• <tr id="yyy80"></tr>
  • <sup id="yyy80"></sup>
  • <tfoot id="yyy80"><noscript id="yyy80"></noscript></tfoot>
  • 99热精品在线国产_美女午夜性视频免费_国产精品国产高清国产av_av欧美777_自拍偷自拍亚洲精品老妇_亚洲熟女精品中文字幕_www日本黄色视频网_国产精品野战在线观看 ?

    Lignocellulose as an insoluble fiber source in poultry nutrition:a review

    2021-12-17 11:54:40IlenheandrgenZentek

    Ilen R?heand Jürgen Zentek

    Abstract

    Extensive research in recent years into the use of various fiber sources in poultry nutrition has led to the perception that dietary fiber is more than a simple diet diluent.Several studies showed that the feeding of insoluble fiber sources such as oat hulls,sunflower hulls or wood shavings may affect digestive physiology and function improving chickens health and growth performance.In this context,the effect of lignocellulose as an insoluble dietary fiber source is increasingly being investigated.Lignocellulose is a component of plant cell walls and consists mainly of the insoluble carbohydrate polymers cellulose and hemicelluloses as well as the phenolic polymer lignin.Lignocellulose is chemically and physicochemically different from other insoluble fiber sources and thus possibly has different effects on poultry compared to traditional fiber sources.Several studies investigated the effect of dietary lignocellulose on growth performance,nutrient digestibility,gastrointestinal tract development and intestinal microbiota in broilers and laying hens.Studies differed in terms of feed formulation and lignocellulose inclusion level as well as products of different suppliers were used.The results obtained are inconsistent;beneficial,indifferent or detrimental effects of feeding lignocellulose were observed,so that a final assessment of lignocellulose as a “novel”insoluble fiber source is difficult.This review article summarizes the results of studies in connection with the feeding of lignocellulose to poultry,compares them with those that have used other insoluble fiber sources and illuminates the possible mechanisms of action.

    Keywords:Fiber,Growth performance,Gut health,Gut morphology,Lignocellulose,Microbiota,Nutrient digestibility,Poultry

    Introduction

    In recent years,there have been increasing scientific reports that dietary fiber can have a positive effect on animal health and productivity.Fiber as feed component in poultry nutrition has traditionally been given little consideration as it has only a low nutritional value from a chemical point of view.However,due to its unique physicochemical properties,several studies showed that insoluble fiber sources may affect digestive tract development and function resulting in improved chicken health and growth performance[1–4].Feeding experiments were mainly carried out with insoluble fiber sources that arise as by-products during industrial production such as oat hulls,sunflower hulls,soybean hulls,wheat bran or wood shavings.In the last decade,research has concentrated on the use of an “innovative”insoluble dietary fiber source,lignocellulose(LC).LC is a constituent of plant cell walls and thus the most abundant and bio-renewable biomass on earth[5].Studies in farm and companion animals showed that dietary LC may have potential effects on digestive physiology and function[6–10].This review gives a comprehensive overview of the effects of dietary LC in poultry.First,the physicochemical properties of LC are described and reference is made to methodological aspects of the incorporation of LC into feed,as this can have a decisive influence on the study results.Next,the results of studies on the effects of dietary LC on growth performance,nutrient digestibility,gastrointestinal tract development and intestinal microbiota are summarized and compared with those observed in feeding experiments using other insoluble fiber sources.In particular,the potential mode of action of insoluble dietary fiber on the digestive physiology of chickens is discussed.In addition,some considerations regarding future research directions and methodological challenges are presented and discussed.

    Chemical composition and physicochemical properties of LC

    Dietary fiber comprises a significant part of plant feedstuffs and is chemically defined as the sum of non-starch polysaccharides(NSP)and lignin[11].From a physiological point of view,dietary fiber comprises any polysaccharide and lignin that is not degraded by endogenous enzymes in the digestive tract,hence reaching the hindgut[12,13].Different types of plants contain different amounts and chemical structures of fibers with varying physical properties[14].Therefore,fiber sources differ in their content of soluble and insoluble NSP.Fiber sources with high levels of insoluble NSP are for example LC,oat hulls,sunflower hulls or wheat bran,while sugar beet pulp or apple pomace contain higher concentrations of soluble NSP[3,13].Lignocellulosic biomass refers to plant dry matter of different origin and is mainly composed of the carbohydrate polymers cellulose and hemicelluloses as well as the phenolic polymer lignin[15,16].The proportional composition of carbohydrate and aromatic polymers of LC may vary depending on the type of lignocellulosic biomass used[17,18].LC applied in animal nutrition is usually derived from forest residues containing different proportions of hard and soft wood as well as bark.A recent study analyzed the chemical composition of three LC products used as fiber additives in animal feed[19].Table 1 shows the chemical and physicochemical characteristics of these LC products compared to that of other insoluble fiber sources,in particular,oat hulls,sunflower hulls and wheat bran.The LC products showed a similar crude fiber content,but differences in the detergent fibers,which allow a rough assessment of the insoluble fractions of cellulose,hemicelluloses and lignin[13,21].Two products(LC2 and LC3)showed a similar cellulose,hemicelluloses and lignin content of~415,~150 and~330g/kg dry matter(DM),respectively,which are close to that reported by Zeitz et al.[22].The LC1 product,however,contained significantly more lignin(~650g/kg DM)and lower amounts of cellulose(~78g/kg DM).All three LC products comprised high proportions of more than 90% insoluble fibers and only small amounts of soluble fibers[19,22].Similarly,oat hulls were mainly composed of insoluble dietary fiber[20]and contained primarily hemicelluloses and cellulose.Sunflower hulls showed slightly lower values for crude and detergent fiber,but the relative distribution of cellulose,hemicelluloses and lignin was similar when compared to LC2 and LC3.Sunflower hulls contained mostly insoluble fiber,but about twice as much soluble fiber compared to LC.Wheat bran had the lowest crude fiber content of all the fiber sources shown and was mainly composed of hemicelluloses.In addition,the proportion of soluble to insoluble dietary fibers in wheat bran was slightly higher compared to LC.With respect to the physicochemical properties,there is a positive correlation between dietary soluble fiber content and digesta viscosity in monogastric animals[13].Due to the low proportion of soluble fibers such as pectins,insoluble dietary fiber sources have little effect on digesta viscosity[3,14].Another important physicochemical feature of dietary fiber is their hydration capacity,which can be characterized by the swelling capacity,the water holding and binding capacity[13].The hydration capacity of a dietary component affects the bulking effect of digesta[23],which in turn could have consequences on digesta retention time and nutrient digestibility[24].LC showed higherhydration capacities and significantly greater swelling properties compared to oat hulls,sunflower hulls and wheat bran[19,20].Finally but yet importantly,the particle size of a fiber source is another key characteristic,which may influence digestive function[23].After processing and fiber breakdown,LC is a powdery material with an average particle size of 80 to 300μm[22,25].This material can then be further processed,so that various LC products are commercially available,e.g.in powdery,crumbled or pelleted form.The particle size of other insoluble fiber sources is usually larger depending on the degree of grinding.

