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Articles

Starch Digestion Rate in the Small Intestine of Broiler Chickens Differs among Feedstuffs¹

Roelof E. Weurding^*,,**2, Albertus Veldman^*, Willem A. G. Veen^*, Petrus J. van der Aar^* and Martin W. A. Verstegen^**

^* Institute for Animal Nutrition, De Schothorst, P.O. Box 533, 8200 AM Lelystad, The Netherlands; Brameco · ZON, P.O. Box 8510, 5605 KM Eindhoven, The Netherlands; and ^** Animal Nutrition Group, Wageningen University & Research Center, P.O. Box 338, 6700 AH Wageningen, The Netherlands

²To whom correspondence should be addressed. E-mail: eweurding{at}schothorst.nl.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Dietary starch is the major energy source for broiler chickens, and knowledge about its digestive behavior can be important. In a digestibility trial with 720 broiler chickens, site, rate and extent of starch digestion were measured for 12 feedstuffs. Starch digestion was determined using the slaughter technique, which involves removal of the small intestine from the recently killed chicken, with manual collection of the contents. Starch digestion coefficients were calculated from remaining starch in three segments of the small intestine and in excreta. Mean retention time in four segments of the small intestine was measured. This enabled calculations for starch digestion rate (k_d). Ileal starch digestibility varied from 33% (potato starch) to 99% (tapioca). Retention time for digesta in the postduodenal small intestine varied from 136 min (barley diet) to 182 min (potato diet). On the basis of starch digestion rates, a distinction was made between slowly digestible starch (k_d < 1 h^-1), gradually digestible starch (k_d:1–2 h^-1) and rapidly digestible starch (k_d > 2 h^-1). Starch from common beans was digested most slowly (k_d: 0.5 h^-1), and starch from tapioca was digested most rapidly (k_d: 4.3 h^-1). Starch digestion rates of potato starch and legume seeds were lower than those of cereal grains and tapioca. Degradation of starch entering the hind gut of the birds did not occur. Milling of corn affected rate, but not the extent of starch digestion. We concluded that site of starch digestion within the small intestine is not an accurate indicator for starch digestion rate.

KEY WORDS: • starch • broiler chickens • digestion rate • retention time

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The apparent metabolizable energy (AME)³ content of broiler feeds is determined by measuring the energy value of the feed minus fecal and urinary energy (1) . The feed industry uses regression equations based on digestible nutrients to predict the AME content of feedstuffs. Studies have shown that the AME of wheat is positively correlated with starch digestibility (2 ,3) and inversely correlated with nonstarch polysaccharides (NSP) content (4) . In The Netherlands, crude protein, crude fat and nitrogen-free extract (NFE) are used in the AME formula (5) . Starch, the major energy supplier for broiler chickens, is not included as such, but it is part of the NFE fraction. This has consequences for the nutritional value, especially when the starch proportion in the NFE fraction is variable.

Incomplete starch digestion in broiler chickens was observed for wheat (2) , barley (6) , peas (7) and isolated starch from several feedstuffs (8) . According to Moran (9) , starch digestibility is a function of granule surface area, starch structure and degree of crystallinity. Granule size differs among feedstuffs, and small granules are generally digested more rapidly than larger granules (10) . Starch granules are composed of amylose and amylopectin molecules. Starch with a high amylose content is often considered to be less digestible. Starch structure is divided in A-, B- and C-type starch. A-type starch is more susceptible to enzymatic attack than B-type starch, and C-type starch is intermediate (11) . Factors not directly related to starch itself may also affect its digestibility. Starch granules can be encapsulated by a rigid protein matrix or by cell walls from the same feedstuff. This reduces the accessibility of starch granules to enzymatic attack (12) . Furthermore, other ingredients of the diet may also affect starch digestion. Soluble NSP in the diet increase digesta viscosity, possibly impairing starch digestion (12 ,13) . Starch digestion is also affected by animal-related factors, including age, feed intake, passage rate and absorption capacity. In human nutrition, much work has been done in the area of starch digestion. In this field, the term resistant starch (RS) was introduced. EURESTA (acronym for European Resistant Starch research group) defined RS as "... the sum of starch and products of starch degradation not absorbed in the small intestine of healthy individuals" (14) . Englyst et al. (15) partitioned RS into three separate fractions, i.e., physically inaccessible starch (RS₁), resistant starch granules (RS₂) and retrograded amylose (RS₃). Differences in the extent of starch digestion might be explained by the existence of an enzyme-resistant starch fraction, differences in starch digestion rate or both. There are indications that starch digestion rates differ among feedstuffs (8) . The dynamics of starch digestion may have considerable nutritional consequences. Rapid or slow starch digestion may elicit different metabolic responses in the animal (e.g., synchronization of protein and starch digestion, effect on insulin response, microbial fermentation). The objective of this experiment was to study the site, rate and extent of starch digestion of 12 different feedstuffs in broiler chickens. These in vivo starch digestion data will also serve as reference values for the development of an in vitro method that simulates passage through the alimentary tract of broiler chickens. Therefore, feedstuffs were selected to cover a wide range in starch characteristics such as starch structure, amylose content and granule size.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Experimental design.

