Review

Energy metabolism in the digestive tract and liver ofcattle: influence of physiological state and nutrition

*

GB Huntington

US Department of Agriculture, Agricultural Research Service, Beltsville, MD 20705, USA

(Received 10 June 1989; accepted 17 October 1989)

Summary ― Major functions of portal-drained viscera (PDV) and liver of cattle include absorptionof digestion products and modification of the body’s supply of intermediary metabolites. The dispro-portionately high metabolic rate of PDV and liver (7-13% of body tissues) is exemplified by their oxy-gen uptake (40-50 % of whole body). Extensive metabolism of glucose, volatile fatty acids and ami-no acids by PDV modulates nutrient supply from the diet such that most responses to diet orphysiological state are a function of level of diet intake. Similarly, blood flow through PDV is highlycorrelated with energy intake across a range of body weight, physiological state or diet composition.Most common dietary responses in metabolite uptake by PDV are changes in uptake of ammoniaand volatile fatty acids, which emphasize the strong energy: nitrogen interrelationship in the rumenand subsequently the rest of the body. The liver (tissue in series with PDV) removes glucose precur-sors and ammonia from its blood supply as part of its functions in gluconeogenesis, ammonia detoxi-fication and urea synthesis. The liver also alters amounts and proportions of amino acids supplied byPDV. Accountable percentages of metabolizable energy from net PDV supply include: organic ac-ids, 41-59 %; amino acids, 5-1 %; and heat energy (from oxygen uptake), 11-22 %.

cattle / gastrointestinal tract / Over / absorption / metabolism

Résumé ― Métabolisme énergétique au niveau du tractus digestif et du foie chez les bovins :influence du stade physiologique et de l’état nutritionnel. La digestion et le métabolisme des nu-triments fournis à l’organisme comptent parmi les principales fonctions du tractus digestif (plus lepancréas, la rate et tissu adipeux mésentérique) et du foie. L activité métabolique intense du tractusdigestif et du foie (qui ne représentent que 7 à 13 % du poids corporel) se traduit par leur consom-mation élevée d’oxygène (40 à 50 % de celle de l’organisme entier). Le métabolisme intense du glu-cose, des acides gras volatils et des acides aminés dans le tractus digestif et le foie module l’apportde nutriments à tel point que la plupart des réponses aux modifications de la composition du régimeet du stade physiologique de l’animal dépendent essentiellement du niveau d’alimentation. Le fluxsanguin à travers le tractus digestif est également étroitement corrélé avec la consommationd’énergie pour une grande gamme de poids vifs, d’états physiologiques et de compositions des ré-gimes. Les variations d’origine alimentaire des prélèvements de nutriments par le tractus digestifconcernent généralement l’ammoniaque et les acides gras volatils, ce qui souligne les relationsétroites azote-énergie dans le rumen et, par suite, dans l’ensemble de l’organisme. Le foie (organeplacé en série après le tractus digestif) prélève les précurseurs du glucose et l’ammoniaque dusang porte pour la néoglucogenèse, la détoxication de l’ammoniaque et la synthèse d’urée. Le foiemodifie également les quantités et les proportions d’acides aminés absorbés. L’ensemble desacides organiques absorbés représente de 41 à 59 % de l’énergie métabolisable, les acides aminésde 5 à 13 % et l’énergie dissipée sous forme de chaleur (déterminée à partir de la consommationd’oxygène du tractus digestif) de 11 à 22 %.

bovin / tractus digestif / foie /absorption l métabolisme

* Presented at the 5th Conference on Nutrition and Feeding of Herbivores, INRA, Paris, 16-17 7March, 1989

INTRODUCTION

Domesticated ruminants are perhaps themost versatile omnivores in the world, ob-taining nutrition from forages, non-proteinnitrogen (N) sources, cereal grains, le-

gumes and feedstuffs of animal origin.Concomitantly, domesticated ruminantsuse their diets with a wide range of partialefficiencies. For example, use ofmetabolizable energy (ME) for growth canbe as low as 0.21 for forages and as highas 0.65 for energetically dense diets [ARC(Agricultural Research Council, 1980)].The ME not retained as tissue or releasedas milk is lost as heat energy (HE).

In most animals, including domesticatedruminants, the portal-drained viscera

(PDV) is the interface between the dietand the organism. The liver, tissue in se-ries with the PDV, is the central metabolicintersection between PDV and the rest ofthe body. Major functions of the splanchnictissues (PDV and liver) are digestion andabsorption of dietary nutrients, supply of aplethora of hormones and immune re-

sponse. This review will focus on digestiveand absorptive functions of cattle. Theterms &dquo;gut&dquo; and &dquo;PDV&dquo; will be used syn-onymously, although PDV includes the

pancreas, spleen and fat associated withthe gut.

