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* North Carolina State University, Raleigh 27614;
and
University of Kentucky, Lexington 40546; and
and
University of Tennessee, Knoxville 37996
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Abstract |
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 Top Abstract INTRODUCTIONRUMINAL FERMENTATION OF STARCHPOSTRUMINAL STARCH DIGESTION AND...POSTABSORPTIVE METABOLISM OF...LITERATURE CITED |
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Key Words: beef • digestion • glucose • grain • metabolism • starch
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INTRODUCTION |
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TopAbstractINTRODUCTIONRUMINAL FERMENTATION OF STARCHPOSTRUMINAL STARCH DIGESTION AND...POSTABSORPTIVE METABOLISM OF...LITERATURE CITED |
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). The annual corn (maize) grain production would fill a square 14.5 km per side to a depth of 1.6 km, or would cover California or Montana to a depth of 0.9 m. Approximately one-half of the grain produced is fed to animals. We calculate that the approximately 14 million feedlot cattle in the United States can account for 15% of the corn fed to livestock, and we expect that essentially all of the sorghum fed to livestock is used for feedlot cattle. That consumption would cover Texas with 10 cm of corn or sorghum.
Cereal grains contain from 57 to 77% of DM as starch, with oats and barley at the lower end of the range, wheat at the top, and corn and sorghum in the middle. Other feedstuffs like grasses and legumes, protein meals, and animal byproducts may contain from 2 to 20% starch (cited by Huntington, 1997).
Because of this tremendous production and high starch content of grains, very small changes in conversion of feed grain to market beef have significant economic effects. At $0.10/kg of corn grain, a 1% improvement in the efficiency of conversion of corn grain to animal body weight would reduce feed costs approximately $23 million for the feedlot industry, or slightly less than $1 per head. Investment of 1% of the potential savings would generate $230,000 annually to support research to attain that 1% improvement.
The purpose of this review is to describe our current understanding of how dietary starch is converted to metabolic substrates to support animal weight gain, with some elucidation of the known limits and potential avenues to improve rates, extents, or efficiency of that conversion. The themes are to maximize ruminal fermentation without precipitating digestive upsets, and to avoid fermentation in the cecum and large intestine.
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RUMINAL FERMENTATION OF STARCH |
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TopAbstractINTRODUCTIONRUMINAL FERMENTATION OF STARCHPOSTRUMINAL STARCH DIGESTION AND...POSTABSORPTIVE METABOLISM OF...LITERATURE CITED |
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). As with other nutrient concentrations, the starch content of a given grain varies with variety, growing conditions, and agronomic practices (Rooney and Pflugfelder, 1986; Philippeau and Michalet-Doreau, 1997). Coefficients of variation from 5 grains from one local source were 2.4% for corn, 3.7% for sorghum, 4.1% for wheat, 5.2% for barley, and 7.1% for oats (Herrera-Saldana et al., 1990). The average and coefficient of variation of starch content of 42 sorghum samples were 68 and 7.1%, respectively (Preston et al., 1991).
Across a wide range of intakes (1 to 5 kg/d) and starch sources (corn, sorghum, and barley), data from 16 studies predict a linear relationship between starch intake and apparent ruminal starch digestion (Harmon et al., 2004); 77% of starch intake is digested in the rumen. Offner and Sauvant (2004) summarized data from 87 studies in which starch intake ranged from 1 to 5.7 kg/d and reported an average ruminal digestibility of starch of 71%. The data of Harmon et al. (2004) came mainly from growing cattle fed high-concentrate diets, and the data set of Offner and Sauvant (2004) included dairy cattle. Both summaries adjusted the data for the random effect of experiment (St-Pierre, 2001). Although we are not certain, the two data sets likely share data from the same studies. We conclude that, within the range of normal rumen function, the amount of starch digested in the rumen is a linear function of intake; however, the range of apparent digestibilities in these two summaries suggests that factors other than intake are affecting starch digestion, and visual appraisal of scatter about the regression line indicates increased variability in starch digestion at greater intakes. Those factors include a variety of ruminal kinetic variables such as liquid and feed particle pools, chemical properties of those pools (pH, osmolarity), rates and extents of particle passage, and outflow of microbial products (Owens and Goetsch, 1986).
