Small Intestinal Starch Digestion Research Articles

327 A Novel, in Vivo Model to Increase Endogenous Pancreatic and Small Intestinal Carbohydrase Activity to Evaluate Limitations of Small Intestinal Starch Digestion in Cattle

Abstract Consumption of high-grain diets by cattle can result in large quantities of starch that flow to the small intestine for potential enzymatic digestion. However, small intestinal starch digestion is potentially limited because of insufficient activity of pancreatic and/or small intestinal carbohydrases. Studies suggest regulation of carbohydrase activity is complex in cattle and perhaps uncoordinated in response to luminal carbohydrate flow. The objective of this experiment was to use postruminal casein infusion and exogenous glucagon-like peptide 2 (GLP-2) administration, known stimulators of pancreatic and small intestinal function, to evaluate their effects on pancreatic and small intestinal carbohydrase activities. Twenty-four Holstein steers [250 ± 23 kg body weight (BW)] were used in a randomized complete block design with a 2 × 2 factorial arrangement of treatments. Steers were fed an alfalfa cube-based diet to limit dietary starch intake. Steers received a continuous abomasal infusion of 4 g/kg of BW of raw corn starch combined with either 0 or 1.33 g/kg of BW of casein for 7 d. Steers received subcutaneous injections in two portions daily of excipient (0.5% bovine serum albumin) or 100 µg/kg of BW GLP-2. This resulted in four treatments: control, casein, GLP-2, and casein + GLP-2. At the end of the 7-d treatment period, steers were slaughtered for pancreatic and small intestinal tissue collection. Data were analyzed using the GLM procedure of SAS. Postruminal casein infusion increased (P ≤ 0.03) pancreatic mass by 12.6%, total pancreatic α-amylase activity by 50%, and postruminal starch disappearance from 96.7% to 99.3%. Exogenous GLP-2 increased (P < 0.01) total small intestinal mass and small intestinal mucosal mass by 1.2 kg and 896 g, respectively. Relative to control, GLP-2 and casein + GLP-2 treatments increased (P = 0.04) total small intestinal α-glucosidase activity (sum of maltase, isomaltase, glucoamylase) by 83.5%. Total small intestinal maltase, isomaltase, and glucoamylase activity was 90%, 100%, and 66.7% greater for GLP-2 and casein + GLP-2 steers compared with control. The use of postruminal casein infusion and exogenous GLP-2 administration resulted in four distinct carbohydrase activity phenotypes. Relative to control, casein increased pancreatic α-amylase activity, GLP-2 increased small intestinal α-glucosidase activity, and the combination of casein and GLP-2 increased both pancreatic α-amylase activity and small intestinal α-glucosidase activity. This approach provides a novel in vivo animal model to selectively increase endogenous carbohydrase activity to overcome complications of coordinating carbohydrase activity with luminal nutrient flows. This model could be useful to understand physiological mechanisms that might improve small intestinal starch assimilation and efficiency of cattle consuming high-grain diets.

Postruminal Casein Infusion and Exogenous Glucagon-Like Peptide 2 Administration Differentially Stimulate Pancreatic α-Amylase and Small Intestinal α-Glucosidase Activity in Cattle

BackgroundIncreasing luminal carbohydrate flow decreases pancreatic α-amylase activity but can increase jejunal maltase activity, suggesting that regulation of carbohydrase activity is perhaps uncoordinated in response to luminal carbohydrate flow. Increasing luminal casein flow increases pancreatic α-amylase activity in cattle, and exogenous glucagon-like peptide 2 (GLP-2) has been shown to increase small intestinal α-glucosidase activity in nonruminants. ObjectivesThe objective was to evaluate the effects of postruminal casein infusion, exogenous GLP-2, or their combination on endogenous pancreatic and small intestinal carbohydrase activity in cattle postruminally infused with starch. MethodsHolstein steers [n = 24; 250 ± 23 kg body weight (BW)] received a continuous abomasal infusion of 3.94 g raw corn starch/kg of BW combined with either 0 or 1.30 g casein/kg of BW. Steers received subcutaneous injections in 2 equal portions daily of excipient (0.5% bovine serum albumin) or 100 μg GLP-2/kg of BW per day. At the end of the 7-d treatment period, steers were slaughtered for tissue collection. Data were analyzed using the MIXED procedure of SAS version 9.4 (SAS Institute Inc.). ResultsPostruminal casein infusion increased (P ≤ 0.03) pancreatic mass by 12.6%, total pancreatic α-amylase activity by 50%, and postruminal starch disappearance from 96.7% to 99.3%. Exogenous GLP-2 increased (P < 0.01) total small intestinal and mucosal mass by 1.2 kg and 896 g, respectively. Relative to control, GLP-2 and casein + GLP-2 increased (P = 0.04) total small intestinal α-glucosidase activity by 83.5%. Total small intestinal maltase, isomaltase, and glucoamylase activity was 90%, 100%, and 66.7% greater for GLP-2 and casein + GLP-2 steers compared with control. ConclusionsCasein increased pancreatic α-amylase activity, GLP-2 increased small intestinal α-glucosidase activity, and the combination of casein and GLP-2 increased both pancreatic α-amylase activity and small intestinal α-glucosidase activity. This novel approach provides an in vivo model to evaluate effects of increasing endogenous carbohydrase activity on small intestinal starch digestion.