Metabolic function and therefore energymetabolism of splanchnic tissues re-

sponds to a variety of environmental stim-uli, including heat (McGuire et al, 1989),cold (Sasaki and Weekes, 1986), fastingor level of feed intake (Lomax and Baird,1983; Ferrell, 1988), diet composition andproductive (physiological) state. The objec-tive of this review is to describe effects ofthe latter two stimuli on function and en-

ergy metabolism of PDV and liver of beefand dairy cattle. Data used were derivedmainly from use of one approach, in vivofluxes across PDV and liver of multi-

catheterized cattle (Huntington et al,1989). Discussion of physiological state

will center on growth or lactation.

PORTAL BLOOD FLOW AND ME IN-TAKE

There is a positive direct relationship be-tween blood flow through PDV and ME in-take in cattle and sheep. Linear (Hunting-ton, 1984a; Weighart et al, 1986) or

curvilinear (Webster et al, 1975; Lomaxand Baird, 1983) regressions of portalblood flow on energy intake likely apply toliver blood flow as well, because liverblood flow derives predominantly fromPDV. Figure 1 is a summary of 16 treat-ment means from studies with cattle fromwhich estimates of average hourly portalblood flow are available, and treatmentswere intakes within diets. The character-istics of the cattle ranged from fasted beefsteers (200 kg) to lactating dairy cows (650kg) eating ME equal to 3 times their main-tenance requirement. Diets ranged fromforage to high concentrate diets. One canregress with high r2 linear or curvilinear re-

lationships among the data points (fig 1),with the bulk of the data between 40-100MJ/d ME intake. Comparison of 11 portalblood flows from studies not used to gener-ate these regressions (Huntington et al,1981; Eisemann and Huntington, 1987;Harmon and Avery, 1987; Gross et al,1988; Harmon et al, 1988; Huntington,1989) showed good predictability. Mean ±SE of predicted divided by observed portalblood flow was 1.00 ± 0.04 for the expo-nential fit and 1.07 ± 0.04 for the linear fit

(fig 1 The ME intakes in the studies usedin the comparison ranged from 40-81 MJ/d.

Daily ME intake is highly correlated withother factors that may influence portal (orhepatic) blood flow, including breed, live

weight (or some exponent thereof) and en-ergy density of the diet. However, direct(Huntington, 1984a) or implied (Weighartet al, 1986) evalution of these factors didnot improve r2. From a more physiologicalperspective, portal blood flow ranges from25-52 mlomin-1okg-1 body weight with amedian value of = 40 in cattle (Carr and Ja-cobson, 1968; Wangness and McGilliard,1972; Harmon and Avery, 1987; Durand etal, 1988; Reynolds et al, 1988).

Teleologically, portal and hepatic bloodflow increase in response to increased MEintake to transport digested nutrients fromthe gut, through the liver, and on to therest of the body. Heart rate and therebycardiac output likewise increase with in-creased intake (Rumsey et al, 1980).Whether or not productive state or dietarycomposition directly alter the proportion ofcardiac output flowing through splanchnictissues remains to be determined. A pre-

liminary report with beef steers (Huntingtonet al, 1988) indicates that these propor-tions are similar in fed and fasted steers,but change in acute response to a p-adrenergic agonist.

OXYGEN UPTAKE BY SPLANCHNICTISSUES

In vivo measurements of oxygen uptake byPDV and liver of cattle provide estimatesof HE attributable to those tissues (tableI). Over a wide range of body weights andproductive states the PDV accounts for

18-25 % and the liver 17-25 % of whole

body oxygen uptake, or energy lost as HE.Similar or appreciably higher proportions ofwhole body oxygen uptake by splanchnictissues have been reported for sheep(Thompson et al, 1978; Burrin et al, 1989).The PDV are 6.4-10 % of body tissues,

and the liver 1-3 % (Smith and Baldwin,1974; Doreau et al, 1985; Jones et al,1985). Therefore, splanchnic tissues ac-count for 35-50 % of HE which is a dispro-portionately high rate of oxidative metabo-lism relative to their contribution to bodymass. Because these proportions or

amounts are substantial, they are at-

tractive targets for ways to improve overallenergetic efficiency of cattle.Absolute rates predictably increase with

lactation (table I) or with increased intake(Webster et al, 1975; Ferrell, 1988; Rey-nolds et al, 1986 Huntington et al, 1988).Diet-related responses in amounts or pro-portions of oxygen uptake by PDV ofsteers fed legume or grass silage(Huntington et al, 1988) eluded statisticalsignificance; however, increased intake ofconcentrate by beef heifers decreased ox-ygen uptake by PDV (Reynolds and

Tyrrell, 1989).