    Table 1 Chemical and physicochemical characterization of different lignocellulose products in comparison with other insoluble fiber sources

    Inclusion of insoluble fiber sources in experimental diets

    In order to investigate the effect of dietary insoluble fiber in chickens,several feeding experiments were designed using different feed formulations.In principle,there are three different options to include fiber sources in diets,as displayed in Fig.1.In “feed formulation 1”,a “control”feed is compared with a “fiber dilution”diet.A “control”poultry diet usually consists of grains,protein sources,plant oils and a premix.The nutrient composition of the“control”diet should meet the nutrient recommendations for chicken diets.The treatment diet “fiber dilution”is based on the “control”diet,but supplemented with the fiber source of interest.As insoluble fiber sources,such as LC,cellulose or wood shavings,have a low nutritive value,the energy and nutrient content of the diet is diluted by the added fiber source.In “feed formulation 2”,the same“control”diet is used,but the fiber containing diet is balanced to be isoenergetic and isonitrogenous.To achieve this,the “iso”diet must be formulated to have increased proportions of fat and protein at the expense of the carbohydrate source.Consequently,the “control”and the “iso”diet show remarkable differences in the feed-and nutrient composition,but show similar energy and protein concentrations.In “feed formulation 3”,a “control sand”and a“fiber dilution”diet is used.The “control sand”diet is based on the “control”feed,but contains a certain percentage of an insoluble ash source,e.g.sand or sepolite.The “fiber dilution”diet is based on the “control sand”diet,but the insoluble ash source is substituted by the insoluble fiber source of interest.Thus,the feed composition is similar with the exception of the components sand and fiber.As a result,the nutrient composition of both diets differs significantly only in terms of crude ash and crude fiber.In summary,the dietary inclusion of an insoluble fiber source is coupled with differences in the feedand nutrient composition of the experimental diets.Therefore,the effects of feeding these diets can be attributed to both,the factor “dietary fiber”and the factor “feed and nutrient composition”(Fig.1).Thus,alterations in animal productivity,nutrient digestibility,digestive tract development or gut microbiota might be related to differences in dietary fiber and/or feed and nutrient composition.Reference is made to this issue in the respective sections of this review.

    Fig.1 Overview of options to include an insoluble fiber source in experimental diets.Detailed legend:The feed and nutrient composition of three different options(feed formulation 1 to 3)to include an insoluble fiber source in experimental diets are presented.Feed formulations vary in terms of dietary fiber content as well as feed-and nutrient composition,which can have potential effects on the animal

    Impact of dietary LC on productivity of chickens

    During the last decade,several studies examined the effect of feeding LC on the productivity of broilers(Table 2)and laying hens(Table 3).Productivity parameters include body weight(BW),average weight gain(AWG),average feed intake(AFI)or average daily feed intake(ADFI),feed conversion ratio(FCR),egg production(EP)and egg weight(EW).Studies differed in terms of feed formulation and LC inclusion level used.In most studies,dietary LC was supplemented on top of feed(“feed formulation 1”,Fig.1);only few used experimental diets based on “feed formulation 2 and 3”(Fig.1).Commonly,lower dietary LC inclusion levels in the range of 0.05% to 2% have been used,while few experiments were performed using relatively high concentrations of dietary LC of 5% to 15%.

    Results obtained in broiler trials using relatively low dietary concentrations of LC are contradictory(Table 2).The feeding of diets supplemented with 0.25% to 2% LC positively affected the FCR of broilers due to an increase in weight gain[26,28].In contrast,broilers fed diets supplemented with 1% LC had an impaired FCR compared to those receiving the control diet[29].However,several studies using similar LC inclusion levels showed no impact of dietary LC on broiler growth performance[22,24,27].Results of a recent study demonstrated that dietary LC concentrations of 0.05% to 0.1% improved FCR of broilers,while the supplementation of 0.2% LC showed no effect on FCR[32].Broilers fed 0.6% LC,which was added at the expense of soybean meal and corn,showed higher BW after 42 d of feeding compared to those fed the control and 0.4% LC[30].Interestingly,the feeding of the same LC concentration of 0.6%,but which was added at the expense of soybean meal only,did not affect final BW of broilers[30]suggesting that the feed composition had a greater impact on BW development than the LC addition.Broilers fed diets diluted with relatively high concentrations of LC of 5% to 15% showed a marked decrease in average daily gain with increasing concentrations of dietary LC,while feed intake tended to increase with increasing concentrations of LC[31].On the contrary,broiler productivity seems to be unaffected by relatively high dietary LC inclusion levels up to 10% when diets were composed to be isoenergetic and isonitrogenous[25,39].

    With respect to feeding experiments with commercial hybrid pullets and laying hens(Table 3),most studies showed that growth and laying performance were not affected by dietary LC inclusion levels of 0.8% to 2%[33–35].However,Sozcu and Ipek[36]demonstrated that the supplementation of 0.05% and 0.1% dietary LC increased BW,ADFI,EP,and EW of laying hens between 18 to 38weeks of age compared to hens fed the control diet.A further increase in the dietary LC concentration to 0.2%,however,led to a decrease in EP and EW[36].In two other studies,the effect of dietary LC was investigated in broiler breeder hens[37]during the laying phase(43 to 55 weeks of age)and in dual purpose hens[38]during the growing(1–22 weeks of age)and laying period(23–52weeks of age).Broiler breeder and dual purpose hens tended to overconsume feed leading to an increased body fat content,which in turn might be related to the observed lower productive efficiency[40–42].Thus,the hypothesis in both studies was that BW and body fat percentage of hens can be reduced by feeding a nutrient-reduced LC-containing diet and that this is accompanied with an improved reproductive performance[37,38].The results showed that dietary LC reduced BW,body fat content[38]and abdominal fat weight of hens[37],which was directly associated with an improved laying performance.