In an in vivo experiment, starch digestion of 12 feedstuffs was studied. The experiment was performed with two batches of 360 female Ross broiler chickens (obtained from Cobroed, Lievelde, The Netherlands), housed in 36 pens. An experimental unit was formed by one pen containing 10 birds. The experiment was conducted in two periods (one batch per period). Each experimental treatment consisted of six replicates equally assigned to the periods.

Animals, housing and diets.

The experimental protocol was in agreement with the standards for animal experiments and was approved by the Ethical Committee of De Schothorst. Newborn chicks were kept in a warm environment (temperature decreased gradually from 30 to 24°C) and received a standard starter diet (supplied by feed cooperative Arkervaart-Twente, Nijkerk, The Netherlands) until they were 14 d old. At this age, chicks were assigned to dietary treatments on the basis of live weight and were transferred to battery cages (treatments equally distributed across floors). The cages were located in a room with 23 h light/d and ambient temperature (± 22°C). The chicks were fed one of 12 experimental diets (supplied by feed cooperative Arkervaart-Twente), varying in starch source until the end of the experiment. Birds were given free access to the diets, which were provided as a mash. Composition of the starch sources examined is presented in Table 1 ; the composition of the experimental diets is presented in Table 2 . All dietary starch originated from these 12 feedstuffs. Feedstuffs were incorporated into the diets to obtain similar starch contents. As can be seen from Table 2 , most diets contained only one starch source. Four diets contained two starch sources. Peas, common beans, horse beans and potato starch were each incorporated to a limited extent into the diets (250–300 g/kg) to avoid digestive disorders. Each of these diets was supplemented with common corn (hammer-milled) to achieve a starch content similar to the other diets. Three different corn diets were fed, differing in variety and milling treatment (Table 1) . All starch sources were milled by a hammer mill to pass a 2.75-mm screen. In addition, as a separate treatment, common corn was also roller-milled. Diets contained Cr₂O₃ as an indigestible marker (Table 2) .

Excreta were collected on d 27 and 28 twice a day (every 4 h) for the determination of total starch digestion to minimize changes in excreta composition after excretion. After collection, excreta were stored in the refrigerator (4°C) until d 29, when they were freeze-dried. On d 29, the birds were killed by an intravenous injection of T61, which is an aqueous solution containing 200 g embutramide, 50 g mebezoniumiodide and 5 g tetracainehydrochloride per liter (Hoechst Veterinär GmbH, München, Germany). Immediately after injection, the small intestine was removed. The mesentery was cut, and jejunum and ileum were separated at Meckel’s diverticulum. Both jejunum and ileum were split into two parts of equal length, namely, anterior jejunum (AJ), posterior jejunum (PJ), anterior ileum (AI) and posterior ileum (PI). Digesta were rinsed out of each segment (without squeezing) with demineralized water (4°C) into separate aluminum containers. Digesta were stored at -20°C and subsequently freeze-dried. After freeze-drying, the samples were preground with a pestle and mortar and subsequently ground in a Retsch mill to pass a 1-mm screen. Samples were analyzed for starch (except the contents of the anterior jejunum) and Cr₂O₃.