Effects of long-term postgastric infusion of casein or glutamic acid on small intestinal starch digestion and energy balance in cattle.

The objective of this experiment was to evaluate effects of postruminal flows of casein or glutamic acid on small intestinal starch digestion and to quantify changes in energy and nutrient balance. Twenty-four steers (body weight = 179 ± 4 kg) were duodenally infused with raw cornstarch (1.46 ± 0.04 kg/d) and either 413 ± 7.0 g casein/d, 121 ± 3.6 g glutamic acid/d or water (control). Measures of small intestinal starch digestion and nutrient excretion were collected across 4 d after 42 d of infusion and measures of respiration via indirect calorimetry were collected across 2 d after 48 d of infusion. Ileal starch flow was least among calves provided casein, but ileal starch flow was not different between glutamic acid or control. Small intestinal starch digestion tended to be greatest among calves provided casein, least for glutamic acid and intermediate for control. Casein increased ileal flow of ethanol soluble oligosaccharides compared to glutamic acid and control. Large intestinal starch digestion was not different among treatments. By design, N intake was greatest among cattle provided casein, intermediate among calves provided glutamic acid and least for control. Nitrogen retention was greater in response to casein compared to control and glutamic acid. Intake of gross energy from feed was similar across treatments, and gross energy from infusate was greatest for casein, intermediate for glutamic acid and least for control. Variation in gross energy intake from feed resulted in no difference in overall gross energy intake across treatments. Similar to measures of small intestinal starch digestion and N retention, casein increased calories of digestible energy and metabolizable energy, compared to glutamic acid and control, which did not differ. Postruminal infusions did not influence methane production, but heat production was greatest in steers infused with casein, intermediate for steers provided glutamic acid, and least for control. Overall, amounts of energy retained by casein tended to be nearly 34% greater than control, but glutamic acid had no impact on energy balance. Improvement in small intestinal starch digestion in response to casein increased energy and N retained; however, glutamic acid did not influence small intestinal starch digestion and energy or N balance in cattle, which seems to suggest that responses in small intestinal starch digestion to greater postruminal flows of glutamic acid become refractory across greater durations of time.

388 Young Scholar Award Talk: Increasing Efficiency of Finishing Beef Production: Optimizing Starch Digestion, Improving Flaking Quality, and Reducing Methane Emissions