The chemical structures of starch granules and their interface with protein moieties in the grain kernel affect both rate and extent of ruminal fermentation (Streeter et al., 1990; Philippeau et al., 2000). Grains contain both amylose and amylopectin, with the former more fermentable than the latter. Starch granules in the floury endosperm are more loosely linked to proteins and are more fermentable than granules embedded in the protein matrix of the vitreous endosperm. Comparisons of dent with flint corn varieties show that although total starch content is similar between dent and flint corn, flint has a greater degree of vitreousness, and slower rates and extents of in vitro, in situ, and in vivo ruminal starch disappearance (Philippeau et al., 1999a,b; Taylor and Allen, 2005a).
Grain processing (moistening, heating, mechanical pressure) disrupts the structure of starch granules, and has been used successfully for several years to improve ruminal fermentability of grains (Theurer et al., 1999). Grains vary in their response to processing, with sorghum > corn > oats = barley > wheat. In general, waxy varieties of corn and sorghum contain only amylopectin (data cited by Huntington, 1997). Comparison of starch granules from waxy or nonwaxy varieties of wheat showed increased amylopectin content and increased physical damage after dry or wet crushing in the waxy variety (Bettge et al., 2000). Therefore, waxy varieties of corn and sorghum may respond more to processing than nonwaxy varieties.
Fermentation and Fermentation Products
Efforts to reliably predict and model ruminal fermentation have been hindered by the complexities of the system. Microbial attack is not an understatement when it comes to describing ruminal fermentation. Although the rumen microbial system evolved to survive and sustain on high-fiber diets, competition for energy-yielding substrates is fierce and altruism is nonexistent. In general, increasing dietary supply of fermentable grain starch is associated with increased production of organic acids, increased production of microbial protein, decreased fiber digestion, decreased ammonia concentrations, and decreased acetate:propionate ratio (Poore et al., 1993; Martin et al., 1999; Philippeau et al., 1999b; Oba and Allen, 2003a,b,c). However, not all these responses are found consistently in all studies, nor do responses satisfy statistical criteria in all studies. For example, Taylor and Allen (2005b) followed changes in ruminal fermentation products associated with their study of floury and vitreous corn described above. Compared with vitreous corn, floury corn caused lower pH, increased (P < 0.14) total VFA, decreased acetate, increased propionate, decreased (P < 0.20) ammonia, and decreased (P < 0.08) branched-chain VFA in ruminal fluid of dairy cows. Neither the amount of NDF digested in the rumen nor supply of microbial protein to the abomasum was affected by grain variety in their study (Taylor and Allen, 2005a).
Opportunities for Improving Understanding of Ruminal Starch Fermentation
Most cattle prefer grain to forages, and will ingest large amounts of grain if given the opportunity to do so. The ruminal microbial population responds rapidly to a supply of readily fermentable carbohydrates, and the resultant production of acids, low ruminal pH, and loss of normal ruminal motility are the main contributors to digestive upsets associated with feeding high-grain diets (Owens et al., 1998). One of the remaining major challenges to avoiding digestive upsets is to link individual an animal’s physiological and metabolic responses to their voluntary eating behavior, and consequently to feeding management. There is an inherent conflict in beef cattle production systems—animals are routinely fed in groups (pens), but income from sale of product is determined for each animal. Therefore, ways to decrease variation within pens, avoid digestive upsets, and improve performance likely will come from improved understanding of the individual animal’s behavior in a pen environment (Schwartzkopf-Genswein et al., 2003). Individual prior experience interacts with genetic variation (Channon et al., 2004) to alter voluntary intake in different ways in group-fed cattle. Physical form of the grain and level of grain in the diet may affect various prehension or rumination activities (Ray and Drake, 1959; Oba and Allen, 2003a). Ruminal microbial species and pH profiles vary in response to source or amount of starch sources (Klieve et al., 2003; Schwartzkopf-Genswein et al., 2004). Mastication activity, rumination patterns, and saliva production vary among grain sources (Beauchemin et al., 1994; Taylor and Allen, 2005c) and processing methods, and ostensibly among individual animals in pens. Feeding ionophores during transition from low- to high-grain diets changes feeding behavior and decreases variation in pen intakes (Gibb et al., 2001). We believe that given more integrated information on individual behavior in pens, one can sort and manage cattle in ways that improve the animals’ well-being as well as their performance in production situations.