Chemical analyses and measurements.

Experimental diets and starch sources were analyzed for contents of dry matter, ash, nitrogen (Dumas), crude fat, crude fiber, starch and free glucose. Starch, free glucose and Cr₂O₃ were determined in experimental diets, and freeze-dried digesta and excreta. Starch was analyzed according to Englyst et al. (15) . Cr₂O₃ content was determined by wet destruction with a mixture of HNO₃/HClO₄ (1:1). The absorption of the hexavalent Cr atom, measured at a wavelength of 357.8 nm, is proportional to the Cr₂O₃ concentration in the sample. Particle size distribution of each starch source was determined by dry sieve analysis. For this measurement, 100 g material was put on top of a set of 7 sieves: 3.15, 2.5, 2.0, 1.4, 1.0, 0.6 and 0.2 mm. Sieves were vibrated with an amplitude of 2 mm for 4 min (with interruptions) and the weight of residues on top of each sieve was determined.

Calculations.

The following variables were calculated in three segments of the small intestine: digestion coefficient of starch (DC_S), absorption coefficient of glucose (AC_G), digestion coefficient of starch free dry matter and mean retention time (MRT). The formula used to calculate the digestion coefficient of starch was as follows:

where DC_s is the digestion coefficient of starch (starch and Cr₂O₃ in g/kg).

Starch was calculated as (total glucose - free glucose) x 0.9. A theoretical glucose absorption was calculated similarly. For this calculation, starch in the numerator was thus replaced by total glucose multiplied by the factor 0.9. It was assumed that all free glucose in digesta and excreta originated from starch. With this assumption, total glucose was related to the Cr₂O₃ content. Collected excreta from the last 2 d of the experimental period were used to calculate total starch digestion. DC_S values for common beans, peas, horse beans and potato starch were calculated from DC_S values of diets E, H, I and K and DC_S values of corn (hammer milled, diet B) which were corrected for differences in MRT. MRT was calculated using the following formula:

where MRT is the mean retention time (min), C is the Cr₂O₃ concentration in the digesta (mg/g), W is the weight of dry gut contents (g), I is the Cr₂O₃ intake over 24 h (mg feed intake · Cr₂O₃ content in feed) and 1440 equals min/d.

By relating the digestion coefficients to the mean retention time, starch digestion rate was calculated. MRT in the duodenum and rectum was not measured. On the basis of results of previous experiments at our institute and in the literature (16 ,17) , MRT in the duodenum was assumed to be 5 min and MRT in the rectum was assumed to be 20 min for all diets. We assumed that starch digestion did not occur prior to the small intestine. The shape of the digestion curve was assumed to follow first-order kinetics and the following equation was used to estimate the digestibility characteristics:

where DC_t is the part of ingested starch digested at time t. Fraction D is the asymptote, which is the potentially digestible starch fraction that will digest at a rate of k_d (h^-1). A k_d-value of 2.00 means a starch digestion rate of 200%/h. The Marquardt method of the PROC NLIN procedure, an iterative curve-fitting procedure (18) , was used to reduce the residual sums of squares associated with the regression model.

Statistical analysis.

Before statistical analysis, the digestion coefficients were transformed by the logit transformation {ln [p/(100 - p)]} to meet statistical assumptions (normal distribution and homogeneity of variance). Differences in DC_S and MRT were tested according to the following statistical model:

where Y_ijkl equals DC_S or MRT, µ is the overall mean, P_i is the period effect (i = 1, 2), F_j is the floor effect (j = 1, 2, 3), SS_k is the effect of starch source (k = 1, ... , 12) and e_ijkl is the error term.

