Energy metabolism in the digestive tract and liver of cattle: influence of physiological state and nutrition, GB Huntington

Tags: Huntington, PDV, Reynolds, ammonia, net absorption, Reynolds PJ, Br J Nutr, Huntington GB, perennial ryegrass, Tyrrell HF, physiological state, white clover, J Nutr, energy metabolism, J Dairy Sci, steers, amino acids, absorption, cattle, diet composition, NZ J Agric Res, OXYGEN UPTAKE, substantial amounts, body tissues, blood flow, animal performance, du, tractus digestif, amino groups, Agricultural Research Service, Holstein cows, VFA, Reynolds CK, le foie
Content: Review Energy metabolism in the digestive tract and liver of cattle: 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 absorption of digestion products and modification of the body's supply of intermediary metabolites. The disproportionately high metabolic rate of PDV and liver (7-13% of body tissues) is exemplified by their oxygen uptake (40-50 % of whole body). Extensive metabolism of glucose, volatile fatty acids and amino acids by PDV modulates nutrient supply from the diet such that most responses to diet or physiological state are a function of level of diet intake. Similarly, blood flow through PDV is highly correlated 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 ammonia and volatile fatty acids, which emphasize the strong energy: nitrogen interrelationship in the rumen and subsequently the rest of the body. The liver (tissue in series with PDV) removes glucose precursors and ammonia from its blood supply as part of its functions in gluconeogenesis, ammonia detoxification and urea synthesis. The liver also alters amounts and proportions of amino acids supplied by PDV. Accountable percentages of metabolizable energy from net PDV supply include: organic acids, 41-59 %; amino acids, 5-1 %; and heat energy (from oxygen uptake), 11-22 %. cattle / gastrointestinal tract / Over / absorption / metabolism Rйsumй ― Mйtabolisme йnergйtique au niveau du tractus digestif et du foie chez les bovins : influence du stade physiologique et de l'йtat nutritionnel. La digestion et le mйtabolisme des nutriments fournis а l'organisme comptent parmi les principales fonctions du tractus digestif (plus le pancrйas, la rate et tissu adipeux mйsentйrique) et du foie. L activitй mйtabolique intense du tractus digestif et du foie (qui ne reprйsentent que 7 а 13 % du poids corporel) se traduit par leur consommation йlevйe d'oxygиne (40 а 50 % de celle de l'organisme entier). Le mйtabolisme intense du glucose, des acides gras volatils et des acides aminйs dans le tractus digestif et le foie module l'apport de nutriments а tel point que la plupart des rйponses aux modifications de la composition du rйgime et du stade physiologique de l'animal dйpendent essentiellement du niveau d'alimentation. Le flux sanguin а travers le tractus digestif est йgalement йtroitement corrйlй avec la consommation d'йnergie pour une grande gamme de poids vifs, d'йtats physiologiques et de compositions des rйgimes. Les variations d'origine alimentaire des prйlиvements de nutriments par le tractus digestif concernent gйnйralement l'ammoniaque et les acides gras volatils, ce qui souligne les relations йtroites azote-йnergie dans le rumen et, par suite, dans l'ensemble de l'organisme. Le foie (organe placй en sйrie aprиs le tractus digestif) prйlиve les prйcurseurs du glucose et l'ammoniaque du sang porte pour la nйoglucogenиse, la dйtoxication de l'ammoniaque et la synthиse d'urйe. Le foie modifie йgalement les quantitйs et les proportions d'acides aminйs absorbйs. L'ensemble des acides organiques absorbйs reprйsente de 41 а 59 % de l'йnergie mйtabolisable, les acides aminйs de 5 а 13 % et l'йnergie dissipйe sous forme de chaleur (dйterminйe а partir de la consommation d'oxygиne du tractus digestif) de 11 а 22 %. bovin / tractus digestif/ foie /absorption l mйtabolisme * Presented at the 5th Conference on Nutrition and Feeding of Herbivores, INRA, Paris, 16-17 March, 1989
