generally found to have a low recovery in the portal vein, but the production of D-3-hydroxybutyrate seems to be underestimated in major parts of the literature. It is therefore necessary to measure portal appearance as well as portal drained viscera metabolism to assess the quantitative as well as the qualitative contribution of SCFA and SCFA metabolites to whole animal metabolism.

Absorption and delivery of nutrients for milk protein synthesis by portal-drained viscera

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.

Site of digestion of starch in the gastrointestinal tract of dairy cows and its effect on milk yield and composition

Physical and chemical processing of feed ingredients and feeding management strategies are major instruments of manipulating amount and site of starch digestion in the gastrointestinal tract. Generally, as rumen escape of starch increases, postruminal starch digestion increases, and there does not appear to be a limitation to intestinal starch digestion. However, the efficiency with which postruminal starch is digested decreases, which represents a limitation that warrants investigation. Even though digestible dietary starch is presented to the intestine, there is no net glucose absorption at the portal vein, and plasma glucose levels remain relatively unaffected. This result may be associated with the large metabolic requirement for postruminally absorbed glucose, which is preferentially used for oxidative metabolism at the visceral tissue level. In addition, peripheral glucose concentration is highly regulated. A possible implication is that the exogenous glucose supply may spare endogenously synthesized glucose for gut metabolism, allowing more to be directed to the mammary gland. Amino acids also may be spared (less metabolism of dietary and tissue amino acids in the gut). Current production studies yield no clear evidence as to the benefits of postruminal digestion of starch to enhance milk yield or to change its composition. However, studies suggest that starch digested postruminally is used more efficiently for milk synthesis than that digested in the rumen.

Net metabolism of volatile fatty acids, D-beta-hydroxybutyrate, nonesterifield fatty acids, and blood gasses by portal-drained viscera and liver of lactating Holstein cows

Net flux of VFA, D-beta-hydroxybutyrate, nonesterified fatty acids, and blood gasses across portal-drained viscera and liver was measured in four lactating Holstein cows fed a 60:40 corn silage: concentrate diet ad libitum and milked at 12-h intervals. Twelve consecutive hourly measurements of net flux (venous-arterial concentration difference times blood flow) were obtained during wk 4 and 8 postpartum for each cow. Milk yield and DM intake averaged 32.2 and 15.6 kg/d. On a net basis, hepatic tissues produced acetate and removed 63 to 101% of other VFA absorbed by portal-drained viscera. Hepatic and portal-drained visceral tissues produced 60 and 40%, respectively, of D-beta-hydroxybutyrate produced by splanchnic tissues. Hepatic tissues removed 9.3% of nonesterified fatty acids in portal vein and hepatic arterial blood. Oxygen use was greater by liver than for portal-drained viscera (3062 vs. 2394 mmol/h). Net portal-drained visceral flux of VFA, D-beta-hydroxybutyrate, alpha-amino nitrogen, L-lactate, and oxygen together accounted for 84.9% of calculated metabolizable energy intake. Net hepatic removal of propionate, L-lactate, and alpha-amino nitrogen maximally accounted for 55.1, 17.4, and 16.5% of carbon in glucose produced by hepatic tissues.

Net portal-drained visceral and hepatic metabolism of glucose, L-lactate, and nitrogenous compounds in lactating holstein cows

Net portal-drained visceral and hepatic flux of glucose, L-lactate, alpha-amino N, NH3N, urea N, glutamate, and glutamine were measured in four Holstein cows. Cows were fed a 60:40 corn silage: concentrate diet ad libitum and milked at 12-h intervals. Six to 16 d postpartum chronic catheters were established in hepatic portal, hepatic, and mesenteric veins and a carotid artery was elevated. Twelve Measurements of net flux, the mathematical product of blood flow (measured by p-aminohippurate dilution) and venous-arterial concentration difference, were obtained for each cow at hourly intervals during 1 d of wk 4 and 8 postpartum. Dry matter, N, and energy digestion trials began 1 to 2 d after blood sampling. Dry matter intake and milk yield averaged 15.6 and 32.2 kg/d. Portal-drained visceral blood flow averaged 80% of hepatic blood flow (2041 L/h). Net flux of NH3N, urea N, and alpha-amino N across portal-drained viscera represented 68, 54, and 51% of N apparently digested. There was net use of glucose by portal-drained viscera. Hepatic glucose production (3.1 kg/d) exceeded calculated mammary glucose requirements. Net hepatic removal of L-lactate, alpha-amino N, and NH3N represented 115, 43, and 101%, respectively, of their net absorption by portal-drained viscera. Net hepatic L-lactate and alpha-amino N removal could account maximally for 17.4 and 16.5% of glucose produced.