NET NUTRIENT FLUX ACROSS PDVAND LIVER

Glucose

In the preponderance of reported studiesencompassing a wide range of diets andintakes there is little if any net glucose ab-sorption from dietary sources (see reviewby Huntington and Reynolds, 1987). Thiswould be expected from consumption of

forage diets, but also appears to be thecase in cattle consuming substantialamounts of starch. For example, dairycows consuming about 5 kg of starch ascorn silage per day had net use of glucose(59 mmol/h) by PDV (Reynolds et al,1988). Post-ruminal infusion of glucose orstarch in a nonlactating cow, beef heifersand dairy steers (Huntington and

Reynolds, 1986; Kreikemeier et al, 1987)

showed that about two-thirds of the glu-cose infused and one-third of the starch in-fused could be accounted for by increasednet absorption of glucose. Ostensibly, therest is used by PDV tissues or further me-tabolized in the lumen of the intestine. Thisis suggested by studies with dairy cows(Pehrson et al, 1981) and beef steers (Tur-geon et al, 1983) which indicate the maxi-mal capacity for starch disappearancefrom the intestine was not exceeded in theinfusion studies cited.

Recent studies with steers (table II) helpexplain how substantial passage of glu-cose (or a-linked glucose polymers) fromthe rumen does not result in net glucoseabsorption. Partition of net glucose fluxacross stomach and post-stomach siteswithin PDV showed net use of glucose byboth sites when the diet was alfalfa hay.When high-concentrate (corn) diet was

fed, however, net use of glucose by stom-ach tissues increased, and net absorptionof glucose by post-stomach tissues wasmeasured, ostensibly in response to starchappearing in the small intestine (table II).Janes et al (1984) reported a similar re-sponse in post-stomach glucose absorp-tion when the diet of sheep was changedfrom forage to high-concentrate. In vitrostudies of rumen mucosa from cattle fed

roughage or a high-concentrate diet con-firm increased glucose uptake by mucosain response to the high-concentrate dietwith concomitant increases in oxidation of

glucose to C02 and formation of lactate

(Harmon, 1986).Ruminants are eminently capable of glu-

coneogenesis to meet their metabolicneeds (Young, 1977). Net hepatic glucoseproduction (3.1 kg/d) of lactating cows pre-viously cited as an example of net glucoseuse by PDV (Reynolds et al, 1988) wasable to meet glucose required for their milklactose synthesis, leaving 0.8 kg/d to meetother glucose requirements. Weighart et al(1986) made similar calculations for lactat-ing cows. In fed cattle propionate is the

major glucose precursor, followed by lac-tate and amino acids (table 111). Studies ofnonlactating and lactating cows (Baird etal, 1980) show how change in physiologi-cal state affects priority of uptake of glu-cose precursors by the liver to ensure aninverse relationship between glucose avail-ability or propionate supply and use of en-dogenous precursors by the gut and liver.Fasting decreased gluconeogenesis andcaused shifts in the sources of carbon from

exogenous sources (propionate) to endog-enous sources (lactate, amino acids andglycerol).

VOLATILE FATTY ACIDS, LACTATEAND KETONES

Volatile fatty acids (VFA) are the predomi-nant source of energy absorbed from die-

tary sources. As do other tissues, the PDVuse VFA as energy sources, which meansthe rates and pattern of VFA production inthe rumen are not the same as their ratesand pattern of absorption. Several studieswith sheep and cattle (Bergman and Wolff,1971; Pethick et al, 1981; Huntington et al,1983; Seal et al, 1989) show that = 33 %of the acetate and 50-80 % of the propion-ate produced in the rumen are metabo-lized by PDV. The liver further alters thedietary supply by removing propionate and4- and 5-carbon VFA from portal blood,and adding acetate from endogenous pro-duction (Lomax and Baird, 1983; Hunting-ton and Eisemann, 1988; Reynolds et al,1988).

There is net production of lactate byPDV; L-lactate predominates, but D-lactateis absorbed by cattle fed high grain diets(Huntington et al, 1981; Harmon et al,1985). As discussed previously, L-lactate isused by the liver for gluconeogenesis(Huntington et al, 1981); D-lactate is oxi-dized (Harmon et al, 1983; Giesecke andStangassinger, 1978, 1979).