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POSTRUMINAL STARCH DIGESTION AND ABSORPTION |
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TopAbstractINTRODUCTIONRUMINAL FERMENTATION OF STARCHPOSTRUMINAL STARCH DIGESTION AND...POSTABSORPTIVE METABOLISM OF...LITERATURE CITED |
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; Harmon, 1992, 1993; Huntington, 1997; Harmon et al., 2004) and will be briefly described here. The process begins in the lumen of the duodenum via the action of pancreatic 
-amylase. -Amylase initiates starch breakdown producing maltose and various branched-chain products commonly referred to as limit dextrins.
It is the role of the pancreas to provide digestive enzymes to ensure complete breakdown of substrates in duodenal digesta and to neutralize the acidic chyme that exits the abomasum through the secretion of bicarbonate in order to provide an optimal environment for enzyme activity. The adequacy of both roles of the pancreas, the secretion of -amylase and bicarbonate, has been questioned in ruminants. The lack of an adaptive response of the ruminant pancreas to increased dietary starch (Kreikemeier et al., 1990) remains a biological enigma and has led to speculation that pancreatic -amylase is the limiting phase of intestinal starch assimilation (Huntington, 1997). Others (Oba and Allen, 2003a,b; Taylor and Allen, 2005b) conclude from their data from dairy cows that the physicochemical characteristic of the corn particles is the primary limiter of intestinal starch digestion.
The second phase of intestinal starch digestion and absorption occurs at the brush border membrane through the action of the brush border carbohydrases. These enzymes have received little attention in ruminant research and have been poorly characterized. There are measurable maltase and isomaltase activities with little or no adaptive response to diet (Janes et al., 1985; Kreikemeier et al., 1990). Kreikemeier and Harmon (1995) analyzed the composition of ileal digesta in steers infused abomasally with glucose, corn dextrins, or cornstarch. They found an accumulation of -glucosides in ileal digesta that were composed mainly of disaccharides with little free glucose. They concluded that under these experimental conditions starch assimilation was limited by brush border -glucosidase activity.
The third and final component of intestinal starch digestion and absorption is the transport of glucose out of the intestinal lumen and into the portal circulation. Although it has been generally thought that glucose crosses the enterocyte brush border membrane via the action of the sodium-dependent glucose transporter SGLT1 (Harmon and McLeod, 2001), the activity of this transporter has shown no adaptive response to luminal substrate in cattle (Bauer et al., 2001; Rodriguez et al., 2004). Despite this lack of adaptive response in glucose transport, abomasally infused glucose readily exits the intestinal lumen (Kreikemeier et al., 1991) suggesting that perhaps mechanisms other than SGLT1 are responsible for glucose transport in cattle (Au et al., 2002).