    Table 2 Impact of dietary lignocellulose on broiler growth performance

    Table 2 Impact of dietary lignocellulose on broiler growth performance(Continued)

    In principle,due to the use of different feed formulations and inclusion levels,it is difficult to make a conclusive statement about the effect of LC on chicken productivity.Few studies showed that similar insoluble fiber sources such as cellulose or wood shavings could have a positive impact on broiler growth performance[43,44].It was suggested that a combination of improved gut function and enhanced nutrient digestibility was responsible for observed beneficial effects[43,44].With regard to dietary LC and lower inclusion levels,it was also hypothesized that digestive physiology and nutrient digestibility might be affected leading to improved growth performance[26,28,30].However,data on the effects of dietary LC on digestive tract development and nutrient digestibility were inconclusive,as described later.Studies generally showed that animal productivity was impaired when diets were supplemented with higher dietary LC concentrations.This observation is explained by the fact that the energy-and nutrient content of diets was considerably reduced by the LC inclusion(“feed formulation 1”,Fig.1)resulting in a lower energy and nutrient intake of chickens impairing growth performance.However,animal productivity seems to be not affectedwhen higher concentrations of LC are included in isoenergetic and isonitrogenious diets(“feed formulation 2”,Fig.1).This phenomenon is already known from former studies,which showed that the crude fiber concentration did not influence growth performance unless it affected the energy content in the diets[45–47].Chickens usually have the ability to covertheir metabolic energy requirement to a certain degree by increasing or decreasing the feed consumption[48,49].Chickens receiving diets diluted by sand or oat hulls up to 20% showed an increased feed intake resulted in a similar energy intake and average daily gain in comparison to those receiving an undiluted control diet[49,50].With respect to higher dietary inclusion levels of powdery LC,this regulatory mechanism seem to be partially restricted[31,38],which might be attributed to the physical form of fine LC fibers and its physical bulking effect[37,38].Accordingly,the use of higher concentrations of powdery LC in feed for broiler breeder and dual purpose hens,might be a reasonable dietary strategy to control feed intake and weight gain ensuring an optimal productive performance.

    Impact of dietary LC on the nutrient digestibility in chickens

    Data regarding the effect of dietary LC on nutrient digestibility in chickens are scarce displaying no clear picture.A direct comparison of results is difficult:on the one hand,diets differed in their feed composition and nutrient content;on the other hand,different dietary LC concentrations were used.The feeding of isoenergetic and isonitrogenous diets containing 0.8% LC increased the true digestibility of protein as well as the apparent and true dietary amino acid digestibility in roosters compared to those fed the control diet[51].Similarly,the same authors observed that the apparent protein digestibility was increased by 5.5% in broilers fed 0.8% dietary LC compared to those receiving the control diet[52].In contrast,the supplementation of 1% or 2% dietary LC did not affect protein and gross energy digestibility in broilers[24,29].Feeding of isoenergetic and isonitrogenous diets with higher LC inclusion levels of 5% and 10% led to a decrease in the apparent ileal digestibility of crude protein and apparent excreta digestibility of organic matter and gross energy while the total tract digestibility of ether extract was not affected[25].The apparent ileal fat digestibility and total tract digestibility of total fatty acids in broilers was also not influenced by the supplementation of 0.25% and 0.5% LC while the feeding of 1% LC resulted in an increased apparent fat digestibility[53].

    If dietary LC has an impact on the digestibility of nutrients,either beneficial or detrimental,the question arises as to how LC might affect the digestive physiology of chickens.Regarding the beneficial effects,it is well known that the feeding of structural components,such as coarse fiber particles,may stimulate digestive function,which is associated with an improved nutrient digestibility[23,54].LC consists of very small fiber particles and thus it remains unclear whether dietary LC may affect the digestive physiology and thus the nutrient digestibility in chickens,as discussed later.Another positive effect of feeding LC might be related to the fat digestibility.Jiménez-Moreno et al.[55]speculated that dietary cellulose might have an effect on micelle formation and lipid absorption,enhancing bile acids recycling and fat absorption.Further research is needed in order to evaluate this hypothesis.With respect to the potential adverse effects,it was assumed that dietary LC might have an abrasive effect on the intestinal mucosa[27,30],thus enhancing endogenous amino acid losses.In this regard,Kluth and Rodehutscord[56]showed that the feeding of increasing concentrations of cellulose up to 8% significantly elevated the inevitable losses of crude protein and amino acids in broilers.Whether this observation also applies to the feeding of increasing concentrations of LC requires further clarification.In summary,based on the studies carried out so far,no statement can be made about whether LC has an impact on nutrient digestibility in chickens.

    Impact of dietary LC on gastrointestinal tract development,intestinal morphology and excreta characteristics of chickens

    Few studies exist evaluating the effect of dietary LC on the gastrointestinal tract development and digestive physiology of broilers and laying hens.Investigations were focused on the gastrointestinal gross morphology,the intestinal histomorphology,and digesta as well as excreta characteristics.

    Effect on gizzard development and function

    In broilers,most studies showed that the feeding of lower dietary LC concentrations up to 2% did not affect the relative weight of the gizzard[22,24,28–30].However,feeding of isoenergetic and isonitrogenous diets containing 5% LC resulted in an increased gizzard weight of slow growing broilers[39].Similarly,the relative gizzard weight was affected by feeding LC in laying hens and pullets[34].Pullets fed 1% LC over a period of 10weeks showed increased relative weights of the gizzard.Moreover,laying hens,aged 31weeks,developed heavier gizzards when fed diets diluted with 0.8% LC after 12 weeks of feeding[34].Interestingly,these effects were not observed in chickens that received these diets for a shorter period,suggesting a time-dependent effect of LC.Studies in quails also showed that the feeding of isoenergetic and isonitrogenous diets containing 3% LC increased the relative gizzard weights[57].Few studies investigated the effect of dietary LC on gizzard pH showing conflicting results.The feeding of diets supplemented with 0.4% and 0.6% LC decreased the gizzard pH of broilers[30],while the feeding of 0.8%,1% and 2% dietary LC had no impact on gizzard pH[22,24].Broilers fed diets containing 0.05% to 0.2% LC showed also a similar gizzard pH compared to those fed the control diet[32].