The ketones [3-hydroxybutyrate and to alesser extent acetoacetate are producedby PDV. About 90 % of butyrate producedin the rumen is oxidized by PDV (Bergmanand Wolff, 1971) and (3-hydroxybutyrate isa major product of that metabolism. Theliver also produces (3-hydroxybutyrate(Heitmann et al, 1987; Reynolds et al,1988). Net uptake of n-butyrate and non-esterified fatty acids by the liver of lactatingcows accounts maximally for 76-83 % ofP-hydroxybutyrate release (Lomax and

Baird, 1983; Reynolds et al, 1988). Fastingcauses the PDV to shift from production to

net use of ketones (Lomax and Baird,1983).

AMINO ACIDS

Like glucose and VFA, the PDV uses ami-no acids from dietary and endogenoussources (Tagari and Bergman, 1978). Forexample, the PDV uses more glutamateand glutamine than is available from die-

tary sources (Harmon and Avery, 1987;Reynolds and Huntington, 1988a). Gluta-mate and glutamine are oxidized, and ami-no groups are transmitted to form alanine,serine and glycine (Bergman and Pell,1985). In general, use of amino acids byPDV is related to the high rate of proteinsynthesis in PDV (Lobley et al, 1980).This use of amino acids by PDV explainsat least in part why it has been difficult toshow effects of diet on either rates or pro-portions of net absorption of amino acidsor a-amino N (Prior et al, 1981; Hunting-ton, 1987; Huntington et al, 1988).

The liver removes amino acids from por-tal supply in amounts that vary among indi-vidual acids, thereby further modulatingthe rates and proportions of amino acidsavailable for the rest of the body (Baird etal, 1975; Bergman and Pell, 1985; Hunt-ington and Eisemann, 1988). The liver is amajor participant in N shuttles among vari-ous tissues that involve alanine, glycine,glutamate, glutamine, arginine, ornithineand citrulline (Bergman and Pell, 1985).The livers of lactating cows removed 43 %of net PDV supply of a-amino N (Reynoldset al, 1988) and the livers of growing beefsteers removed 24 % (Huntington andEisemann, 1988). Liver removal of a-

amino N decreased in steers changedfrom hay to a high-grain diet, resulting in

greater splanchnic release with constantPDV absorption (Huntington, 1989). Gueri-no et al (1988) increased PDV absorption

of a-amino N abomasal infusion of caseininto steers fed a high-grain diet, but liverremoval increased correspondingly and

splanchnic release of a-amino did not

change.

AMMONIA AND UREA

Ammonia and urea are significant compo-nents in overall N metabolism of cattle (ta-ble IV). Over a variety of diets, N digestibil-ity ranged from 61-72 % of N intake. UrineN and retained N varied with intake and

productive state. Net PDV production ofammonia N ranged from 16―95 % of N in-take and was directly related to N intake.Net removal of urea N by PDV (transferfrom blood to the lumen of the gut) rangedfrom 15-37 % of N intake, and net produc-tion of a-amino N ranged from 18-36 % ofN intake. With the exception of beef heifersfed the high-concentrate diet (table IV), netabsorption of ammonia N was equal to orgreater than net absorption of a-amino N.Net transfer of urea N and net absorptionof a-amino are generally similar.

Diet composition affected site of urea Nflux across PDV of steers (table V). Insteers fed hay, urea was transferred pre-dominantly to the post-stomach. When thesame steers were fed a high-concentratediet, urea flux shifted to the stomach (ru-men). Earlier work with 15N similarlyshowed transfer of urea N was predomi-nantly to the lower gut of sheep fed forage(Nolan and Leng, 1972). Bunting et al

(1989) used radioisotopes to show in-creased protein intake of calves increasedproduction of ammonia in the rumen, in-creased urea synthesis, and doubled thepercentage of total gastrointestinal urea

degradation occurring in the rumen. Net

production of ammonia N in steers fedtwice daily was 2.7: 1 stomach: post-stomach for the hay diet and about 1:1 for

the high-concentrate diet (measurementsmade at meal time). In steers fed 12meals/d net production of ammonia N wasabout 2:1 stomach: post-stomach on bothdiets (table V), suggesting that about 2/3 ofabsorbed ammonia N emanated from rumi-nal fermentation and tissue metabolism,and one-third from metabolism of N sourc-es in the lower gut.

The liver receives directly the ammoniaN produced by PDV and essentially re-

moves all of it from blood (Huntington andEisemann, 1988; Reynolds et al, 1988;Huntington, 1989). Net liver removal canaccount maximally for 70 to 80 % of ureaN released (Huntington and Eisemann,1988; Reynolds et al, 1988; Huntington,1989). The capacity of a healthy liver to re-move ammonia is not exceeded with nor-mal diets (Symonds et aI, 1981; Orzechow-ski et al, 1987), even when PDV

production is substantial.