Optimizing Site of Digestion Advantages of the Small Intestine
To determine the impact of site of digestion on energetic efficiency, McLeod et al. (2001) determined energy retention in steers receiving ruminal vs. abomasal infusions of hydrolyzed starch for 28 d. From these animals they estimated partial efficiencies of infused substrates using indirect calorimetry. Abomasal infusion resulted in a partial efficiency of 0.60 whereas ruminal infusions resulted in a partial efficiency of 0.48. It was known that the amounts of starch infused abomasally in these experiments would not exceed the capacity of the small intestine to digest the starch and absorb glucose (Branco et al., 1999). These data demonstrate that an energetic advantage can be gained if starch is digested and absorbed in the small intestine. At the very least we should avoid digestion in the large intestine because of its low efficiency of nutrient use (Owens et al., 1986; Harmon and McLeod, 2001). It is clear that processing, particularly steam flaking, enhances digestive efficiency. At least a portion of this increased efficiency probably results from increased intestinal digestion. Starch that escapes digestion in the small intestine is extensively fermented in the large intestine (Philippeau et al., 1999b), indicating that additional starch is potentially available in the small intestine. It has not been demonstrated that additional amylolytic activity would improve digestion in the small intestine (Remillard et al., 1990), suggesting that either mucosal enzymes or glucose transport may be limiting assimilation. To apply these concepts in practice we need an accurate depiction of small intestinal digestion. We need to predict starch flow and disappearance from the small intestine at the time of diet formulation. Based on our current methods, adequate means of reliably assessing small intestinal digestion are not available. Without the ability to accurately describe and predict small intestinal starch digestion we cannot optimize starch digestive efficiency. We can only hope to avoid fecal starch excretion through grain processing and nutritional management.
Another factor complicating our ability to predict small intestinal starch digestion and glucose absorption is changing visceral metabolism. Just because starch disappears in the small intestine does not mean it contributes to absorbed glucose (Kreikemeier et al., 1991). To determine how site of starch digestion interacts with visceral glucose metabolism, steers were infused either ruminally or postruminally with starch (Harmon et al., 2001). Visceral nutrient flux and glucose metabolism were measured during an intravenous infusion of 14C-glucose. Based on an estimated small intestinal digestibility of 90% for the infused starch (Branco et al., 1999), estimates of net portal-drained visceral glucose absorption indicated that from 38 to 56% of the infused carbohydrate was digested and absorbed as glucose. However, when the estimates were corrected for changes in visceral glucose metabolism, this estimate increased to 77% of infused carbohydrate digested and absorbed as glucose. Measuring glucose net absorption and visceral metabolism may provide more meaningful and precise estimates of small intestinal starch availability, upon which we can begin to build much-needed dietary predictions and thereby improve digestive efficiency. We need data such as these in beef cattle with high levels of feed consumption.
Efficiency of Digestion
To assess the impact of changes in site of digestion, we calculated the yield of digestible energy for each region of the gastrointestinal tract and summed these to estimate overall energy yield. Values for efficiencies were from Harmon and McLeod (2001) and were as follows: ruminal 80%, small intestinal 97%, and large intestinal 62%. Digestibility in the large intestine was held at 44% (Harmon et al., 2004) and ruminal and small intestinal digestibility coefficients were varied to generate the data in Figure 1. This data analysis demonstrated that shifting digestion from the rumen to the small intestine decreased the efficiency of energy yield when small intestinal digestibility was less than 75%. When digestibility in the small intestine was above 75%, the efficiency of energy yield was increased by shifting digestion from the rumen to the small intestine. Using the data summarized previously (Harmon et al., 2004) we fit a rising logistic model to develop a more kinetic-based model to describe small intestinal digestion (Figure 2). Using this relationship, we conclude that small intestinal digestion was only above 70% at very low intakes, when 700 g/d or less starch reaches the small intestine. This suggests that advantages in digestive efficiency through increased intestinal starch digestion can only be obtained at low intakes, or perhaps with highly processed feeds such as steam-flaked corn. The low digestibility and low energetic efficiency of large intestinal digestion dictate that little starch should escape the small intestine or declines in efficiency will be noted.