    In general,coarsely ground fiber sources such as oat,soybean and pea hulls,or wood shavings,containing primarily insoluble NSP,are known to increase gizzard size and weight[43,58,59].An increased gizzard weight might be an indicator of enhanced gizzard function[23,54,60,61].Several feeding experiments using different coarsely ground fiber sources showed that an increased gizzard weight was accompanied with a lower gizzard pH[59,61–63]suggesting an enhanced proventricular secretion of hydrochloric acid.Moreover,an increased gizzard activity is related to an increased gastrointestinal reflux and pancreatic enzyme secretion[64–67].Furthermore,the feed passage rate might be affected by feeding structural fiber components improving nutrient digestibility and growth performance of chickens.In this regard,it is generally accepted that the feeding of coarsely ground,insoluble fibers increases the feed passage rate[23].However,in poultry it is suggested that the feeding of moderate amounts of structural dietary fiber reduces the digesta transit time,as fiber particles may accumulate in the gizzard[23,58].A recent study proved that digesta transit time of broilers was not affected by feeding diets containing finely ground LC or oat hulls[29].The question arises whether the fiber inclusion itself,the particle size of the fiber source or a combination of both factors are responsible for observed effects on gizzard development and digestive function.In this regard,Jiménez-Moreno et al.[55]investigated the impact of type and particle size of dietary fiber on gizzard weight of broilers(Fig.2).Diets contained different fiber sources,in particular cellulose,oat hulls and sugar beet pulp at inclusion levels of 3%.Furthermore,diets differed in terms of particle size distribution indicated by different geometric mean diameters(GMD).Results indicated that broilers fed diets showing the highest GMD,namely coarsely ground oat hulls and sugar beet pulp,developed the greatest gizzard weights,while broilers fed diets having the lowest GMD,cellulose and finely ground sugar beet pulp,showed the lowest gizzard weights(Fig.2).It was concluded that dietary cellulose did not stimulate gizzard function due its lack of physical structure[55].Thus,it seems obvious that particle size of the fiber source,rather than fiber inclusion itself,is the determining factor that stimulates gizzard development.In this regard,it has been suggested that feed particles should be at least larger than 1mm to enhance gizzard development[54,68].Based on this,it can therefore be assumed that fine-fiber LC,similar to cellulose,has little effect on gizzard development and function.In this context,it would be interesting to investigate whether the physical form or the macrostructure of LC might have an impact on gizzard development.Therefore,future studies should investigate the effects of the physical form of LC and that of the overall feed structure on digestive physiology in chickens.

    Fig.2 Impact of dietary fiber and particle size on the gizzard weight of broilers.Detailed legend:Effect of type and particle size of dietary fiber on the relative gizzard weight of broilers according to Jiménez-Moreno et al.[55];CEL=cellulose-fed broilers;OH=oat hulls-fed broilers;SBP=sugar beet pulp-fed broilers;the geometric mean diameter of diets is indicated above the respective bar

    Effect on the intestinal gross morphology

    Only few studies investigated the impact of dietary LC on the development of the small and large intestine,but most of them showed no effect of LC feeding on the intestinal gross morphology.In broilers,the feeding of relatively low dietary LC inclusion levels of 0.4% and 0.6% did not affect the relative weight of the intestine[30].In agreement with this,broilers fed diets supplemented with 1% LC had similar relative duodenal,jejunal and cecal weights compared to those fed the control diets[29].Relative lengths of small intestinal segments were also comparable between broilers fed 0.25% to 1% dietary LC and those offered the control diet[69].Similarly,the absolute length of the small intestine and cecum of broilers was not influenced by feeding isoenergetic and isonitrogenous diets containing 2% and 4% LC.In another study,the feeding of isoenergetic and isonitrogenous diets containing 3% LC increased the relative weight of the jejunum and ileum of quails[57].Pullets fed 1% LC over a period of 10 weeks showed comparable relative weights of the small intestine compared to those fed the control diet[34].The feeding of diets supplemented with LC concentrations of 1% to 2% also showed no impact on the absolute cecal weight and length of laying hens[35]nor on the relative cecal weight of pullets[34].The feeding of isoenergetic and isonitrogenous diets containing higher concentrations of LC up to 5% also had no impact on the absolute length of the small intestine and cecum of slow growing broilers[39].In contrast,the feeding of energy-and nutrient reduced diets containing 10% LC,led,in relation to the BW,to increased weights of the small and large intestine[70].

    Reasons for alterations in length or weight of intestinal organs due to feeding insoluble fiber sources are not fully understood.In general,it is supposed that an increase in the intestinal size and length but also an enlargement of the intestinal mucosa contributes to a higher intestinal weight[71].Several studies demonstrated that chickens fed different insoluble fiber sources at varying inclusion levels showed increased relative digestive tract weights[63,70,72]implying a fiber-related effect on intestinal organ development[63,72].However,in those studies dietary fiber inclusion also led to a decrease of the chicken’s BW[63,70,72].Therefore,the hypothesis that dietary fiber influenced organ weight development in those chickens is not valid as data on intestinal weight and length are related to the BW.Only considering studies in which chickens had similar empty BW,results on the effect of fiber on intestinal tract development are conflicting.On the one hand,it has been reported that feeding of isoenergetic and isonitrogenous diets containing 3% insoluble fiber sources,such as oat and soybean hulls,increased the digestive tract weight of broilers[59,61,73,74].On the other hand,the feeding of isoenergetic and isonitrogenous diets containing 3% oat hulls,soybean hulls or cellulose did not affect the relative weight of intestinal organs[55,75,76].Similarly,the feeding of diets,supplemented with 10% oat hulls or cellulose,did also not affect the relative weight of intestine of broilers[77,78].Regarding possible fiber-effects on intestinal tract development,it was suggested that an enlargement of the digestive tract might be a consequence of physical distension caused by luminal swelling of the ingested fiber sources[59,63,72].Further research is needed in order to clarify whether and why dietary insoluble fiber may have an impact on gut gross morphology.