INTRODUCTION Domesticated ruminants are perhaps the most versatile omnivores in the world, obtaining nutrition from forages, non-protein nitrogen (N) sources, cereal grains, legumes and feedstuffs of animal origin. Concomitantly, domesticated ruminants use their diets with a wide range of partial efficiencies. For example, use of metabolizable energy (ME) for growth can be as low as 0.21 for forages and as high as 0.65 for energetically dense diets [ARC (Agricultural Research Council, 1980)]. The ME not retained as tissue or released as milk is lost as heat energy (HE). In most animals, including domesticated ruminants, the portal-drained viscera (PDV) is the interface between the diet and the organism. The liver, tissue in series with the PDV, is the central metabolic intersection between PDV and the rest of the body. Major functions of the splanchnic tissues (PDV and liver) are digestion and absorption of dietary nutrients, supply of a plethora of hormones and immune response. This review will focus on digestive and absorptive functions of cattle. The terms &dquo;gut&dquo; and &dquo;PDV&dquo; will be used synonymously, although PDV includes the pancreas, spleen and fat associated with the gut. Metabolic function and therefore energy metabolism of splanchnic tissues responds to a variety of environmental stimuli, including heat (McGuire et al, 1989), cold (Sasaki and Weekes, 1986), fasting or level of feed intake (Lomax and Baird, 1983; Ferrell, 1988), diet composition and productive (physiological) state. The objec- tive of this review is to describe effects of the latter two stimuli on function and en- ergy metabolism of PDV and liver of beef and dairy cattle. Data used were derived mainly from use of one approach, in vivo fluxes 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 INTAKE There is a positive direct relationship between blood flow through PDV and ME intake in cattle and sheep. Linear (Huntington, 1984a; Weighart et al, 1986) or curvilinear (Webster et al, 1975; Lomax and Baird, 1983) regressions of portal blood flow on energy intake likely apply to liver blood flow as well, because liver blood flow derives predominantly from PDV. Figure 1 is a summary of 16 treat- ment means from studies with cattle from which estimates of average hourly portal blood flow are available, and treatments were intakes within diets. The character- istics of the cattle ranged from fasted beef steers (200 kg) to lactating dairy cows (650 kg) eating ME equal to 3 times their maintenance requirement. Diets ranged from forage to high concentrate diets. One can regress with high r2 linear or curvilinear relationships among the data points (fig 1), with the bulk of the data between 40-100 MJ/d ME intake. Comparison of 11 portal blood 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 portal blood flow was 1.00 ± 0.04 for the exponential fit and 1.07 ± 0.04 for the linear fit (fig 1The ME intakes in the studies used in the comparison ranged from 40-81 MJ/ d. Daily ME intake is highly correlated with other factors that may influence portal (or hepatic) blood flow, including breed, live
weight (or some exponent thereof) and energy density of the diet. However, direct (Huntington, 1984a) or implied (Weighart et al, 1986) evalution of these factors did not improve r2. From a more physiological perspective, portal blood flow ranges from 25-52 m1 olkomgin-- body weight with a median value of = 40 in cattle (Carr and Jacobson, 1968; Wangness and McGilliard, 1972; Harmon and Avery, 1987; Durand et al, 1988; Reynolds et al, 1988). Teleologically, portal and hepatic blood flow increase in response to increased ME intake to transport digested nutrients from the gut, through the liver, and on to the rest of the body. Heart rate and thereby cardiac output likewise increase with increased intake (Rumsey et al, 1980). Whether or not productive state or dietary composition directly alter the proportion of cardiac output flowing through splanchnic tissues remains to be determined. A pre-
liminary report with beef steers (Huntington et al, 1988) indicates that these propor- tions are similar in fed and fasted steers, but change in acute response to a padrenergic agonist. OXYGEN UPTAKE BY SPLANCHNIC TISSUES In vivo measurements of oxygen uptake by PDV and liver of cattle provide estimates of HE attributable to those tissues (table I). Over a wide range of body weights and productive 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 of whole body oxygen uptake by splanchnic tissues 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 account for 35-50 % of HE which is a disproportionately high rate of oxidative metabolism relative to their contribution to body mass. Because these proportions or amounts are substantial, they are attractive targets for ways to improve overall energetic efficiency of cattle. Absolute rates predictably increase with lactation (table I) or with increased intake (Webster et al, 1975; Ferrell, 1988; Reynolds et al, 1986 Huntington et al, 1988). Diet-related responses in amounts or pro- portions of oxygen uptake by PDV of steers fed legume or grass silage (Huntington et al, 1988) eluded statistical significance; however, increased intake of concentrate by beef heifers decreased oxygen uptake by PDV (Reynolds and Tyrrell, 1989).