Net portal absorption of volatile fatty acids and L(+)-lactate by lactating Holstein cows

Net absorption of L-lactate and VFA from the portal drained viscera of first lactation Holstein cows was measured at 4, 8, (four cows), 12, 16, and 20 wk (two cows) of lactation. Chronic indwelling catheters were installed 7 to 14 d postpartum in appropriate vessels to measure blood flow and net nutrient absorption. Cows were fed a completely mixed, 60:40 (dry basis) corn silage:supplement diet and milked every 12 h. Average metabolizable energy intake was 2.8X maintenance and mean milk production was 24.0 kg. Net absorption of lactate, any of the VFA, or their total was not affected by week postpartum. Net absorption of L-lactate, VFA, and alpha-amino N accounted in sum for 53.6% of metabolizable energy intake; contributions of each component to energy in absorbed nutrients were acetate and propionate, 29.5% each; alpha-amino N, 23.4%; L-lactate, 5.4%; n-butyrate, 5.3%; 2-methylbutyrate, 2.8%, and i-butyrate, i-valerate, and n-valerate, 1.2 to 1.6% each. Comparison of paired samples of blood and plasma showed that blood cells contribute to the transport of acetate, propionate, i-butyrate, and 2-methyl-butyrate but not of n-butyrate, i-valerate, or n-valerate.

Effects of dietary protein on milk, rumen, and blood parameters in dairy cattle fed low fiber diets

Eighteen multiparous and 9 primiparous Holstein cows were used to determine the effects of a 13 and 23% crude protein concentrate on milk fat depression during early lactation. Beginning on d 22 postpartum, cows were fed a high fiber diet (27% acid detergent fiber) for 3 wk and then switched to a low fiber diet (9 to 10% acid detergent fiber) for 6 wk. Crude protein percentages calculated from dry matter consumption were 13.5 and 17.9% during the high fiber period and 12.7 and 22.3% during the low fiber period. Daily milk and fat yields for both primiparous and multiparous cows were greater for the high protein treatment. The magnitude of decline in milk fat percentage (from high to low fiber) was greater for the low protein treatment, as determined by nonlinear regression. The high protein treatment was more effective in reducing the severity of milk fat depression in primiparous cows than in multiparous cows. Dietary crude protein had no effect on milk protein or solids-not-fat percentages, rumen volatile fatty acid molar proportions, or serum acetate concentration. The mechanism by which the high protein ration minimized the fat depression response to low fiber rations by primiparous cows is unknown.

Influence of dietary energy intake and substrate addition on the in vitro metabolism of glucose and glutamine in rumen epithelial tissue

Ten Holstein steers were fed either 14.2 or 26.2 Mcal ME for 28 days prior to investigating the effect of dietary energy on epithelial metabolism. Rumen papillae were incubated in vitro with glucose (5 mM) or glutamine (1 mM) as well as additional energy substrates. Increased dietary intake increased production of 14CO2 from glucose and glutamine, increased uptake and net lactate production from glucose, and decreased net glutamate and alanine production from glutamine. At these substrate concentrations, rates of glucose oxidation to 14CO2 were sevenfold higher than glutamine.

Net absorption of glucose and nitrogenous compounds by lactating Holstein cows

Net absorptions from portal-drained viscera of glucose, ammonia nitrogen, alpha-amino nitrogen, and urea nitrogen were measured in four Holstein cows in first lactations. Cows were fed for ad libitum consumption a completely mixed, 60:40 corn silage:supplement diet (dry basis) in two equal feedings at 12-h intervals daily. Portal blood flow (dye dilution) and net absorption (portal-arterial difference times portal blood flow) were measured at hourly intervals for 12 h for all cows 4 and 8 wk postpartum and for two cows 4, 8, 12, 16, and 20 wk postpartum. Apparent digestibilities of dry matter and nitrogen were measured immediately after net absorption. Milk yield and composition were measured with absorption and digestion. Mean daily dry matter intake and milk yield were 14.1 +/- .6 and 23.8 +/- 1.8 kg. Portal blood flow was not affected by time of day or weeks postpartum; mean was 1371 +/- 123 liters/h. Net absorption of glucose was negative, indicating no uptake of glucose from dietary sources. Means (g/day) for nitrogenous sources were: nitrogen intake 387, fecal nitrogen 121, urinary nitrogen 146, milk nitrogen 116, net ammonia nitrogen absorption 149, net alpha-amino nitrogen absorption 137, and net loss of urea nitrogen from blood to the gut 93. Net absorption of amino nitrogen and alpha-amino nitrogen in blood was greater than in plasma; corresponding losses of urea nitrogen and glucose to the gut were greater from blood than plasma.