    Effects on the intestinal mucosal development

    Few studies showed that dietary LC might affect the morphology of the intestinal mucosa of chickens.Sarikahn et al.[26]showed that ileal villus height and villus height to crypt depth ratio were increased in broilers fed diets containing 0.25%,0.5% and 0.75% LC.Similarly,broilers receiving diets supplemented with up to 2% LC had an increased jejunal villus height,villus apparent surface area and villus height to crypt depth ratio compared to those fed the control diet[32].In contrast,duodenal and jejunal villus height and crypt depth were reduced in broilers fed diets supplemented with 0.5% LC while the inclusion of 1% LC showed no effect on villus morphology[69].Interestingly,a different observation was made in the ileum of those chickens as increasing concentrations of dietary LC led to an increase in villus height and crypt depth[69].The feeding of 0.6% LC,which was included in the diet at the expense of soybean meal,resulted in an increased villus height and width as well as crypt depth in the duodenum,jejunum and ileum of broilers[30].However,the same inclusion level of 0.6% dietary LC,supplemented at the expense of 0.3% soybean meal and 0.3% corn,did not affect duodenal and jejunal villus height and villus width of broilers[30]implying that intestinal mucosal development was influenced by feed composition rather than dietary LC inclusion. Abdollahi et al. [29] showed that the supplementation of 1% dietary LC had no impact on histomorphological parameters in the duodenum and jejunum of broilers.In laying hens,the feeding of 0.05% and 0.1% dietary LC increased jejunal villus height and width,villus apparent surface area and villus height to crypt depth ratio,while a further increase in dietary LC concentration of 0.2% decreased observed histomorphological parameters[36].Dual purpose hens fed diets supplemented with 10% LC showed an enhanced mucosal development of the colorectum indicated by a greater villus area and a higher villus and crypt mucosal enlargement factor[70].Interestingly,the colorectal villus surface was negatively correlated with the short chain fatty acid(SCFA)concentration in the cecum of hens[70].Another study showed that the feeding of 3% dietary LC included in isoenergetic and isonitrogenous diets increased the villus height and villus height to crypt depth ratio in the jejunum and ileum of quails[57].

    An enlargement of the intestinal surface area due to longer or increased numbers of intestinal villi is generally associated with an increased intestinal nutrient absorption [79,80]and thus an improved nutrient utilization.However,the development of the intestinal microarchitecture strongly depends,among other things,on the concentration of enteral nutrients and thus on the nutrient content of the diet[81,82].It has been suggested that chickens fed high-fiber diets suffer from a nutrient deficiency and thus try to enhance nutrient and bacterial metabolite absorption by increasing the mucosal surface area[70,82,83].Thus,it has to be emphasized that both the dietary nutrient content and/or the fiber inclusion might affect the mucosal development of the intestine(Fig.1).However,effects on intestinal mucosal development were also observed in studies using isoenergetic and isonitrogenous diets or lower dietary fiber inclusion levels[26,30,74],so that similar enteral nutrient concentrations can be expected.The potential mode of action of dietary insoluble fiber on intestinal mucosal development in chickens is still unknown.Whether specific chemical and physicochemical properties of the fiber source or changes in the intestinal microbiota due to fiber feeding could be responsible requires further clarification.

    Effects on excreta quality

    Studies in broilers indicated that dietary LC inclusion might have a positive effect on litter quality.Litter moisture content was lower in broilers fed diets supplemented with 0.6%,0.8%,1%,and 2% LC compared to litter of control-fed broilers[24,30,52].Similarly,litter moisture content was also reduced in quails fed 3% dietary LC[57].The litter DM content usually correlates with the DM content of the excreta.Consistent with the latter,laying hens fed 10% LC for 52 weeks had a higher excreta DM content at 10,17 and 22 weeks of age compared to those fed the control diet[70].However,studies in broilers showed no impact of dietary LC on excreta scoring or excreta DM[22,25].

    In comparison with other insoluble fiber sources,LC has a moderate to high water holding capacity[19].It has been speculated that the water holding capacity and the digesta retention time might be increased in LC fed chickens resulting in increased luminal water absorption and higher excreta DM content[24].In addition to the hydration capacity,digesta-and excreta DM might be also affected by the particle size of the fed fiber source.Excreta score was improved in broilers fed coarsely ground wood shavings at a ratio of 6:100(w/w),while finely ground cellulose-and control-fed broilers showed comparable excreta scores[43].Authors speculated that coarsely ground fibers might hold larger amounts of water reducing the solubilisation of NSP than finely ground fiber particles[43].Further research is needed in order to clarify whether and why insoluble fiber sources might reduce excreta DM and thus improve litter quality.

    Impact of dietary LC on the gut microbiota

    It is well known that dietary fiber can modulate the gut microbiota in humans and animals,which in turn might have consequences on the intestinal health[84,85].Thus,few studies evaluated the impact of dietary LC on the intestinal microbiota in chickens.Alterations in the microbial composition can be accompanied with changes in the production of bacterial metabolites;vice versa,shifts of the intestinal bacterial metabolite profile are a clear indicator for a modification of the composition and activity of intestinal bacteria.Therefore,investigations were focused on both,bacteria residing in the avian intestinal tract and the concentration of intestinal bacterial metabolites.Table 4 shows the impact of dietary LC on the concentration of SCFA in the intestine of chickens.In general,results are conflicting,which may be explained by differences in the used study design,in particular regarding the used feed formulation,LC inclusion level and LC product.

    Few studies used the same LC product,but different LC inclusion levels and feed formulations[25,27,30,70].The feeding of diets supplemented with LC at relatively low inclusion levels of 0.25% to 0.6% reduced counts of Escherichia coli and Clostridium perfringens and increased those of Bifidobacterium spp.and lactic acid bacteria in the ileum and cecum of broilers[30].Similarly,ileal counts of Lactobacillus spp.as well as ileal and cecal counts of Bifidobacterium spp.were elevated in broilers fed diets supplemented with 0.25%,0.5% and 1% LC[27].Ileal and cecal counts of Escherichia coli and Clostridium spp.were also reduced in broilers receiving 0.25% and 0.5% dietary LC[27].In the same experiment,however,the intestinal SCFA profile was generally not affected by LC feeding(Table 4),with the exception of broilers receiving 0.5% LC,which showed increased total SCFA concentrations in the ileum and cecum[27].Two further studies evaluated the effect of relatively high concentrations of dietary LC on bacterial composition and metabolites[25,70].The feeding of diets diluted with 10% dietary LC had generally no impact on cecal microbial composition in dual pupose hens,but reduced the cecal concentration of SCFAs and ammonia[70].Similarly,cecal bacterial metabolites were reduced in broilers fed isoenergetic and isonitrogenous diets containing 10% LC[25].Moreover,increasing concentrations of dietary LC decreased counts of Escherichia/Hafnia/Shigella[25].Four recent studies using a potential more fermentable LC product also displayed conflicting results regarding the effect of dietary LC on the gut microbiota in broilers and laying hens[22,24,35,86].The feeding of isoenergetic,isonitrogenous diets containing 2% and 4% LC increased the cecal microbial diversity and the abundance of butyrateproducing bacteria in free-range chickens,while the luminal concentration of butyrate,acetate and propionate was not affected[86].In contrast,broilers and laying hens fed diets which were supplemented with 0.8% and 1% LC,showed no alterations in the overall cecal microbial diversity[22,35].Sun at al[35].showed that the feeding of 1% LC increased the relative abundance of lactate-and butyrate-producing bacteria in the cecum of laying hens,which was accompanied with higher concentrations of cecal SCFAs.In contrast,the total amountof cecal SCFAs was not affected in broilers fed diets supplemented with 0.8% of the same LC product[22].Moreover,LC-fed broilers had a lower cecal abundance of the bacteria families Ruminococcaceae and Lactobacillaceae as well as a higher abundance of Clostridiaceae,Enterobacteriaceae,Peptostreptococcaceae and Erysipelotrichaceae[22].Diets diluted with 1% and 2% LC had in general no effect on detected bacteria except that counts of Ruminococcus spp.were increased and those of Clostridium spp.reduced in the cecum of broilers fed 2% LC[24].