The effects were tested using the general linear model of the SAS package (18) . Differences were considered significant when P < 0.05. Mean comparisons were made using the least-square means option and least-square means were transformed back to the original scale for presentation.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Differences among starch sources were found in each segment, but they were most pronounced in the upper small intestine (Table 3). Starch digestion occurred mainly in the upper small intestine. On average, 90% of digested starch in cereal grains was digested before the ileum and 98% before the posterior ileum. These values were lower for common beans (50 and 87%, respectively), peas (71 and 91%, respectively), horse beans (70 and 92%, respectively) and potato starch (60 and 77%, respectively). A large difference indicates a gradual starch digestion along the small intestine. Tapioca starch digestion was almost complete in the upper small intestine. A substantial proportion of ingested starch from peas and beans was digested in the ileum (23–36%). A smaller proportion of potato starch (13%) and cereal starch (6–13%) was digested in this part of the small intestine.

Differences in potential starch digestibility (D) between feedstuffs (Table 4) were similar to those in ileal starch digestibility (Table 3) . Fractional starch digestion rates (k_d) differed considerably among feedstuffs. It is clear that a low extent of starch digestion is combined with a slow starch digestion. Total MRT in the jejunum and ileum (Table 5) varied from 136 min (barley diet) to 182 min (potato starch diet). MRT in the AJ was considerably shorter than in the three following segments. DC_S and MRT were negatively correlated (PJ: r = -0.76; AI: r = -0.74; PI: r = -0.57; n = 12). The MRT of the diet with roller-milled corn tended to be shorter than that of the diet with hammer-milled corn (P = 0.11).

Mean particle size varied from 0.3 (potato starch) to 1.7 mm (roller-milled corn). Most starch sources had a mean particle size between 0.8 and 1.2 mm (Table 6).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

This experiment clearly illustrated that for most feedstuffs, starch digestibility is high, but incomplete at the end of the ileum. The undigested starch fraction in the starch sources studied varied from 1% for tapioca pellets to 67% for raw potato starch. Undigested starch fractions of cereal grains and legume grains were between these two extremes. Cereal grains had an undigested starch fraction between 2 and 6%. In legume grains, this fraction varied from 19 to 28%. Undigested starch may serve as a substrate for bacteria present in the hind gut. However, starch fermentation is energetically less efficient than enzymatic starch digestion in the small intestine (21) . In our experiment, total starch digestion (measured in excreta) was the same as ileal starch digestion, indicating that the undigested starch fraction was not fermented in the hind gut. This observation is consistent with the observation that the undigested starch fraction is similar for conventional and germ-free chicks (22) . Short-chain fatty acid production has been observed in the broiler intestinal tract, mainly in the ceca (23) , implying fermentation of carbohydrates. In our experiment, starch digestion was calculated from remaining starch in the digesta, but fermentation of starch by microbes cannot be excluded. The differences in the undigested starch fractions among feedstuffs can be explained in part by the starch characteristics of the feedstuffs. Starch in cereal grains has an A-type structure, starch in legume grains and tapioca has a C-type structure and starch in potatoes has a B-type structure (24) . Amylose to amylopectin ratios are highest in legume grains (±0.33), followed by cereal grains (±0.25) (except rice) and are lowest in tapioca, potatoes and rice (±0.20) (25) . For some cereal grains, genotypes exist with higher (high amylose varieties) or lower (waxy varieties) amylose contents. Finally, starch granules in potatoes are larger than in most cereals and tapioca (25) . We assume that these distinctions in starch characteristics also applied to the batches used in this experiment.

Estimated potential starch digestibility D (Table 4) was similar to actual ileal starch digestion (Table 3) for cereal grains and tapioca. It appears that the undigested starch fraction in these feedstuffs is truly indigestible. However, in the case of legume grains and potato starch, estimated potential starch digestibility (D) was higher than ileal starch digestion. Apparently, digestion of potentially digestible starch was incomplete due to a combination of a slow starch digestion rate in these feedstuffs and the relatively short retention time in the gastrointestinal tract of broiler chickens.