Abstract The objectives of my dissertation research are to develop strategies to increase the efficiency of finishing beef production by optimizing the site and extent of starch digestion, improving flaking quality and consistency, and reducing methane emissions. Post-flaking sampling and handling techniques such as sifting steam-flaked corn samples with a sieve (P &amp;lt; 0.01), air equilibration time (P &amp;lt; 0.01), and storage temperature during shipping (P &amp;lt; 0.01) all influenced enzymatic starch availability of steam-flaked corn. Inducing starch retrogradation of steam-flaked corn decreased the rate (P &amp;lt; 0.01), but not the extent (P = 0.80), of ruminal dry matter degradation and thus, energetic efficiency was predicted to increase by up to 3.4% because of increased postruminal flow and digestion of starch in the small intestine. If producing steam-flaked corn is not feasible for smaller capacity feedlots, finely ground corn has a similar ruminal starch solubility to steamflaked corn (38.1% vs. 44.4, respectively, P &amp;gt; 0.05), suggesting an alternative processing method to increase feeding value in finishing diets. Duodenal infusion of glucoamylase (fungal origin) increased (P = 0.02) small intestinal starch disappearance in steers, while duodenal infusion of porcine α-amylase did not (P = 0.41), supporting the hypothesis that small intestinal carbohydrases are the primary hydrolytic limit to small intestinal starch digestion. Comparative sequencing analyses revealed a missing region of the stalk region of the sucrase-isomaltase protein sequence (AA 47-60) which is important for intracellular sorting of the sucrase subunit to the apical membrane. The absence of this region of the protein likely explains the absence of intestinal sucrase activity and quantitative limitations of starch digestion in the ruminant small intestine. Feeding decoquinate with Yucca schidigera extract for 7-d decreased (P &amp;lt; 0.01) ruminal acetate proportion and tended to increase ruminal (P = 0.08) propionate proportion, suggesting potential for hydrogen shifts which could contribute to reductions in methane emissions with longer feeding periods. The findings of my dissertation research provide: 1) post-flaking sampling and handling recommendations for commercial feedlots and laboratories to improve estimates of starch availability and flaking consistency, 2) a mechanistic target (sucrase-isomaltase) for limitations in small intestinal starch digestion, 3) practical feeding strategies to shift the site of starch digestion to the small intestine to increase feed efficiency (retrogradation), 4) new processing alternatives for small feedlot producers (fine ground corn inclusion), and 5) new information on how combinations of feed additives could potentially decrease methane production in feedlot cattle.

Nutritional and Physiological Constraints Contributing to Limitations in Small Intestinal Starch Digestion and Glucose Absorption in Ruminants

Increased efficiency of nutrient utilization can potentially be gained with increased starch digestion in the small intestine in ruminants. However, ruminants have quantitative limits in the extent of starch disappearance in the small intestine. The objective is to explore the nutritional and physiological constraints that contribute to limitations of carbohydrate assimilation in the ruminant small intestine. Altered digesta composition and passage rate in the small intestine, insufficient pancreatic α-amylase and/or small intestinal carbohydrase activity, and reduced glucose absorption could all be potentially limiting factors of intestinal starch assimilation. The absence of intestinal sucrase activity in ruminants may be related to quantitative limits in small intestinal starch hydrolysis. Multiple sequence alignment of the sucrase-isomaltase complex gives insight into potential molecular mechanisms that may be associated with the absence of intestinal sucrase activity, reduced capacity for intestinal starch digestion, and limitations in the efficiency of feed utilization in cattle and sheep. Future research efforts in these areas will aid in our understanding of small intestinal starch digestion and glucose absorption to optimize feeding strategies for increased meat and milk production efficiency.

Duodenal Infusions of Starch with Casein or Glutamic Acid Influence Pancreatic and Small Intestinal Carbohydrase Activities in Cattle

Small intestinal starch digestion in ruminants is potentially limited by inadequate production of carbohydrases. Previous research has demonstrated that small intestinal starch digestion can be improved by postruminal supply of casein or glutamic acid. However, the mechanisms by which casein and glutamic acid increase starch digestion are not well understood. The objective of this experiment was to evaluate the effects of duodenal infusions of starch with casein or glutamic acid on postruminal carbohydrase activities in cattle. Twenty-two steers [mean body weight (BW) = 179±4.23kg] were surgically fitted with duodenal and ileal cannulas and limit-fed a soybean hull-based diet containing small amounts of starch. Raw cornstarch (1.61±0.0869kg/d) was infused into the duodenum alone (control), or with 118±7.21g glutamic acid/d, or 428±19.4g casein/d. Treatments were infused continuously for 58 d and then steers were killed for tissue collection. Activities of pancreatic (α-amylase) and intestinal (maltase, isomaltase, glucoamylase, sucrase) carbohydrases were determined. Data were analyzed as a randomized complete block (replicate group) design using the GLM procedure of SAS to determine effects of infusion treatment. Duodenal casein infusion increased (P <0.05) pancreatic α-amylase activity by 290%. Duodenal glutamic acid infusion increased (P <0.03) duodenal maltase activity by 233%. Duodenal casein infusion increased jejunal maltase (P=0.02) and glucoamylase (P=0.03) activity per gram protein by 62.9% and 97.4%, respectively. Duodenal casein infusion tended to increase (P=0.10) isomaltase activity per gram jejunum by 38.5% in the jejunum. Sucrase activity was not detected in any segment of the small intestine. These results suggest that small intestinal starch digestion can be improved in cattle with increased small intestinal flow of casein through increases in postruminal carbohydrase activities.

RUMINANT NUTRITION SYMPOSIUM: Effects of postruminal flows of protein and amino acids on small intestinal starch digestion in beef cattle.