    Table 4 Impact of dietary lignocellulose on the concentration of intestinal short chain fatty acids(SCFA)

    Based on the studies carried out so far,no uniform picture can be drawn as to whether and to what extent dietary LC influences the intestinal microbiota in chickens.It is generally agreed that insoluble fiber sources such as LC,cellulose or wood shavings,are not extensively degraded by intestinal bacteria residing in the avian digestive tract[14,85,87].On the one hand,this is due to the anatomical peculiarities of the chicken’s digestive tract,which is relatively short,resulting in a short feed passage rate.In addition,several studies suggest that only small and soluble fiber fractions can enter the cecum[66,88,89],which appears to be the main site for bacterial fermentation of fiber in chickens[90,91].On the other hand,there is evidence that the cellulolytic activity of bacteria in the chicken’s hindgut seems to be low[92–94].Consequently,it is assumed that the impact of insoluble fiber on intestinal bacterial composition and activity appears to be minimal[14,95].However,some authors speculated that LC could be fermented in the cecum of chickens as intestinal bacterial composition or SCFA profile had changed due to dietary LC inclusion[22,27,35].Furthermore,some studies used an “eubiotic”LC product,which might have a higher susceptibility to microbial fermentation than the standard LC product[22,35,86,96,97].Moreover,it was suggested that dietary LC may have an abrasive effect on the intestinal mucosa and adhering bacteria[27,30]or that phenolic compounds of lignin exhibit antimicrobial properties[22,27].

    The major problem in answering the question of whether dietary insoluble fibers generally have an effect on the intestinal microbiota of chickens refers to the experimental diets chosen to study that effect.With reference to Fig.1,most studies that investigated the effect of insoluble fiber on gut microbiota,chose experimental diets based on “feed formulation 1”,and a few those based on “feed formulation 2”.Depending on the amount of dietary fiber added,there are corresponding changes in the nutrient composition between the control and the fiber containing diet(Fig.1).Alterations in the nutritional composition of the feed result in changes of the amount of substrate that reaches the large intestine and can be fermented by resident bacteria[25].As a consequence,changes in the dietary nutrient composition may influence the gut microbiota and bacterial fermentation pathways[25],making it difficult to distinguish between nutrient composition-and fiber related effects.The best way to study the effect of insoluble fiber on gut microbiota is possibly to use feed variants according to“feed formulation 3”(Fig.1).Feed and nutrient composition of control and fiber diets are very similar,although it cannot be ruled out that even the inclusion of an insoluble ash sources might affect the gut microbiota.

    Conclusions

    In conclusion,several studies were performed in order to evaluate the effect of dietary LC as an insoluble fiber source in poultry nutrition.Data on the impact of LC on growth performance,nutrient digestibility,digestive tract development and gut microbiota in chickens are inconsistent and do not allow a conclusive assessment.One of the reasons for this is that a direct comparison of results is difficult as studies differed in terms of feed formulations,LC inclusion levels and LC products.In future research,more attention should be paid to the type of feed formulation used in order to better distinguish the effects of dietary fiber from those of the feed and nutrient composition.In addition,the mode of action of LC in the digestive tract should be examined more closely,with particular reference to its chemical and physicochemical properties.

    Abbreviations

    LC:Lignocellulose;NSP:Non-starch polysaccharides;BW:Body weight;

    AWG:Average weight gain;AFI:Average feed intake;ADFI:Average daily

    feed intake;FCR:Feed conversion ratio;EP:Egg production;EW:Egg weight;DM:Dry matter;GMD:Geometric mean diameter;SCFA:Short chain fatty acids

    Supplementary Information

    The online version contains supplementary material available at https://doi.org/10.1186/s40104-021-00594-y.

    Additional file 1.Overview on LC products used in the different

    studies.Description of data:Additional information on LC used in the different studies including supplier information and product name.

    Acknowledgements

    Not applicable.

    Authors’contributions

    IR and JZ wrote and approved the manuscript.

    Funding

    Open Access funding enabled and organized by Projekt DEAL.

    Availability of data and materials

    Not applicable.

    Declarations

    Ethics approval and consent to participate

    Not applicable.

    Consent for publication

    Not applicable.

    Competing interests

    The authors declare that we have no competing interests.