ENERGETIC SUMMATION OF PDV ANDLIVER METABOLISM

Baird et al (1975) first published a compre-hensive summation of energy flux by PDVof a lactating cow. Energy absorbed asVFA, lactate and ketones, and amino acidssummed to 135 MJ/d, which was 84 % ofcalculated ME intake. Further studies withsteers and dairy cows include HE from ox-ygen uptake by PDV (table VI). In the

steers, energy from net absorption plus HEaccounted for 75-91 % of measured MEintake of legume or grass silage. Not all

energy sources were measured in all stud-ies with dairy cows, but in lactating cowsnutrient absorption plus HE accounted for85% of ME intake. In both steers and cowsthe largest single source of energy was ac-etate followed by propionate, except forfirst lactation cows in which the order wasreversed. The third largest source (again

except for first lactation cows) was HEfrom oxygen uptake and fourth was energyfrom amino acids. Other VFA, L-lactateand (3-hydroxybutyrate supplied lesseramounts of energy. L-lactate plus VFA ac-counted for 41-57 % of ME intake (tableVI). A similar percentage of ME intake wasattributable to L-lactate and VFA in beefheifers fed a high-concentrate diet (46%;Huntington and Prior, 1983). Sources ofME not accounted for in table VI includeheat of fermentation, absorption of long-chain fatty acids, and absorption of othernitrogenous compounds such as nucleicacids and peptides (Webb, 1986).

Dietary effects on sources of energy ab-sorbed or HE by PDV were slight in thesteers. Steers fed alfalfa silage absorbedmore energy as branched-chain VFA andn-valerate than steers fed grass silage (ta-ble VI). Comparison of differences be-tween intake within silages suggests somedietary effects; compared to the grass si-lage, the alfalfa silage caused a greater in-crement as acetate (40 vs 30% of the in-crement) and a lesser increment as HE

(15 vs 26 % of the increment) (Huntingtonet al, 1988). This may explain in part someof the metabolism behind greater efficien-cy of energy use by ruminants fed le-

gumes compared to grasses (Rattray andJoyce, 1970; Tyrrell et al, 1982; Thompsonet al, 1985; Varga et al, 1987).

Nonlactating and lactating cows werefed the same diet, but at different intakesto support production (table VI). Comparedto non-lactating cows, lactating cows ab-sorbed a greater percentage of ME intakeas propionate (16 vs 10%). The percent-age of ME absorbed as acetate was less

by cows in first lactation (16%) than drycows (20%) or older lactating cows (22%).

CONCLUSIONS

Five major points can be derived from in-formation in this review. First, blood flowthrought PDV of cattle is highly and posi-tively correlated with their ME intake,which provides a transport for increasedabsorption of nutrients. Second, the PDVand liver are metabolically active at ratesdisproportionately greater than their contri-bution to body tissue mass; together, theycan account for one-half of HE. Third, alto-ugh cattle derive little if any glucose direct-ly from dietary sources, they are metaboli-cally designed and tuned to synthesizeglucose, which provides a use for propion-ate and lactate absorbed by PDV. Fourth,nonprotein N sources are significant partic-ipants in overall N metabolism which is or-chestrated by the liver. Fifth, dietary or

physiological effects on nutrient absorptionand liver metabolism are most evident for

VFA, ammonia and urea, which emphasiz-es the close and perhaps obligate interre-lation between energy and N metabolismin cattle.

These points show the central role gutand liver tissues play in energy metabo-lism, both by regulating and modulatingdietary energy sources and by virtue of

their high energy expenditures. It followsthen that a small change in mode or effi-ciency of splanchnic metabolism couldhave a major effect on the whole animal’sresponse to physiological state or nutrition.

REFERENCES

Agricultural Research Council (1980) Nutrient

Requirements of Livestock. CommonwealthAgricultural Bureaux, UK

Baird GD, Lomax MA, Symonds HW, Shaw SR(1980) Net hepatic and splanchnic metabo-lism of lactate, pyruvate and propionate in

dairy cows in viw in relation to lactation andnutrient supply. Biochem J 186, 47-57

Baird GD, Symonds HW, Ash R (1975) Someobservations on metabolite production andutilization in vivo by the gut and liver of adultdairy cows. J Agric Sci (Camb) 85, 281-296

Bergmann EN, Pell JM (1985) Integration ofamino acid metabolism in the ruminant. In:Herbivore Nutrition in the Subtropics and Tro-pics (Gilchrist FMC, Mackie RI, eds) TheScience Press, Johannesburg, S Africa, 613-628