Many nutritionists adopt feeding strategies designed to increase ruminal starch fermentation because ruminal capacity for starch degradation often exceeds amounts of starch able to be digested in the small intestine of cattle. However, increases in fermentable energy supply are positively correlated with increased instances of metabolic disorders and reductions in DMI, and energy derived by cattle subsequent to fermentation is less than that derived when glucose is intestinally absorbed. Small intestinal starch digestion (SISD) appears to be limited by α-glycohydrolase secretions and a precise understanding of digestion of carbohydrates in the small intestine remains equivocal. Interestingly, small intestinal α-glycohydrolase secretions are responsive to luminal appearance of milk-specific protein (i.e., casein) in the small intestine of cattle, and SISD is increased by greater postruminal flows of individual AA (i.e., Glu). Greater flows of casein and Glu appear to augment SISD, but by apparently different mechanisms. Greater small intestinal absorption of glucose has been associated with increased omental fat accretion even though SISD can increase NE from starch by more than 42% compared to ruminal starch degradation. Nonetheless, in vitro data suggest that greater glucogenicity of diets can allow for greater intramuscular fat accretion, and if greater small intestinal absorption of glucose does not mitigate hepatic gluconeogenesis then increases in SISD may provide opportunity to increase synthesis of intramuscular fat. If duodenal metabolizable AA flow can be altered to allow for improved SISD in cattle, then diet modification may allow for large improvements in feed efficiency and beef quality. Few data are available on direct effects of increases in SISD in response to greater casein or metabolizable Glu flow. An improved understanding of effects of increased SISD in response to greater postruminal flow of Glu and casein on improvements in NE and fates of luminally assimilated glucose could allow for increased efficiency of energy use from corn and improvements in conversion of corn grain to beef. New knowledge related to effects of greater postruminal flow of Glu and casein on starch utilization by cattle will allow nutritionists to more correctly match dietary nutrients to cattle requirements, thereby allowing large improvements in nutrient utilization and efficiency of gain among cattle fed starch-based diets.

749 Effects of postruminal flows of protein and amino acids on small intestinal starch digestion in cattle

A large amount of cattle in the United States are fed starch-based diets because limitations to efficient production of meat and milk by cattle are frequently associated with limits in NE. Many nutritionists adopt feeding strategies designed to increase ruminal starch fermentation because ruminal capacity for starch degradation often exceeds amounts of starch able to be digested in the small intestine of cattle. However, increases in fermentable energy supply are positively correlated with increased instances of metabolic disorders and reductions in DMI, and energy derived by cattle subsequent to fermentation is less than that derived when glucose is intestinally absorbed. Apparently, small intestinal starch digestion (SISD) is limited by α-glycohydrolase secretions and a precise understanding of digestion of carbohydrates in the small intestine remains equivocal. Interestingly, small intestinal α-glycohydrolase secretions are responsive to luminal appearance of high-quality protein (i.e., casein) in the small intestine of cattle, and SISD is increased by greater postruminal flows of individual AA (i.e., Glu). Indeed, greater flows of casein and Glu appear to augment SISD but by apparently different mechanisms. Greater small intestinal absorption of glucose has been associated with increased omental fat accretion even though SISD can increase NE from starch by more than 42% compared to ruminal starch degradation. Nonetheless, in vitro data suggest that greater glucogenicity of diets can allow for greater intramuscular fat accretion, and if greater small intestinal absorption of glucose does not mitigate hepatic gluconeogenesis then increases in SISD may provide opportunity to increase synthesis of intramuscular fat. If duodenal metabolizable AA flow can be altered to allow for improved SISD in cattle, then diet modification may allow for large improvements in feed efficiency and beef quality. Few data are available on direct effects of increases in SISD in response to greater casein or metabolizable Glu flow. An improved understanding of effects of increased SISD in response to greater postruminal flow of Glu and casein on improvements in NE and fates of luminally assimilated glucose could allow for increased efficiency of energy use from corn and improvements in conversion of corn grain to beef. New knowledge related to effects of greater postruminal flow of Glu and casein on starch utilization by cattle will allow nutritionists to more correctly match dietary nutrients to cattle to allow large improvements in nutrient utilization and efficiency of gain among cattle fed starch-based diets.