NET NUTRIENT FLUX ACROSS PDV AND LIVER Glucose In the preponderance of reported studies encompassing a wide range of diets and intakes there is little if any net glucose absorption from dietary sources (see review by Huntington and Reynolds, 1987). This would be expected from consumption of forage diets, but also appears to be the case in cattle consuming substantial amounts of starch. For example, dairy cows consuming about 5 kg of starch as corn silage per day had net use of glucose (59 mmol/h) by PDV (Reynolds et al, 1988). Post-ruminal infusion of glucose or starch in a nonlactating cow, beef heifers and 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 increased net absorption of glucose. Ostensibly, the rest is used by PDV tissues or further me- tabolized in the lumen of the intestine. This is suggested by studies with dairy cows (Pehrson et al, 1981) and beef steers (Turgeon et al, 1983) which indicate the maximal capacity for starch disappearance from the intestine was not exceeded in the infusion studies cited. Recent studies with steers (table II) help explain how substantial passage of glucose (or a-linked glucose polymers) from the rumen does not result in net glucose absorption. Partition of net glucose flux across stomach and post-stomach sites within PDV showed net use of glucose by both sites when the diet was alfalfa hay. When high-concentrate (corn) diet was fed, however, net use of glucose by stomach tissues increased, and net absorption of glucose by post-stomach tissues was measured, ostensibly in response to starch appearing in the small intestine (table II). Janes et al (1984) reported a similar response in post-stomach glucose absorption when the diet of sheep was changed from forage to high-concentrate. In vitro studies of rumen mucosa from cattle fed
roughage or a high-concentrate diet confirm increased glucose uptake by mucosa in response to the high-concentrate diet with concomitant increases in oxidation of glucose to C20 and formation of lactate (Harmon, 1986). Ruminants are eminently capable of gluconeogenesis to meet their metabolic needs (Young, 1977). Net hepatic glucose production (3.1 kg/d) of lactating cows previously cited as an example of net glucose use by PDV (Reynolds et al, 1988) was able to meet glucose required for their milk lactose synthesis, leaving 0.8 kg/d to meet other glucose requirements. Weighart et al (1986) made similar calculations for lactating cows. In fed cattle propionate is the major glucose precursor, followed by lactate and amino acids (table 111). Studies of nonlactating and lactating cows (Baird et al, 1980) show how change in physiological state affects priority of uptake of glucose precursors by the liver to ensure an inverse relationship between glucose availability or propionate supply and use of endogenous precursors by the gut and liver. Fasting decreased gluconeogenesis and caused shifts in the sources of carbon from exogenous sources (propionate) to endogenous sources (lactate, amino acids and glycerol).
VOLATILE FATTY ACIDS, LACTATE AND KETONES Volatile fatty acids (VFA) are the predomi- nant source of energy absorbed from die- tary sources. As do other tissues, the PDV use VFA as energy sources, which means the rates and pattern of VFA production in the rumen are not the same as their rates and pattern of absorption. Several studies with 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 propionate produced in the rumen are metabolized by PDV. The liver further alters the dietary supply by removing propionate and 4- and 5-carbon VFA from portal blood, and adding acetate from endogenous production (Lomax and Baird, 1983; Huntington and Eisemann, 1988; Reynolds et al, 1988).