Bergman EN, Wolff JE (1971) Metabolism of vo-latile acids by liver and portal-drained visce-ra in sheep. Am J Physiol221, 586-592

Bunting LD, Boling JA, MacKown CT (1989) Ef-fect of protein level on nitrogen metabolism inthe growing bovine. I. Nitrogen recycling andintestinal protein supply in calves. J Anim

Sci 67, 810-819 9

Burrin DG, Ferrell CL, Britton RA (1989) Effectof feed intake of lambs on visceral organgrowth and metabolism. In: Proc llth EAAPSymposium on Energy Metabolism of FarmAnimals, Lunteren, Netherlands (Van Der Ho-ning Y, Close WH, eds), Eur Assoc AnimProd Publ No. 43, 103-106

Carr SB, Jacobson DR (1968) Method for mea-surement of gastrointestinal absorption innormal animals, combining portal-carotid dif-ference and telemetered portal flow by Dop-pler shift. J Dairy Sci 51, 721-729

Doreau M, Robelin J, Lestrade A (1985) Effectsof physiological state and body fatness on di-gestive-tract weight and composition in the

dairy cow. Livestock Prod Sci 12, 379-385Durand DD, Bauchart, LeFaivre J, Donnat JP

(1988) Method for continuous measurementof blood metabolite hepatic balance in cons-cious preruminant calves. J Dairy Sci 71,1632-1637

Eisemann JH, Huntington GB (1987) Metabo-lism in two tissue beds of steers and effectsof clenbuterol. J Anim Sci 65 (suppl. 1 ), 250(abstr)

Ferrrel CL (1988) Contribution of visceral organsto animal energy expenditures. J Anim Sci(suppl 3), 23-34

Giesecke D, Stangassinger M (1978) Untersu-chungen zur Genese und Biochemie derPansenacidose. V. Kinetic der eliminationvon D(-) Milchsaure aus dem Blut. Zbl VetMed A25, 327-337

Giesecke D, Stangassinger M (1979) Untersu-chungen zur Genese und Biochemie derPansenacidose. VII. Oxydationsrate und

quantitative Gluconeogenesis aus d-Lactat-(!4C) bei Ziegen. Zbl VetMedA26, 85-94

Gross KL, Harmon DL, Avery TB (1988) Net por-tal nutrient flux in steers fed diets containingwheat and sorghum grain alone or in combi-nation. J Anim Sci 66, 543-551

Guerino F, Huntington G, Erdman R (1988) Netportal and hepatic flux of nitrogenous com-pounds by beef steers abomasally infusedwith casein. J Anim Sci 66 (suppl 1), 486(abstr)

Harmon DL (1986) Influence of dietary energyintake and substrate addition on the in vitrometabolism of glucose and glutamine inrumen epithelial tissue. Comp Biochem Phy-sio185B, 643-647

Harmon DL, Avery TB (1987) Effects of dietarymonensin and sodium propionate on net nu-trient flux in steers fed a high-concentratediet. J Anim Sci 65, 1610-1616 6

Harmon DL, Avery TB, Huntington GB, Rey-nolds PJ (1988) Influence of ionophore addi-tion to roughage and high-concentrate dietson portal blood flow and net nutrient in cattle.Can J Anim Sci 68, 419-429

Harmon DL, Britton RA, Prior RL (1983) In-fluence of diet on glucose turnover and ratesof gluconeogenesis, oxidation and turnoverof D(-)-lactate in the bovine. J Nutr 113,1842-1850

Harmon DL, Britton RA, Prior RL, Stock RA(1985) Net portal absorption of lactate andvolatile fatty acids in steers experiencing glu-cose-induced acidosis or fed a 70% concen-trate diet ad libitum. J Anim Sci 60, 500-569

Heitmann RN, Dawes DJ, Sensenig SC (1987)Hepatic ketogenesis and peripheral ketonesbody utilization in the ruminant. J Nutr 117,1174-1180

Huntington G (1984a) Relationship of portalblood flow to metabolizable energy intake ofcattle. Can J Anim Sci 64 (suppl), 16 (abstr)

Huntington GB (1984b) Net absorption of glu-cose and nitrogenous compounds by lacta-ting Holstein cows. J Dairy Sci 67, 1919-1927

Huntington GB (1987) Net absorption from por-tal-drained viscera of nitrogenous com-

pounds by beef heifers fed on diets differingin protein solubility or degradability in therumen. Br J Nutr 57, 109-114 4

Huntington GB (1989) Hepatic urea synthesisand site and rate of urea removal from bloodof beef steers fed alfalfa or a high concen-trate diet. Can J Anim Sci 69, 215-223