A mechanistic model of small intestinal starch digestion and glucose uptake in the cow

The high contribution of postruminal starch digestion (up to 50%) to total-tract starch digestion on energy-dense, starch-rich diets demands that limitations to small intestinal starch digestion be identified. A mechanistic model of the small intestine was described and evaluated with regard to its ability to simulate observations from abomasal carbohydrate infusions in the dairy cow. The 7 state variables represent starch, oligosaccharide, glucose, and pancreatic amylase in the intestinal lumen, oligosaccharide and glucose in the unstirred water layer at the intestinal wall, and intracellular glucose of the enterocyte. Enzymatic hydrolysis of starch was modeled as a 2-stage process involving the activity of pancreatic amylase in the lumen and of oligosaccharidase at the brush border of the enterocyte confined within the unstirred water layer. The Na+-dependent glucose transport into the enterocyte was represented along with a facilitative glucose transporter 2 transport system on the basolateral membrane. The small intestine is subdivided into 3 main sections, representing the duodenum, jejunum, and ileum for parameterization. Further subsections are defined between which continual digesta flow is represented. The model predicted nonstructural carbohydrate disappearance in the small intestine for cattle unadapted to duodenal infusion with a coefficient of determination of 0.92 and a root mean square prediction error of 25.4%. Simulation of glucose disappearance for mature Holstein heifers adapted to various levels of duodenal glucose infusion yielded a coefficient of determination of 0.81 and a root mean square prediction error of 38.6%. Analysis of model behavior identified limitations to the efficiency of small intestinal starch digestion with high levels of duodenal starch flow. Limitations to individual processes, particularly starch digestion in the proximal section of the intestine, can create asynchrony between starch hydrolysis and glucose uptake capacity.

Increases in duodenal glutamic acid supply linearly increase small intestinal starch digestion but not nitrogen balance in cattle.

Small intestinal starch digestion (SISD) in cattle is often limited; however, greater postruminal flow of high-quality protein (e.g., casein) can increase SISD, and Glu can mimic responses to casein for SISD. We evaluated effects of increasing Glu flows to the duodenum on SISD and N retention in cattle. Cattle received (DM basis) continuous duodenal infusion of raw cornstarch (1.5 ± 0.08 kg/d) and 0, 30.9 ± 0.6, 62.4 ± 1.2, or 120.4 ± 3.4 g/d Glu or 387.9 ± 17.5 g/d casein. As expected, the positive control (i.e., casein) increased ( = 0.05) SISD. Interestingly, SISD linearly increased ( = 0.02) with increasing amounts of Glu. Starch flow to the ileum linearly decreased ( = 0.04) in response to greater postruminal Glu and tended to decrease ( = 0.07) with duodenal casein infusion. Ileal flow of ethanol-soluble starch was not affected by duodenal Glu ( = 0.16) or casein ( = 0.42). There was a tendency ( = 0.08) for a quadratic response to Glu for ileal glucose flow with greater flows for intermediate levels of Glu, but casein had no effect ( = 0.81) on glucose flows to the ileum. Greater postruminal supplies of Glu (linear, = 0.05) and casein ( = 0.02) decreased fecal starch flow. Postruminal starch digestion was increased by both casein ( = 0.03) and Glu (linear, = 0.05). Nitrogen intake from feed was not different among treatments ( ≥ 0.23). By design, infusate N increased from 0 to 13 ± 1.5 g/d with greater amounts of Glu, and casein provided 61 ± 1.3 g N/d. Urinary N excretion was not affected ( ≥ 0.30) by postruminal Glu flow, but urine N was increased by casein ( < 0.01). Glutamic acid did not affect N retention ( ≥ 0.34), but casein increased N retention ( < 0.01). However, N retained as a percent of N intake (26.7 ± 1.7%) was not different when cattle were provided Glu ( ≥ 0.16) or casein ( = 0.38).

Influence of Steam-Pelleting Temperatures and Grain Variety of Finely-Ground, Sorghum-Based Broiler Diets on Small Intestinal Starch and Nitrogen Digestion Dynamics in Broiler Chickens