There is net production of lactate by PDV; L -lactate predominates, but D -lactate is absorbed by cattle fed high grain diets (Huntington et al, 1981; Harmon et al, 1985). As discussed previously, L -lactate is used by the liver for gluconeogenesis (Huntington et al, 1981);D-lactate is oxidized (Harmon et al, 1983; Giesecke and Stangassinger, 1978, 1979). The ketones [3-hydroxybutyrate and to a lesser extent acetoacetate are produced by PDV. About 90 % of butyrate produced in the rumen is oxidized by PDV (Bergman and Wolff, 1971) and (3-hydroxybutyrate is a major product of that metabolism. The liver also produces (3-hydroxybutyrate (Heitmann et al, 1987; Reynolds et al, 1988). Net uptake of n-butyrate and nonesterified fatty acids by the liver of lactating cows accounts maximally for 76-83 % of P-hydroxybutyrate release (Lomax and Baird, 1983; Reynolds et al, 1988). Fasting causes the PDV to shift from production to
net use of ketones (Lomax and Baird, 1983). AMINO ACIDS
of a-amino N abomasal infusion of casein into steers fed a high-grain diet, but liver removal increased correspondingly and splanchnic release of a-amino did not change.
Like glucose and VFA, the PDV uses amino acids from dietary and endogenous sources (Tagari and Bergman, 1978). For example, the PDV uses more glutamate and glutamine than is available from dietary sources (Harmon and Avery, 1987; Reynolds and Huntington, 1988a). Glutamate and glutamine are oxidized, and amino groups are transmitted to form alanine, serine and glycine (Bergman and Pell, 1985). In general, use of amino acids by PDV is related to the high rate of protein synthesis in PDV (Lobley et al, 1980). This use of amino acids by PDV explains at least in part why it has been difficult to show effects of diet on either rates or pro- portions of net absorption of amino acids or a-amino N (Prior et al, 1981; Huntington, 1987; Huntington et al, 1988). The liver removes amino acids from por- tal supply in amounts that vary among individual acids, thereby further modulating the rates and proportions of amino acids available for the rest of the body (Baird et al, 1975; Bergman and Pell, 1985; Huntington and Eisemann, 1988). The liver is a major participant in N shuttles among various tissues that involve alanine, glycine, glutamate, glutamine, arginine, ornithine and citrulline (Bergman and Pell, 1985). The livers of lactating cows removed 43 % of net PDV supply of a-amino N (Reynolds et al, 1988) and the livers of growing beef steers removed 24 % (Huntington and Eisemann, 1988). Liver removal of aamino N decreased in steers changed from hay to a high-grain diet, resulting in greater splanchnic release with constant PDV absorption (Huntington, 1989). Guerino et al (1988) increased PDV absorption
AMMONIA AND UREA Ammonia and urea are significant components in overall N metabolism of cattle (table IV). Over a variety of diets, N digestibility ranged from 61-72 % of N intake. Urine N and retained N varied with intake and productive state. Net PDV production of ammonia N ranged from 16―95 % of N intake and was directly related to N intake. Net removal of urea N by PDV (transfer from blood to the lumen of the gut) ranged from 15-37 % of N intake, and net production of a-amino N ranged from 18-36 % of N intake. With the exception of beef heifers fed the high-concentrate diet (table IV), net absorption of ammonia N was equal to or greater than net absorption of a-amino N. Net transfer of urea N and net absorption of a-amino are generally similar. Diet composition affected site of urea N flux across PDV of steers (table V). In steers fed hay, urea was transferred predominantly to the post-stomach. When the same steers were fed a high-concentrate diet, urea flux shifted to the stomach (rumen). Earlier work with 1N5 similarly showed transfer of urea N was predominantly to the lower gut of sheep fed forage (Nolan and Leng, 1972). Bunting et al (1989) used radioisotopes to show increased protein intake of calves increased production of ammonia in the rumen, increased urea synthesis, and doubled the percentage of total gastrointestinal urea degradation occurring in the rumen. Net production of ammonia N in steers fed twice daily was 2.7: 1 stomach: poststomach for the hay diet and about 1:1 for
the high-concentrate diet (measurements made at meal time). In steers fed 12 meals/d net production of ammonia N was about 2:1 stomach: post-stomach on both diets (table V), suggesting that about 2/3 of absorbed 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 ammonia N produced by PDV and essentially removes all of it from blood (Huntington and Eisemann, 1988; Reynolds et al, 1988; Huntington, 1989). Net liver removal can account maximally for 70 to 80 % of urea N 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; Orzechowski et al, 1987), even when PDV production is substantial.