Huntington GB, Eisemann JH (1988) Regulationof nutrient supply by gut and liver tissues. JAnim Sci 66 (suppl 3), 35-48

Huntington G, Eisemann J, Whitt J (1988) Pro-portions of whole body blood flow and oxy-gen uptake attributable to gut and liver ofbeef steers. J Anim Sci 66 (suppl 1), 147(abstr)

Huntington GB, Prior RL (1983) Digestion andabsorption of nutrients by beef heifers fed ahigh concentrate diet. J Nutr 113, 2280-2288

Huntington GB, Prior RL (1985) Net absorptionof amino acids by portal-drained viscera andhind half of beef cattle fed a high-concentrate diet. J Anim Sci 60, 1491-1499

Huntington GB, Prior RL, Britton RA (1981) Glu-cose and lactate absorption and metabolicinterrelationships in steers changed from lowto high concentrate diets. J Nutr 111, 1164-1172

Huntington GB, Reynolds CK (1987) Oxygenconsumption and metabolite flux of bovine

portal-drained viscera and liver. J Nutr 117,1167-1173

Huntington GB, Reynolds PJ (1986) Net absorp-tion of glucose, L-lactate, volatile fatty acidsand nitrogenous compounds by bovine givenabomasal infusions of starch or glucose. JDairy Sci 69, 2428-2436

Huntington GB, Reynolds CK, Stroud BH (1989)Techniques for measuring blood flow in

splanchnic tissues of cattle. J Dairy Sci 72,1 583-1 595

Huntington GB, Reynolds PJ, Tyrrell HF (1983)Net absorption and ruminal concentrations ofmetabolites in nonpregnant dry Holsteincows before and after intraruminal acetic in-fusion. J Dairy Sci 66, 1901-1908

Huntington GB, Tyrrell HF (1985) Oxygen cons-umption by portal-drained viscera of cattle:comparison of analytical techniques and rela-tionship to whole body oxygen consumption.J Dairy Sci 68, 2727-2731

Huntington GB, Varga GA, Glenn BP, Waldo DR(1988) Net absorption and oxygen consump-tion by Holstein steers fed alfalfa or orchard-grass silage at equalized intakes. J Anim Sci66, 1292-1302

Janes AN, Parker DS, Weekes TEC, ArmstrongDG (1984) Mesenteric venous blood flow andnet absorption of glucose in sheep fed driedgrass or ground maize-based diets. J AgricSci (Camb) 103, 549-553

Jones SDM, Rompala RE, Jeremiah LE (1985)Growth and composition of the empty body insteers of different maturity types fed concen-trate or forage diets. J Anim Sci 60, 427-433

Kreikemeier KK, Harmon DL, Avery TB, BrandtRT (1987) Starch and glucose utilization inthe small intestine of steers. J Anim Sci 66(suppl 1 ), 469 (abstr)

Lobley GE, Milne V, Lovie J, Reeds PJ, PennieK (1980) Whole body and tissue protein syn-thesis in cattle. BrJNuir43, 491-502

Lomax MA, Baird GD (1983) Blood flow and nu-trient exchange across the liver and gut ofthe dairy cow. Br J Nutr 49, 481-496

McGuire MA, Beede DK, DeLorenzo MA, WilcoxCJ, Huntington GB, Reynolds CK, Collier RJ(1989) Effects of thermal stress and level offeed intake on portal plasma flow and netfluxes or metabolites in lactating Holsteincows. J Anim Sci 67, 1050-1060

Nolan JV, Leng RA (1972) Dynamic aspects ofammonia and urea metabolism in sheep. BrJ JNutr 27, 177-194

Orzechowski A, Motyl T, Pierzynowski G, BareyW (1987) Hepatic capacity for ammonia re-moval in sheep. J Vet Med A34, 108-112 2

Pehrson B, Johnson U, Knutsson M (1981) Thedigestion of starch in the small intestines ofdairy cows. Zbl Vet Med A 28, 241-246

Pethick DW, Lindsay DB, Barker PJ, NorthropAJ (1981) Acetate supply and utilization bythe tissues of sheep in vivo. Br J Nutr 46, 97-109

Prior RL, Huntington GB, Britton RA (1981) In-fluence of diet on amino acid absorption inbeef cattle and sheep. J Nutr 111, 2212-2222

Rattray PV, Joyce JP (1970) The nutritive valueof white clover and perennial ryegrass for

young sheep. I. Nitrogen retention studiesand associated animal performance. NZ JAgric Res 13, 778-791