The effects of sorghum grain variety and conditioning temperature at which broiler diets were steam-pelleted on small intestinal digestion dynamics of starch and nitrogen were assessed. A 2×3 factorial array of treatments comprised two sorghum varieties, with white (Liberty) and red (Venture) pericarps and three conditioning temperatures (70, 80, 90°C). Both sorghums were finely-ground through a 2.0 mm hammer-mill screen prior to incorporation into the diets. Each of the dietary treatments was offered to six replicate cages (6 birds per cage) from 7 to 28 days post-hatch. Digesta samples were collected at day 28 for determination of starch and nitrogen digestibilities and mean retention time in the proximal jejunum, distal jejunum, proximal ileum and distal ileum. The digestion dynamics of starch and nitrogen were determined using an exponential mathematical model to relate digestion coefficients with mean retention times in each small intestinal segment. Increasing conditioning temperatures generated a 'concav e response' in starch digestion rates in white sorghum diets; whereas, there was a 'convex response' in red sorghum diets. Thus, at the intermediate 80°C conditioning temperature, starch digestion rates were at a minimum value for white, but at a maximum value for red sorghum-based diets, which is consistent with the significant sorghum type×temperature interaction (p<0.05). Conditioning temperatures increased potential digestible starch (r = 0.336, p<0.05), predicted glucose absorption (r = 0.468, p<0.01), retention time in the distal ileum (r = 0.362, p<0.05) and the entire small intestine (r = 0.371, p<0.05). There was a quadratic relationship between potential digestible starch (p<0.05) and Feed Conversion Ratio (FCR), which indicated that the optimal FCR was attained with a potential digestible starch value of 0.862. These results suggested that starch and protein digestion may influence feed conversion efficiency.

Digestion site of starch from cereals and legumes in lactating dairy cows

The effect of grinding and rolling ( i.e. processing) of cereals and legumes ( i.e. source) on site of starch digestion in lactating dairy cows was tested according to a 2×2 factorial design using a dataset derived from an overall dataset compiled from four experiments conducted at our laboratory that investigated effects of various processing techniques on digestion kinetics. Data originated from four experiments with cows cannulated in the rumen, duodenum and ileum, and fed rations composed of the respective starch source and grass–clover silage balanced with soy bean meal and a mineral premix. In addition to apparent digestibilities of starch in the rumen, small intestine and hind gut, true ruminal starch digestibilties and duodenal flow of microbial starch was estimated by regression analysis. There was an interaction between starch source and processing, as the apparent and true ruminal digestibility of starch was decreased by rolling for legumes, whereas the three other source by processing combinations did not differ. The duodenal flow of microbial starch was estimated to 276 g/d as the intercept in the regression analysis. Apparent ruminal digestibilities of starch seemed to underestimate true ruminal digestibility in rations with low starch intake due to a relatively higher contribution of microbial starch to total duodenal starch flow compared to rumen escape feed starch. The small intestinal and total tract digestibility of legume starch was lower compared with starch from cereals. Rolling reduced the small intestinal and total tract digestibility of starch from both cereals and legumes compared to grinding. Reductions in ruminal starch digestion were generally not associated with increases in small intestinal starch digestion, but were associated with increases in hind gut and reductions in total tract digestion of starch. Starch from legumes had lower ruminal and small intestinal digestibilities compared to starch from barley and wheat, indicating that action of bacterial and pancreatic amylase is limited by the same factors. Rolling did not provide sufficient physical processing of legume seeds to obtain a high digestibility of starch before the hind gut.

Effects of supplemental fat source on nutrient digestion and ruminal fermentation in steers1

Five Holstein steers (235 kg of BW) fitted with ruminal, duodenal, and ileal cannulas were used in a 5 x 5 Latin square design experiment to determine the effects of supplemental fat source on site and extent of nutrient digestion and ruminal fermentation. Treatments were diets based on steam-flaked corn containing no supplemental fat (control) or 4% (DM basis) supplemental fat as tallow, dried full-fat corn germ (corn germ), corn oil, or flax oil. Fat supplementation decreased (P < 0.08) ruminal starch digestion but increased (P < 0.03) small intestinal starch digestion as a percentage of intake. Feeding corn germ decreased (P < 0.09) ruminal starch digestion and increased (P < 0.03) large intestinal starch digestion compared with steers fed corn oil. Large intestinal starch digestion was less (P < 0.04), and ruminal NDF digestion was greater (P < 0.09) for steers fed tallow compared with steers fed other fat sources. Small intestinal (P < 0.08) and total tract NDF digestibilities were greater (P < 0.02) for steers fed corn germ than for those fed corn oil. Feeding tallow increased total ruminal VFA (P < 0.03) and NH(3) (P < 0.07) concentrations compared with steers fed the other fat sources. Feeding corn germ led to a greater (P < 0.02) rate of ruminal liquid outflow compared with corn oil. A diet x hour interaction (P < 0.04) occurred for ruminal pH, with steers fed corn oil having the greatest ruminal pH 18 h after feeding, without differences at other time points. Fat supplementation increased (P < 0.09) ruminal concentrations of Fusobacterium necrophorum. Duodenal flow of C18:3n-3 was greater (P < 0.01) for steers fed flax oil compared with those fed corn oil. Feeding corn germ led to less (P < 0.01) ruminal biohydrogenation of fatty acids compared with corn oil. Steers fed tallow had greater small intestinal digestibility of C14:0 (P < 0.02) and C16:1 (P < 0.04) than steers fed the other fat sources. Fat supplementation decreased (P < 0.06) small intestinal digestibility of C18:0. Feeding corn germ decreased (P < 0.10) small intestinal digestibility of C18:1 compared with corn oil. It appears that source of supplemental fat can affect the site and extent of fatty acid and nutrient digestion in steers fed diets based on steam-flaked corn.