ENERGETIC SUMMATION OF PDV AND LIVER METABOLISM Baird et al (1975) first published a comprehensive summation of energy flux by PDV of a lactating cow. Energy absorbed as VFA, lactate and ketones, and amino acids summed to 135 MJ/d, which was 84 % of calculated ME intake. Further studies with steers and dairy cows include HE from oxygen uptake by PDV (table VI). In the steers, energy from net absorption plus HE accounted for 75-91 % of measured ME intake of legume or grass silage. Not all energy sources were measured in all stud- ies with dairy cows, but in lactating cows nutrient absorption plus HE accounted for 85% of ME intake. In both steers and cows the largest single source of energy was acetate followed by propionate, except for first lactation cows in which the order was reversed. The third largest source (again
except for first lactation cows) was HE from oxygen uptake and fourth was energy from amino acids. Other VFA, L -lactate and (3-hydroxybutyrate supplied lesser amounts of energy. L -lactate plus VFA accounted for 41-57 % of ME intake (table VI). A similar percentage of ME intake was attributable to L -lactate and VFA in beef heifers fed a high-concentrate diet (46%; Huntington and Prior, 1983). Sources of ME not accounted for in table VI include heat of fermentation, absorption of longchain fatty acids, and absorption of other nitrogenous compounds such as nucleic acids and peptides (Webb, 1986). Dietary effects on sources of energy absorbed or HE by PDV were slight in the steers. Steers fed alfalfa silage absorbed more energy as branched-chain VFA and n-valerate than steers fed grass silage (table VI). Comparison of differences between intake within silages suggests some dietary effects; compared to the grass silage, the alfalfa silage caused a greater increment as acetate (40 vs 30% of the increment) and a lesser increment as HE (15 vs 26 % of the increment) (Huntington et al, 1988). This may explain in part some of the metabolism behind greater efficiency of energy use by ruminants fed legumes compared to grasses (Rattray and Joyce, 1970; Tyrrell et al, 1982; Thompson et al, 1985; Varga et al, 1987). Nonlactating and lactating cows were fed the same diet, but at different intakes to support production (table VI). Compared to non-lactating cows, lactating cows absorbed a greater percentage of ME intake as propionate (16 vs 10%). The percent- age of ME absorbed as acetate was less by cows in first lactation (16%) than dry cows (20%) or older lactating cows (22%).
CONCLUSIONS Five major points can be derived from information in this review. First, blood flow throught PDV of cattle is highly and positively correlated with their ME intake, which provides a transport for increased absorption of nutrients. Second, the PDV and liver are metabolically active at rates disproportionately greater than their contribution to body tissue mass; together, they can account for one-half of HE. Third, altough cattle derive little if any glucose directly from dietary sources, they are metabolically designed and tuned to synthesize glucose, which provides a use for propionate and lactate absorbed by PDV. Fourth, nonprotein N sources are significant participants in overall N metabolism which is orchestrated by the liver. Fifth, dietary or physiological effects on nutrient absorption and liver metabolism are most evident for VFA, ammonia and urea, which emphasizes the close and perhaps obligate interre- lation between energy and N metabolism in cattle. These points show the central role gut and liver tissues play in energy metabolism, both by regulating and modulating dietary energy sources and by virtue of their high energy expenditures. It follows then that a small change in mode or efficiency of splanchnic metabolism could have a major effect on the whole animal's response to physiological state or nutrition. REFERENCES Agricultural Research Council (1980) Nutrient Requirements of Livestock. Commonwealth Agricultural Bureaux, UK