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POSTABSORPTIVE METABOLISM OF PRODUCTS OF DIGESTION AND ABSORPTION |
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TopAbstractINTRODUCTIONRUMINAL FERMENTATION OF STARCHPOSTRUMINAL STARCH DIGESTION AND...POSTABSORPTIVE METABOLISM OF...LITERATURE CITED |
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). Metabolic requirements for glucose are linked to production demands and priorities; they can be estimated or calculated as a function of energy intake (ME) across a wide range of body weights, breeds, or productive purposes, at least in cattle (Herbein et al., 1978; Russell et al., 1986; Wieghart et al., 1986).
Regressions shown in Figures 3 to 5 were adjusted for groups (growing cattle or dairy cows) as described by St-Pierre (2001). Figure 3 shows 3 linear regression lines that represent glucose irreversible loss from 31 data points from dairy cattle (Bauman et al., 1988; Veenhuizen et al., 1988; Amaral et al., 1990; Knowlton et al., 1998; Rigout et al., 2002; Lemosquet et al., 2003) and growing cattle (Herbein et al., 1978; Schmidt and Keith, 1983; Armentano et al., 1984; Lyle et al., 1984; Richards, 1999). Glucose irreversible loss represents glucose carbon that is used by the animal and will not be recycled (for example, excretion as milk lactose, or exhalation as CO2). The regression lines in Figure 4 represent liver gluconeogenesis from 49 data points from dairy cattle (Baird et al., 1980; Lomax and Baird, 1983; Reynolds et al., 1988, 1992, 2003; Casse et al., 1994; DeVisser et al., 1997; Benson et al., 2002) and growing cattle (Reynolds et al., 1988, 1992, 1994, 2003; Harmon et al., 1991; Krehbiel et al., 1992; Eisemann et al., 1996; Eisemann and Huntington, 1994; Taniguchi et al., 1995; Huntington et al., 1996; Richards, 1999; Lozano et al., 2000). The regression lines in Figure 5 represent propionate’s contribution to liver gluconeogenesis from the same database used for liver gluconeogenesis.
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Most published information supports the general concept that gluconeogenesis is a positive function of energy intake. Intensive work with ruminal or duodenal infusion of glucose or propionate into lactating dairy cows (Rigout et al., 2002; Lemosquet et al., 2003) and work with hydrolyzed starch infusion into beef steers (Richards, 1999) suggests that direct use of absorbed glucose by visceral tissues is equal to 28% of glucose available for absorption in the small intestine. In our view, the work with lactating cows cited above shows a similar response in glucose irreversible loss to duodenal infusion of glucose or to ruminal infusion of propionate. Almost none of the published information specifies or discriminates among dietary sources of energy; however, extensive analysis of data from cattle and sheep indicates that the presence of corn grain in the diet will increase glucose turnover (the sum of irreversible loss and recycling) more than isoenergetic diets that contain grains other than corn (Ortigues-Marty et al., 2003). This underlines the fact that ruminants will continue to synthesize glucose even when they are provided a diet rich in starch, largely through use of glucogenic ruminal fermentation products. The relationship between liver propionate uptake and liver glucose production exemplifies the link between precursor supply to the liver and glucose production. Comparison of regressions in Figures 3
to 5 also indicates that propionate does not supply all the carbon for liver glucose production or whole body irreversible glucose loss.
Glucose Precursors
Several products of starch digestion and absorption are key carbon sources for gluconeogenesis. Propionate is quantitatively most important; the slopes of lines in Figures 4 and 5 for growing cattle indicate that net propionate uptake by the liver accounts for about 70% (100 x 14 ÷ 20) of net liver glucose production. Direct measures show a range of values for maximal theoretical contribution of propionate to hepatic gluconeogenesis (Table 1). Propionate is followed by L-lactate and glucogenic amino acids. Propionate and lactate are direct fermentation products, and amino acids are, at least in part, from microbial protein. However, amino acids and lactate may arrive in the liver as immediate products of the Cori cycle (lactate to and from glucose in the gut or peripheral tissues) or deamination or transamination of glucogenic amino acids (again in the gut or peripheral tissues). Because data in Table 1 represent maximal theoretical calculations based on 1 glucose unit produced from 2 units of the precursor, the sums may be greater than 100%. Transition dairy cows likely use relatively more lactate and glycerol and less propionate to support gluconeogenesis immediately before and after calving (Reynolds et al., 2003), because they mobilize body tissues to support the onset of lactation.