Factors affecting intestinal starch digestion in ruminants: A review

The process of starch assimilation in the ruminant is complex and remains an avenue by which increases in production efficiency can be gained. Ruminal starch digestion is typically 0.75–0.80 of starch intake. Starch that escapes fermentation and flows to the small intestine may be more resistant to enzymatic digestion and on average 0.35–0.60 of starch entering the small intestine is degraded there. Of the fraction that escapes small intestinal digestion an additional 0.35–0.50 is degraded in the large intestine. This suggests that limitations to small intestinal starch digestion do exist. This review summarizes available information describing the digestive and absorptive processes occurring in the small intestine with an emphasis on nutritional factors that influence these processes. A review of experiments measuring small intestinal starch digestion indicates that small intestinal digestion is either highly variable or poorly determined whereas ruminal and large intestinal digestion are much more clearly described. These data indicate that improvements in methodologies are needed before we can accurately describe processes occurring in the small intestine and formulate diets to optimize site of starch digestion. Key words: Ruminant, starch, glucose, small intestine, digestion

Application of a dynamic mechanistic model of small intestinal starch digestion in the dairy cow

The high contribution of postruminal starch digestion (&gt;50%) to total tract starch digestion on certain energy dense diets (Mills et al. 1999) demands that limitations to small intestinal starch digestion are identified. Therefore, a dynamic mechanistic model of the small intestine was constructed and evaluated against published experimental data for abomasal carbohydrate infusions in the dairy cow. The mechanistic structure of the model allowed the current biological knowledge to be integrated into a system capable of identifying restrictions to dietary energy recovery from postruminal starch delivery.

Abomasal glucose, maize starch and maize dextrin infusions in cattle: small-intestinal disappearance, net portal glucose flux and ileal oligosaccharide flow.

Three castrated male Holstein cattle (423 (SD 19)kg live weight) fitted with elevated carotid artery, portal, and mesenteric venous catheters, and abomasal and ileal cannulas were used to study small-intestinal starch digestion. The cattle were infused abomasally with water (275 ml/h), glucose (66 g/h), maize dextrin (66 g/h) or maize starch (66 g/h) in an incomplete Latin square design, with eight infusion periods. Infusion with carbohydrate resulted in higher arterial glucose concentrations and greater net portal glucose flux than when cattle were infused with water. Arterial glucose concentration and net portal glucose flux were highest when glucose was infused. In the small intestine, 85% of abomasally-infused glucose, 78% of infused dextrin, and 66% of infused starch disappeared. Of the carbohydrate that disappeared in the small intestine, that which could be accounted for as net portal glucose flux was 73% for glucose, 60% for dextrin, and 57% for starch. Ileal digesta contained unpolymerized glucose, and short-chain soluble alpha-glucoside. Of the infused dextrin flowing past the ileum (14 g/h), 0.3 g/h was glucose, 6.2 g/h was soluble alpha-glucoside, and 7.5 g/h was insoluble alpha-glucoside. Of the infused starch flowing at the ileum (22.2 g/h), 0.9 g/h was glucose, 5.3 g/h was soluble alpha-glucoside, and 15.9 g/h was insoluble alpha-glucoside. The average chain lengths of the soluble alpha-glucosides in ileal digesta were 2.07 and 2.36 for dextrin and starch infusions respectively, indicating mostly di- and to a lesser extent trisaccharides. We conclude that (1) when 66 g raw starch is presented to the small intestine per h, about half of the intestinal disappearance appears as glucose in the portal vasculature, and (2) alpha-1,4 glucosidase (EC 3.2.1.20) activity at the brush border is the rate-limiting step to small-intestinal starch digestion in cattle.