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Huntington GB (1984b) Net absorption of glucose and nitrogenous compounds by lactating Holstein cows. J Dairy Sci 67, 1919- 1927 Huntington GB (1987) Net absorption from portal-drained viscera of nitrogenous compounds by beef heifers fed on diets differing in protein solubility or degradability in the rumen. Br J Nutr 57, 109-114 Huntington GB (1989) Hepatic urea synthesis and site and rate of urea removal from blood of beef steers fed alfalfa or a high concen- trate diet. Can J Anim Sci 69, 215-223 Huntington GB, Eisemann JH (1988) Regulation of nutrient supply by gut and liver tissues. J Anim Sci 66 (suppl 3), 35-48 Huntington G, Eisemann J, Whitt J (1988) Proportions of whole body blood flow and oxygen uptake attributable to gut and liver of beef steers. J Anim Sci 66 (suppl 1),147 (abstr) Huntington GB, Prior RL (1983) Digestion and absorption of nutrients by beef heifers fed a high concentrate diet. J Nutr 113, 2280-2288 Huntington GB, Prior RL (1985) Net absorption of amino acids by portal-drained viscera and hind half of beef cattle fed a high- concentrate diet. J Anim Sci 60, 1491-1499 Huntington GB, Prior RL, Britton RA (1981) Glucose and lactate absorption and metabolic interrelationships in steers changed from low to high concentrate diets. J Nutr 111, 1164- 1172 Huntington GB, Reynolds CK (1987) Oxygen consumption and metabolite flux of bovine portal-drained viscera and liver. J Nutr 117, 1167-1173 Huntington GB, Reynolds PJ (1986) Net absorption of glucose, L-lactate, volatile fatty acids and nitrogenous compounds by bovine given abomasal infusions of starch or glucose. J Dairy 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 of metabolites in nonpregnant dry Holstein cows before and after intraruminal acetic in- fusion. J Dairy Sci 66, 1901-1908
Huntington GB, Tyrrell HF (1985) Oxygen consumption by portal-drained viscera of cattle: comparison of analytical techniques and relationship to whole body oxygen consumption. J Dairy Sci 68, 2727-2731 Huntington GB, Varga GA, Glenn BP, Waldo DR (1988) Net absorption and oxygen consumption by Holstein steers fed alfalfa or orchardgrass silage at equalized intakes. J Anim Sci 66, 1292-1302 Janes AN, Parker DS, Weekes TEC, Armstrong DG (1984) Mesenteric venous blood flow and net absorption of glucose in sheep fed dried grass or ground maize-based diets. J Agric Sci (Camb) 103, 549-553 Jones SDM, Rompala RE, Jeremiah LE (1985) Growth and composition of the empty body in steers of different maturity types fed concentrate or forage diets. J Anim Sci 60, 427-433 Kreikemeier KK, Harmon DL, Avery TB, Brandt RT (1987) Starch and glucose utilization in the small intestine of steers. J Anim Sci 66 (suppl 1), 469 (abstr) Lobley GE, Milne V, Lovie J, Reeds PJ, Pennie K (1980) Whole body and tissue protein synthesis in cattle. BrJNuir43, 491-502 Lomax MA, Baird GD (1983) Blood flow and nutrient exchange across the liver and gut of the dairy cow. Br J Nutr 49, 481-496 McGuire MA, Beede DK, DeLorenzo MA, Wilcox CJ, Huntington GB, Reynolds CK, Collier RJ (1989) Effects of thermal stress and level of feed intake on portal plasma flow and net fluxes or metabolites in lactating Holstein cows. J Anim Sci 67, 1050-1060 Nolan JV, Leng RA (1972) Dynamic aspects of ammonia and urea metabolism in sheep. BrJ Nutr 27, 177-194 Orzechowski A, Motyl T, Pierzynowski G, Barey W (1987) Hepatic capacity for ammonia removal in sheep. J Vet Med A34, 108-112 Pehrson B, Johnson U, Knutsson M (1981) The digestion of starch in the small intestines of dairy cows. Zbl Vet Med A 28, 241-246 Pethick DW, Lindsay DB, Barker PJ, Northrop AJ (1981) Acetate supply and utilization by the tissues of sheep in vivo. Br J Nutr 46, 97- 109 Prior RL, Huntington GB, Britton RA (1981) Influence of diet on amino acid absorption in beef cattle and sheep. J Nutr 111, 2212-2222