Starch digestion in the upper small intestine was greatest for tapioca pellets, which had a very fine particle size distribution (Table 6) . Thus, the relative surface area was large. Furthermore, the pelleting process may have gelatinized part of the starch in the product (26) . Digestion coefficients in the AI showed the same ranking among feedstuffs as in the PI. The ranking of feedstuffs based on digestion coefficients in the PJ was different from that in the ileum. Waxy corn, sorghum and rice starch were slow starters compared with starch in other cereals. Wheat starch was digested to the same extent as barley starch in the PJ. Tapioca starch was digested very rapidly (98% was digested before the ileum). The SD calculated indicated that starch digestion coefficients within feedstuffs were less variable in the posterior parts of the small intestine compared with the anterior parts. AC_G is the proportion of ingested starch that has been theoretically absorbed in the form of glucose. The difference between DC_S and AC_G was most pronounced for common beans in all segments of the small intestine. The slow digestion of starch in this feedstuff (Table 4) ensures a constant release of glucose. When the majority of a starch is digested in the anterior parts of the small intestine, then most glucose will already be absorbed before the digesta reaches the more posterior parts. Another explanation may be that the absorptive capacity of the small intestine was reduced due to damage to the intestinal wall caused by lectins (27) .

The negative correlation between DC_S and MRT indicates that feedstuffs with a low starch digestion coefficient stay longer in the small intestine. Starch digestion data from Table 3 are end points that are determined by starch digestion rate and MRT. Thus, the site of starch digestion is not an accurate indicator of starch digestion rate because passage rate is also affected by diet.

Starch digestion rates varied considerably among feedstuffs. Common beans and raw potato starch had an extremely slow starch digestion (k_d: 0.5 h^-1). Starch from horse beans and peas was also digested slowly (k_d: 1.0 h^-1). This slow digestion is undoubtedly associated with the incomplete starch digestion in the birds. Waxy corn, sorghum and rice displayed a gradual starch digestion (k_d: 1.8–2.0 h^-1), whereas wheat, barley and hammer milled corn showed a rapid starch digestion (k_d: 2.5 h^-1). Roller-milled corn (k_d: 3.1 h^-1) and tapioca pellets (k_d: 4.3 h^-1) had extremely high starch digestion rates. No difference in starch digestion rates for wheat, barley and hammer-milled corn was observed. Wheat starch, however, was digested to a lesser extent. Apparently wheat contained a larger indigestible starch fraction.

Different milling treatments did not affect ileal starch digestibility of corn. In both cases, starch digestibility was 97%. Site of starch digestion also was not different for these two treatments (Table 3) . The starch digestion rate of roller-milled corn, however, was higher than that of hammer-milled corn. On the basis of the differences in mean particle size (Table 6) , a more rapid starch digestion was expected for hammer-milled corn. It is possible that the two milling treatments of corn not only resulted in different particle size distributions, but also changed other particle properties (shape of particles), thereby affecting feed passage rate (see Table 5 ) and starch digestion rate.

We conclude that ileal starch digestion varies considerably among different feedstuffs. Most of these differences originate in the upper small intestine. Ileal starch digestibility is therefore related to starch digestion rate. Some feedstuffs have the same ileal starch digestion, but differ in starch digestion rate. Other feedstuffs have the same starch digestion rate, but differ in ileal starch digestion. For most feedstuffs, ileal starch digestion was high, but incomplete, and no further starch digestion occurred in the hind gut. This experiment shows that the site of starch digestion is not an accurate indicator of starch digestion rate. Starch digestion rate varies among native starches, as present in the feedstuffs. This provides the opportunity to manipulate the availability of glucose throughout the day. The practical relevance of starch digestion rate in broiler nutrition has to be established.

	ACKNOWLEDGMENTS

 
The authors thank Arnold Dijkstra for all starch analyses.

	FOOTNOTES

 
¹ Funded by the Dutch feed co-operatives and the Dutch Technology Foundation STW (project no. WPR66.4050).

³ Abbreviations used: AC_G, absorption coefficient of glucose; AI, anterior ileum; AJ, anterior jejunum; AME, apparent metabolizable energy; D, potentially digestible starch fraction; DC_S, digestion coefficient of starch; k_d, fractional starch digestion rate; MRT, mean retention time; NFE, nitrogen-free extract; NSP, nonstarch polysaccharides; PI, posterior ileum; PJ, posterior jejunum; RS, resistant starch.

Manuscript received March 13, 2001. Initial review completed April 9, 2001. Revision accepted June 21, 2001.

	LITERATURE CITED

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