    Received:11 December 2020 Accepted:11 April 2021

    婷婷六月久久综合丁香| 村上凉子中文字幕在线| 国产免费男女视频| 国产成人一区二区三区免费视频网站| a级毛片a级免费在线| 国产免费av片在线观看野外av| 在线观看免费视频日本深夜| 日本免费a在线| 国产欧美日韩一区二区三| 天天躁夜夜躁狠狠躁躁| 亚洲va日本ⅴa欧美va伊人久久| 国产1区2区3区精品| 欧美日韩亚洲国产一区二区在线观看| 搡老妇女老女人老熟妇| 他把我摸到了高潮在线观看| 一区福利在线观看| 一a级毛片在线观看| 啦啦啦免费观看视频1| 美女午夜性视频免费| 一区福利在线观看| 亚洲激情在线av| 精品国产美女av久久久久小说| 日韩欧美三级三区| av有码第一页| 亚洲精品国产精品久久久不卡| 日韩三级视频一区二区三区| 日韩三级视频一区二区三区| 美女高潮到喷水免费观看| 久久久久久国产a免费观看| 成人免费观看视频高清| 视频区欧美日本亚洲| 久久中文字幕一级| 制服丝袜大香蕉在线| 国产高清有码在线观看视频 | 动漫黄色视频在线观看| 日本黄色视频三级网站网址| 色哟哟哟哟哟哟| 亚洲全国av大片| 巨乳人妻的诱惑在线观看| 国产精品香港三级国产av潘金莲| 亚洲激情在线av| 久久99热这里只有精品18| 国产精品,欧美在线| 亚洲成av片中文字幕在线观看| 婷婷六月久久综合丁香| 岛国视频午夜一区免费看| 日韩三级视频一区二区三区| 国产三级黄色录像| 中文字幕另类日韩欧美亚洲嫩草| 夜夜爽天天搞| 国产亚洲av嫩草精品影院| av福利片在线| 欧美色欧美亚洲另类二区| 亚洲第一欧美日韩一区二区三区| www.精华液| 国产精品 欧美亚洲| 婷婷丁香在线五月| 欧洲精品卡2卡3卡4卡5卡区| 级片在线观看| 91大片在线观看| 一级毛片高清免费大全| 免费无遮挡裸体视频| 欧美精品亚洲一区二区| 亚洲电影在线观看av| 日本免费一区二区三区高清不卡| 欧美一区二区精品小视频在线| 日本五十路高清| 波多野结衣av一区二区av| 国产精品香港三级国产av潘金莲| 婷婷六月久久综合丁香| 亚洲国产欧洲综合997久久, | 国内久久婷婷六月综合欲色啪| 黑人欧美特级aaaaaa片| 国产1区2区3区精品| 欧美激情久久久久久爽电影| 亚洲三区欧美一区| 国产成人欧美| 午夜免费成人在线视频| 美女大奶头视频| 亚洲片人在线观看| 美女高潮喷水抽搐中文字幕| 一二三四社区在线视频社区8| 亚洲熟妇熟女久久| a级毛片a级免费在线| 欧美成人性av电影在线观看| 午夜免费激情av| 国产成人av教育| aaaaa片日本免费| 老司机福利观看| 男女那种视频在线观看| 国产1区2区3区精品| 搞女人的毛片| 国产精品免费视频内射| 久久久久国产精品人妻aⅴ院| 亚洲午夜精品一区,二区,三区| 精品国产亚洲在线| 熟女电影av网| 日韩欧美 国产精品| 国产野战对白在线观看| av在线播放免费不卡| 18禁国产床啪视频网站| 色综合站精品国产| 国产精品久久久久久人妻精品电影| 一级片免费观看大全| 久久性视频一级片| 久久这里只有精品19| 亚洲第一欧美日韩一区二区三区| 欧美另类亚洲清纯唯美| 人妻丰满熟妇av一区二区三区| 午夜精品在线福利| tocl精华| 97超级碰碰碰精品色视频在线观看| 欧美中文综合在线视频| 成人18禁高潮啪啪吃奶动态图| 真人做人爱边吃奶动态| АⅤ资源中文在线天堂| av在线天堂中文字幕| 久久久久国内视频| 日本撒尿小便嘘嘘汇集6| 99久久综合精品五月天人人| 丁香六月欧美| 婷婷亚洲欧美| 丝袜在线中文字幕| 婷婷精品国产亚洲av在线| 午夜福利一区二区在线看| 啦啦啦 在线观看视频| 丝袜美腿诱惑在线| 欧美zozozo另类| av福利片在线| 国产精品永久免费网站| 欧美最黄视频在线播放免费| 国产成人精品久久二区二区免费| 欧美日韩瑟瑟在线播放| 精品电影一区二区在线| avwww免费| 视频区欧美日本亚洲| 视频在线观看一区二区三区| 18禁黄网站禁片午夜丰满| 色综合欧美亚洲国产小说| 淫妇啪啪啪对白视频| 亚洲一区中文字幕在线| 老熟妇仑乱视频hdxx| 久久亚洲精品不卡| 国产一区二区激情短视频| 大型av网站在线播放| 大香蕉久久成人网| 中出人妻视频一区二区| 中文字幕久久专区| 色老头精品视频在线观看| 自线自在国产av| 免费av毛片视频| 极品教师在线免费播放| 91国产中文字幕| 国产精品 欧美亚洲| 熟妇人妻久久中文字幕3abv| 精品国产亚洲在线| 久久狼人影院| videosex国产| 一边摸一边抽搐一进一小说| 很黄的视频免费| 欧美大码av| 在线十欧美十亚洲十日本专区| 国产高清视频在线播放一区| www日本黄色视频网| 欧美激情极品国产一区二区三区| 免费搜索国产男女视频| 欧美日韩福利视频一区二区| 99久久综合精品五月天人人| 国产99久久九九免费精品| 久热爱精品视频在线9| 无遮挡黄片免费观看| 午夜a级毛片| 波多野结衣高清作品| 亚洲一码二码三码区别大吗| 中文字幕精品亚洲无线码一区 | 国产黄片美女视频| 免费看a级黄色片| 视频区欧美日本亚洲| 禁无遮挡网站| 国产久久久一区二区三区| 亚洲一区二区三区不卡视频| 不卡一级毛片| 欧美黑人精品巨大| 午夜亚洲福利在线播放| 国产真实乱freesex| 亚洲天堂国产精品一区在线| 色婷婷久久久亚洲欧美| 国产爱豆传媒在线观看 | 香蕉av资源在线| 日韩av在线大香蕉| xxxwww97欧美| 亚洲欧美日韩高清在线视频| 手机成人av网站| 国产三级在线视频| 欧美日韩中文字幕国产精品一区二区三区| 成年人黄色毛片网站| 级片在线观看| 欧美精品啪啪一区二区三区| 大型黄色视频在线免费观看| 国产精品二区激情视频| 人成视频在线观看免费观看| 