Rattray PV, Joyce JP (1970) The nutritive value of white clover and perennial ryegrass for young sheep. I. Nitrogen retention studies and associated animal performance. NZ J Agric Res 13, 778-791 Reynolds CK, Huntington GB (1988a) Partiton of portal-drained visceral net flux in beef steers. I. Blood flow and net flux of oxygen, glucose and nitrogenous compounds across stomach and post-stomach tissues. Br J Nutr 60, 553-562 Reynolds PJ, Huntington GB (1988b) Net portal absorption of volatile fatty acids and L(+)lactate by lactating Holstein cows. J Dairy Sci 71, 124-133 Reynolds CK, Huntington GB, Tyrrell HF, Reynolds PJ (1986) Splanchnic tissue and whole animal oxygen consumption by lactating Holstein cows. J Dairy Sci 69 (suppl 1),193 (abstr) Reynolds CK, Huntington GB, Tyrrell HF, Reynolds PJ (1988) Net portal-drained visceral and hepatic metabolism of glucose, L-lactate and nitrogenous compounds in lactating Holstein cows. J Dairy Sci 71, 1803-1812 Reynolds CK, Tyrrell HF (1989) Effects of fo- rage to concentrate ratio and intake on visce- ral tissue and whole body energy metabolism of growing beef heifers. In: Proc 11 th EAAP Symp, Energy Metabolism of Farm Animals, Lunteren, Netherlands (Van Der Honing Y, Close WH, eds). Eur Assoc Animal Prod No 43, 151-154 Rumsey TS, Tyrrell HF, Moe PW (1980) Effect of diethylstilbestrol and synovex-S on fasting metabolism measurement of beef steers. J Anim Sci 50, 160-165 Sasaki Y, Weekes TEC (1986) Metabolic responses to cold. In: Control of Digestion and Metabolism in Ruminants. Proc 6th Intl Symp, Ruminant Physiol (Milligan LP, Grovum WL, Dobson A, eds) Prentice-Hall, Englewood Cliffs, NJ, 326-343 Seal CJ, Sarker A, Parker DS (1989) Rumen propionate production rate and absorption of fermentation end-products into the portal vein of 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 organs and tissues in cattle. J Dairy Sci 50, 1055- 1060
Symonds HW, Mather DL, Colles KA (1981) The maximum capacity of the liver of the adult dairy cow to metabolize ammonia. Br J Nutr 46, 481-486 Tagari H, Bergman EN (1978) Intestinal disappearance and portal blood appearance of amino acids in sheep. J Nutr 108, 790-803 Thompson GE, Maneson W, Clarke PL, Bell AW (1978) Acute cold exposure and the metabolism of glucose and some of its precursors in the liver of the fed and fasted sheep. Quart J Exp Physiol63, 189-199 Thomson DJ, Beever DE, Haines MJ, Cammell SB, Evans RT, Dhanoa MS, Austin AR (1985) Yield and composition of milk from Freisan cows grazing either perennial ryegrass or white clover in early lactation. J Dairy Res 52, 17-311 Tyrrell HF, Thomson DJ, Waldo DR, Goering HK (1982) Energy retention and utilization of grass and legume by cattle. Proc Nutr Soc 41, 23A (abstr) Turgeon OA Jr, Brink DR, Britton RA (1983) Corn particle size mixtures, roughage level and starch utilization in finishing steer diets. J Anim Sci 57, 739-749 Varga GA, Tyrrell HF, Waldo DR, Huntington GB, Glenn BP (1987) Effect of alfalfa or orchardgrass silages on energy and nitrogen utilization for growth by Holstein steers. In: Energy Metabolism of Farm Animals (Moe PW, Tyrrell HF, Reynolds PJ, eds) EAAP Publ 32, Rowman & Littlefield, Totowa, NJ, 86-89 Wangness PJ, McGilliard AD (1972) Measurement of portal blood flow by dye-dilution. J Dairy Sci 55, 1439-1446 Webb KE Jr (1986) Amino acid and peptide absorption from the gastrointestinal tract. Fed Proc 45, 2268-2271 Webster AJF, Osuji PO, White F, Ingram JF (1975) The influence of food intake on portal blood flow and heat production in the digestive tract of sheep. BrJ Nutr 34, 125-139 Weighart M, Slepetis R, Elliot JM, Smith DF (1986) Glucose absorption and hepatic gluconeogenesis in dairy cows fed diets varying in forage content. J Nutr 116, 839-850 Young JW (1977) Gluconeogenesis in cattle: significance and methodology. J Dairy Sci 60, 1-15

GB Huntington

File: energy-metabolism-in-the-digestive-tract-and-liver-of-cattle.pdf
Author: GB Huntington
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