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make two points about gluconeogenesis in beef steers. First, as steers became older and fatter, the maximal theoretical contribution of propionate to gluconeogenesis reduced in favor of increased contribution of gluconeogenic amino acids (Eisemann et al., 1996). Second, in steers fed high-concentrate, flaked-sorghum grain diets, decreasing flake density (increased ruminal fermentation rate) increased the supply of propionate, which was directly linked to increased gluconeogenesis (Lozano et al., 2000).
Studies of the effect of insulin on net hepatic glucose release and net uptake of glucose precursors show that increased insulin supply in euglycemic, hyperinsulinemic beef steers will reduce liver glucose production to the point where all glucose production could be derived from propionate that was removed by the liver (Eisemann and Huntington, 1994). Intravenous infusion of propionate to lactating dairy cows reduced net hepatic uptake of lactate, and switched hepatic pyruvate flux from net uptake to net output (Baird et al., 1980). Intravenous infusion of lactate did not change net hepatic uptake of propionate. Calculations that account for recycling of radioactivity of compounds infused intravenously into a sheep indicate that 95% of propionate removed by the liver is used for gluconeogenesis (Steinhour and Bauman, 1988).
These results plus in vitro data with hepatocytes from ruminating calves (Donkin and Armentano, 1995) imply that propionate use for gluconeogenesis is independent of insulin’s control, and that there is a high metabolic priority for the liver to remove propionate from blood and use it to satisfy the need for gluconeogenesis. Insulin-independent use of propionate for gluconeogenesis also supports the observations that high-starch diets (corn, wheat, or sorghum) that promote production of propionate in the rumen likely will promote greater rates of gluconeogenesis. The relative insensitivity to control by insulin also implies that use of ionophores or other feed additives that promote propionate production should spare other glucose precursors, such as glycerol or glucogenic amino acids.
Glucose Use
In addition to exemplifying the relationship between energy intake and glucose metabolism in cattle, Figures 3 to 5 shed some light on the recycling of glucose in response to production needs. Liver glucose production (Figure 4) represents 75% or more of glucose total entry rate (the sum of glucose that enters the bloodstream from all sources; Brockman, 1993). It does not include glucose absorbed from the gut or gluconeogenesis in kidneys or other extrahepatic tissues. However, it does include gluconeogenesis from dietary precursors (propionate, lactate, glucogenic amino acids) as well as recycling of glucose carbon. Possible avenues of recycling include glycogen production and glycogenolysis in the liver or muscle, movement of carbon through synthesis and catabolism of glucogenic amino acids, or movement of carbon through lipid metabolism in the form of glycerol. Therefore, the energy delivered from high-starch diets moves through many metabolites to meet the needs of carbohydrate, protein, and lipid metabolism.
Glucose oxidation to CO2 in growing Holstein steers consuming a 30% concentrate diet (44.4% of irreversible loss; Veenhuizen et al., 1988) and lactating dairy cows that need glucose for milk lactose synthesis (17.2% of irreversible loss; Bauman et al., 1988) show that normally functioning ruminants have more than adequate gluconeogenic capacity to meet glucose irreversible loss as well as to meet other requirements for metabolic balance, or homeostasis. Conversion of propionate to glucose in the liver (the slopes of the lines in Figure 5) is at least equal to increasing glucose irreversible loss (the slopes of the lines in Figure 4), indicating that the major dietary glucose precursor keeps pace with minimal glucose need as ME intake increases. The growing gap between liver gluconeogenesis and irreversible loss at greater ME intakes (the slopes of the lines in Figures 3 and 4) indicates that recycling of carbon to support liver gluconeogenesis is metabolically more significant in lactating dairy cows than in growing animals.