Reynolds CK, Huntington GB (1988a) Partitonof portal-drained visceral net flux in beefsteers. I. Blood flow and net flux of oxygen,glucose and nitrogenous compounds acrossstomach and post-stomach tissues. Br J Nutr60, 553-562

Reynolds PJ, Huntington GB (1988b) Net portalabsorption of volatile fatty acids and L(+)-lactate by lactating Holstein cows. J Dairy Sci71, 124-133

Reynolds CK, Huntington GB, Tyrrell HF, Rey-nolds PJ (1986) Splanchnic tissue and wholeanimal oxygen consumption by lactating Hol-stein cows. J Dairy Sci 69 (suppl 1), 193(abstr)

Reynolds CK, Huntington GB, Tyrrell HF, Rey-nolds PJ (1988) Net portal-drained visceraland hepatic metabolism of glucose, L-lactateand nitrogenous compounds in lactating Hol-stein cows. J Dairy Sci 71, 1803-1812 2

Reynolds CK, Tyrrell HF (1989) Effects of fo-

rage to concentrate ratio and intake on visce-ral tissue and whole body energy metabolismof growing beef heifers. In: Proc 11 th EAAPSymp, Energy Metabolism of Farm Animals,Lunteren, Netherlands (Van Der Honing Y,Close WH, eds). Eur Assoc Animal Prod No43, 151-154

Rumsey TS, Tyrrell HF, Moe PW (1980) Effectof diethylstilbestrol and synovex-S on fastingmetabolism measurement of beef steers. JAnim Sci 50, 160-165

Sasaki Y, Weekes TEC (1986) Metabolic res-ponses to cold. In: Control of Digestion andMetabolism in Ruminants. Proc 6th Intl

Symp, Ruminant Physiol (Milligan LP, Gro-vum WL, Dobson A, eds) Prentice-Hall, En-glewood Cliffs, NJ, 326-343

Seal CJ, Sarker A, Parker DS (1989) Rumenpropionate production rate and absorption offermentation end-products into the portal veinof forage and forage-concentrate fed cattle.Proc Nutr Soc 48, 143 A (abstr)

Smith NE, Baldwin RL (1974) Effects of breed,pregnancy and lactation on weight of organsand tissues in cattle. J Dairy Sci 50, 1055-1060

Symonds HW, Mather DL, Colles KA (1981) Themaximum capacity of the liver of the adult

dairy cow to metabolize ammonia. Br J Nutr46, 481-486

Tagari H, Bergman EN (1978) Intestinal disap-pearance and portal blood appearance ofamino acids in sheep. J Nutr 108, 790-803

Thompson GE, Maneson W, Clarke PL, Bell AW(1978) Acute cold exposure and the metabo-lism of glucose and some of its precursors inthe liver of the fed and fasted sheep. Quart JExp Physiol63, 189-199

Thomson DJ, Beever DE, Haines MJ, CammellSB, Evans RT, Dhanoa MS, Austin AR

(1985) Yield and composition of milk fromFreisan cows grazing either perennial rye-grass or white clover in early lactation. J

Dairy Res 52, 17-31 1

Tyrrell HF, Thomson DJ, Waldo DR, Goering HK(1982) Energy retention and utilization of

grass and legume by cattle. Proc Nutr Soc41, 23A (abstr)

Turgeon OA Jr, Brink DR, Britton RA (1983)Corn particle size mixtures, roughage leveland starch utilization in finishing steer diets. JAnim Sci 57, 739-749

Varga GA, Tyrrell HF, Waldo DR, HuntingtonGB, Glenn BP (1987) Effect of alfalfa or or-chardgrass silages on energy and nitrogenutilization for growth by Holstein steers. In:

Energy Metabolism of Farm Animals (MoePW, Tyrrell HF, Reynolds PJ, eds) EAAPPubl 32, Rowman & Littlefield, Totowa, NJ,86-89

Wangness PJ, McGilliard AD (1972) Measure-ment of portal blood flow by dye-dilution. JDairy Sci 55, 1439-1446

Webb KE Jr (1986) Amino acid and peptide ab-sorption from the gastrointestinal tract. FedProc 45, 2268-2271

Webster AJF, Osuji PO, White F, Ingram JF(1975) The influence of food intake on portalblood flow and heat production in the diges-tive tract of sheep. Br J Nutr 34, 125-139

Weighart M, Slepetis R, Elliot JM, Smith DF(1986) Glucose absorption and hepatic glu-coneogenesis in dairy cows fed diets varyingin forage content. J Nutr 116, 839-850

Young JW (1977) Gluconeogenesis in cattle: si-gnificance and methodology. J Dairy Sci 60,1-15 5