Small intestinal starch digestion in steers: effect of various levels of abomasal glucose, corn starch and corn dextrin infusion on small intestinal disappearance and net glucose absorption.

Eight Holstein steers (four at 300 kg, four at 406 kg) fitted with an elevated carotid artery, hepatic portal and mesenteric venous catheters, and abomasal and ileal cannulas were used in several 4 x 4 Latin square experiments to evaluate small intestinal starch digestion. They were fed alfalfa hay at 1.5% of BW and abomasally infused with water or glucose, corn starch or corn dextrin (one carbohydrate per Latin square) at 20, 40 or 60 g/h, with subsequent determination of small intestinal disappearance and net portal glucose absorption. Increasing the amount of all three carbohydrates infused abomasally increased the amount of carbohydrate disappearing in the small intestine. Increased infusion of glucose caused a continual increase (linear, P less than .01) in net glucose absorption, whereas net glucose absorption for starch and dextrin was maximal at the 20 g/h infusion (quadratic, P less than .05). With the 60 g/h infusion, 94% of the glucose but only 38% of starch and 29% of small intestinal dextrin disappearance could be accounted for as net glucose absorption, leaving a large portion of starch and dextrin disappearance unaccounted for. Of the infused starch and dextrin passing the ileum, 5.8 and 7.3%, respectively, was unpolymerized glucose, indicating that, at least in the distal small intestine, complete starch hydrolysis exceeded the capacity for glucose disappearance. It is concluded that only about 35% of the raw corn starch disappearing in the steer's small intestine resulted in net portal glucose absorption.

Small intestinal starch, dextrin, and glucose digestion in steers (1988)

Three Holstein steers (930 lb) were surgically fitted with abomasal and ileal cannulae, portal and mesenteric venous catheters, and an elevated carotid artery and used to study small intestinal starch digestion. Water, corn starch (66 g/hr), corn dextrin (66 g/hr), or glucose (66 g/hr) were continuously infused into the abomasum. Small intestinal disappearance of corn dextrin (57 g/hr) and glucose (57 g/hr) were higher (P<.05) than that of starch (48 g/hr). The percentage of carbohydrate disappearance accounted for as net portal glucose flux was 52, 54, and 72% for corn starch, corn dextrin, and glucose, respectively. Small intestinal starch utilization in the bovine may be limited by starch granular characteristics, enzyme activity, and glucose transport across the small intestine.; Dairy Day, 1988, Kansas State University, Manhattan, KS, 1988;

Effect of dietary corn starch intake on ruminal, small intestinal and large intestinal starch digestion in cattle.

In Exp. 1, 24 yearling Holstein steers averaging 340 kg were fed either an alfalfa hay diet at a maintenance level of metabolizable energy (ME) intake or corn silage-corn diets at one, two or three times maintenance ME intake. After a 42-day adjustment period, steers were fed individually, and digestibilities of total alpha-glucosides, starch oligosaccharides and glucose were determined at 2-week intervals, with chromic oxide used as an indicator. Steers fed the alfalfa hay diet had higher (P less than .05) total tract digestibilities of total alpha-glucosides and starch than steers fed the corn diets. Fecal starch (percentage of dry matter) in steers fed the corn diets increased (P less than .05) from approximately 11 to 31% as level of ME intake increased from one to three times maintenance. Starch digestibilities for the corn diets fed at one, two and three times maintenance were 81.4, 76.4 and 76.0%, respectively. However, these trends toward reduced starch digestibilities were not significant. There appeared to be no apparent adaption of alpha-glucoside digestibility in the total digestive tract among steers fed different levels of corn over the intervals observed. In Exp. 2, four Holstein steers (350 KH) were each fitted with duodenal and ileal reentrant cannulas and fed either a low or a high level of corn. Alpha-glucoside intakes for animals given the low and high levels of cord averaged 1.7 and 3.2 kg, respectively. Steers fed the high level of corn digested more (p less than .05) alpha-glucoside in the total tract (2.9 vs 1.6 kg), reticulo-rumen (2.3 vs 1.2 kg) and large intestine level of corn. Steers fed the high level of corn also digested more corn in the small intestine (.415 vs .221 kg) than steers fed the lower level; however, differences were not significant. Although there were trends toward lower partial digestion coefficients (expressed as a percentage of alpha-glucoside presented to that segment) in the total tract, reticulo-rumen and small intestine for steers fed the high corn diet, the magnitude of the differences was not significant.