Veenhuizen et al. (1988) fed steers 600 g/d of sodium propionate; they increased gluconeogenesis from propionate, increased irreversible loss of glucose by 59%, increased oxidation of glucose to CO2, and increased the percentage of CO2 supplied by oxidation of glucose from 7.8 to 13.1%. Increased oxidation of glucose to CO2 accounted for essentially all of the increased irreversible loss of glucose by the steers. Amaral et al. (1990) increased glucose supply for lactating cows by intravenous infusion of glucose (up to 737 g/d) and increased milk production by 6% (not statistically significant), increased irreversible loss of glucose by 53%, decreased gluconeogenesis from propionate, and increased the percentage of CO2 supplied by oxidation of glucose from 4.1 to 6.8%. Knowlton et al. (1998) increased potential glucose supply in lactating cows by abomasal infusion of 1,500 g/d of partially hydrolyzed starch and increased milk production by 5% (P < 0.07), increased irreversible loss of glucose by 21%, and increased the percentage of CO2 supplied by oxidation of glucose from 5.4 to 7%. Taken together, these results indicate that glucose supply was not a major limitation to growth or milk production in these studies, and that it is difficult to show with statistical confidence that an enhanced glucose supply will push the metabolic system in an energetically efficient manner.
Therefore, these intensive metabolic studies tell us that normally functioning ruminants (not ketotic or otherwise diseased) are capable of synthesizing sufficient glucose to match irreversible losses, and will oxidize glucose in excess of their metabolic requirement. However, there may be some benefit by sparing amino acids that otherwise would have been used to support gluconeogenesis. Propionate production in the rumen represents a relatively efficient capture of energy for subsequent production of glucose, but there has to be a concomitant demand (pull) to avoid oxidation of extra glucose.
Site of Starch Digestion Affects Glucose and Energy Metabolism
As would be expected, shifting starch digestion from the rumen to the small intestine increases portal-drained visceral (digestive tract, pancreas, spleen and mesenteric fat) net glucose absorption (Taniguchi et al., 1995; Reynolds et al., 1998; Richards, 1999). Richards (1999) determined that shifting the site of digestion of 800 g/d of starch from the rumen to the small intestine increased use of glucose by portal-drained visceral tissue from 120 to 281 g/d. However, as indicated by oxygen consumption in beef steers (Richards, 1999) and lactating dairy cows (Reynolds et al., 1998), the increase in glucose use by the portal-drained visceral tissues is not associated with an increase in energy use by those tissues, indicating that other energy substrates are conserved.
Taniguchi et al. (1995) and Richards (1999) both infused 800 g/d of starch into either the rumen or the abomasum of beef steers, and both measured blood flow and net nutrient flux across splanchnic tissues. Taniguchi et al. (1995) reported increases only in net portal-drained visceral acetate and valerate absorption, whereas Richards (1999) reported increases in net butyrate and valerate absorption with ruminal starch infusion, in which increases in ruminal VFA production would be expected. When the heat of combustion estimates are applied to each of the energy substrates absorbed across, and oxygen consumed by, the portal-drained viscera, there was no difference in energy released by the portal-drained viscera due to site of starch digestion in either of the above experiments.
Despite differences in substrates absorbed and utilized by the portal-drained viscera, the supply of major precursors reaching the liver for gluconeogenesis was not affected by the site of starch digestion. Consequently, as noted previously, liver glucose production was similar for both sites of starch digestion in steers (Taniguchi et al., 1995; Richards, 1999). Taniguchi et al. (1995) reported that metabolism of glucose in the liver of ruminally infused calves resulted in decreased glucose and lactate, but increased acetate and ß-hydro-xybutyrate, releases to peripheral tissues, which is similar to the findings of Richards (1999), except in that experiment acetate was greater with abomasal starch infusion. Both experiments infused the same quantity of starch, but Richards (1999) supplied a partially hydrolyzed form, which resulted in greater net glucose absorption. In lactating dairy cows, Reynolds et al. (1998) infused starch into the abomasum of lactating cows and increased net glucose absorption, but did not affect liver glucose production when compared with a water infusion. However, starch infusion into the rumen increased liver glucose production.
If one examines the total energy available for use by peripheral tissues (total splanchnic flux), there are the differences in substrates available, as described above. Greater quantities of glucose available with postruminal starch digestion result in greater quantities of total glucose irreversible loss and peripheral tissue use by steers (Richards, 1999). However, if we look at energy available for use by peripheral tissues, the data of Taniguchi et al. (1995) and Richards (1999) differ in quantities of glucose available from postruminal digestion. The quantity of acetate available for peripheral metabolism was greater when more glucose was absorbed post-ruminally, which would agree with the concept that in portal-drained viscera, acetate use is associated primarily with oxidative metabolism rather than incorporation into mesenteric fat.
Oxidation of acetate to CO2 can represent up to 25% of the whole-body acetate turnover on forage-based diets (Pethick et al., 1981). Bartley and Black (1966) and Knowlton et al. (1998) both discuss the possibility that postruminal starch digestion causes preferential use of glucose by the portal-drained viscera tissues, thereby reserving amino acids and VFA for other functions. A numerical increase in acetate flux between a basal diet control and the postruminal starch infusion of Taniguchi et al. (1995) would indicate that the additional glucose supply does result in conservation of acetate. These differences resulted in 635 kcal/d greater peripheral energy availability for the ruminal treatment in Taniguchi et al. (1995) and 1,759 kcal/d greater energy for the abomasal digestion in Richards (1999).
In an attempt to evaluate the metabolic consequences of changing site of starch digestion, we created a scenario for a finishing beef steer (Table 2). Glucose irreversible loss and liver gluconeogenesis were calculated using the equations for growing cattle shown in Figures 3 and 4. The diet contained 90% grain, which provided 6,300 g of starch daily. We used the proportions of Veenhuizen et al. (1988) and Amaral et al. (1990) for transfer of ruminal propionate to blood glucose as the basis for an estimate of 45% of carbon from ruminally digested starch appearing as blood glucose. In contrast, approximately 70% of available starch digested in the small intestine appears as blood glucose (Richards, 1999). Therefore, we assumed that ruminal conversion of carbon from starch digested in the rumen to blood glucose is 45/70, or approximately 64% as efficient as the conversion in the small intestine.
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). Using the assumptions and calculations in Table 2, at least 16% of the 6,300 g of starch consumed needs to be fermented in the rumen to accommodate the animal’s requirements for glucose irreversible loss solely from dietary starch. In that physiologically untenable situation, over 4 kg of starch would be either digested in the large intestine or excreted in the feces.
Need for Further Research
The relationship between maximal glucose production (supply) and the most efficient use of ME for carcass growth (demand) is not clear. We believe that there is likely a minimal level of glucose that must be oxidized, but clear identification of that level is needed. Also needed is more basic information on the relationship between supply of glucose and other oxidizable substrates at production levels of energy intake.
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Footnotes |
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2 Corresponding author: Gerald_Huntington{at}ncsu.edu
Received for publication August 11, 2005. Accepted for publication October 6, 2005.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONRUMINAL FERMENTATION OF STARCHPOSTRUMINAL STARCH DIGESTION AND...POSTABSORPTIVE METABOLISM OF...LITERATURE CITED |
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, 0.6; 
, 0.65; 
, 0.70; x, 0.75; 
, 0.80; •, 0.85; and +, 0.90. The figure shows that when small intestinal digestibility is less than 0.75, decreasing ruminal digestibility results in less efficient energy capture. Conversely, when small intestinal digestibility is greater than 0.75, decreasing ruminal digestibility results in more efficient energy capture.