老司机午夜福利在线观看视频| 在线国产一区二区在线| 国产亚洲精品久久久久久毛片| 亚洲国产看品久久| 国产精品精品国产色婷婷| 热99re8久久精品国产| 日韩大尺度精品在线看网址| 久热爱精品视频在线9| 18禁黄网站禁片免费观看直播| 国产一区二区在线av高清观看| 欧美色视频一区免费| svipshipincom国产片| 欧美绝顶高潮抽搐喷水| 亚洲真实伦在线观看| 成人免费观看视频高清| 大香蕉久久成人网| 一本精品99久久精品77| 午夜两性在线视频| 一区二区日韩欧美中文字幕| 两个人视频免费观看高清| 很黄的视频免费| 男男h啪啪无遮挡| 国产一级毛片七仙女欲春2 | 欧美日韩乱码在线| 桃红色精品国产亚洲av| 国产成人欧美在线观看| 观看免费一级毛片| 色婷婷久久久亚洲欧美| 精品国产超薄肉色丝袜足j| av天堂在线播放| 国产一区二区三区在线臀色熟女| 国产亚洲av嫩草精品影院| 日韩一卡2卡3卡4卡2021年| 窝窝影院91人妻| 天天躁夜夜躁狠狠躁躁| 久久99热这里只有精品18| 色播亚洲综合网| 久久久精品国产亚洲av高清涩受| 中出人妻视频一区二区| 淫妇啪啪啪对白视频| 午夜两性在线视频| 午夜免费成人在线视频| 香蕉av资源在线| 国产成人影院久久av| 亚洲一区高清亚洲精品| 级片在线观看| 国产视频一区二区在线看| 欧美又色又爽又黄视频| 成人亚洲精品一区在线观看| 日韩欧美一区视频在线观看| 成人免费观看视频高清| 99热只有精品国产| 日日干狠狠操夜夜爽| 国产成人啪精品午夜网站| 黑人巨大精品欧美一区二区mp4| 国产精品久久久人人做人人爽| 久久久久久久久久黄片| 一个人免费在线观看的高清视频| 亚洲人成伊人成综合网2020| 观看免费一级毛片| 亚洲中文字幕日韩| 精品久久久久久,| 午夜福利免费观看在线| 18禁裸乳无遮挡免费网站照片 | 久久久久久久久中文| 亚洲欧美精品综合一区二区三区| 此物有八面人人有两片| 欧美黄色片欧美黄色片| 黄频高清免费视频| 特大巨黑吊av在线直播 | 999久久久国产精品视频| 国产午夜精品久久久久久| 午夜福利免费观看在线| 亚洲五月天丁香| 免费在线观看黄色视频的| 色综合婷婷激情| 妹子高潮喷水视频| 久久精品aⅴ一区二区三区四区| a级毛片在线看网站| 最新在线观看一区二区三区| 亚洲精品av麻豆狂野| 精品少妇一区二区三区视频日本电影| 在线观看www视频免费| 在线免费观看的www视频| 母亲3免费完整高清在线观看| 久9热在线精品视频| 看片在线看免费视频| 啦啦啦观看免费观看视频高清| 天天一区二区日本电影三级| 韩国精品一区二区三区| 久久久久久大精品| 国产真实乱freesex| 一本精品99久久精品77| 日韩精品青青久久久久久| 天堂动漫精品| 午夜精品在线福利| 亚洲午夜理论影院| 麻豆成人av在线观看| 国产一区在线观看成人免费| 欧美激情高清一区二区三区| 欧美人与性动交α欧美精品济南到| 亚洲精品av麻豆狂野| 亚洲一区二区三区色噜噜| 国产欧美日韩精品亚洲av| 国产精品久久视频播放| 人人澡人人妻人| 又紧又爽又黄一区二区| 男人舔女人下体高潮全视频| 欧美成人一区二区免费高清观看 | 久久久久久免费高清国产稀缺| 搡老岳熟女国产| 香蕉丝袜av| 免费在线观看黄色视频的| 国产精品一区二区免费欧美| 欧美精品亚洲一区二区| 一级a爱片免费观看的视频| 国产亚洲精品久久久久5区| 国产成人一区二区三区免费视频网站| 国内精品久久久久精免费| 午夜激情av网站| 日韩 欧美 亚洲 中文字幕| 中文字幕另类日韩欧美亚洲嫩草| 女警被强在线播放| 国产成人av激情在线播放| 日本黄色视频三级网站网址| 久久精品国产清高在天天线| 亚洲五月天丁香| 欧美一级毛片孕妇| 午夜日韩欧美国产| 老司机靠b影院| 午夜免费观看网址| 久久久久九九精品影院| 欧洲精品卡2卡3卡4卡5卡区| 成人国产综合亚洲| avwww免费| 日本a在线网址| 日韩欧美国产在线观看| 亚洲av五月六月丁香网| 日韩欧美国产一区二区入口| 国产精品一区二区三区四区久久 | 亚洲欧美一区二区三区黑人| 欧美午夜高清在线| 国产成人精品久久二区二区91| 日韩一卡2卡3卡4卡2021年| 黄色毛片三级朝国网站| 18禁观看日本| 波多野结衣高清作品| 禁无遮挡网站| 50天的宝宝边吃奶边哭怎么回事| 日本黄色视频三级网站网址| 亚洲,欧美精品.| 中国美女看黄片| 丰满人妻熟妇乱又伦精品不卡| 老司机在亚洲福利影院| 国产不卡一卡二| 亚洲aⅴ乱码一区二区在线播放 | 操出白浆在线播放| 国产99久久九九免费精品| 每晚都被弄得嗷嗷叫到高潮| 日本精品一区二区三区蜜桃| 成人手机av| 精品一区二区三区av网在线观看| 男人舔女人的私密视频| 成人亚洲精品一区在线观看| 中文字幕人妻熟女乱码| 夜夜躁狠狠躁天天躁| 国产一级毛片七仙女欲春2 | 一边摸一边抽搐一进一小说| 欧洲精品卡2卡3卡4卡5卡区| 免费无遮挡裸体视频| 国产精品香港三级国产av潘金莲| 欧美zozozo另类| 久久人妻av系列| 亚洲国产高清在线一区二区三 | 亚洲精品美女久久av网站| 成年人黄色毛片网站| 欧美一级a爱片免费观看看 | 女性被躁到高潮视频| 中文字幕人成人乱码亚洲影| 性色av乱码一区二区三区2| 精品日产1卡2卡| 老司机靠b影院| 天堂√8在线中文| 男人操女人黄网站| 人妻丰满熟妇av一区二区三区| 亚洲国产高清在线一区二区三 | 国产精品二区激情视频| 这个男人来自地球电影免费观看| 黑人欧美特级aaaaaa片| 久久久久久免费高清国产稀缺| 长腿黑丝高跟| 中文字幕人妻熟女乱码| 人人妻人人看人人澡| 国产亚洲精品第一综合不卡| 亚洲av电影在线进入| 久99久视频精品免费| 夜夜躁狠狠躁天天躁| 欧美亚洲日本最大视频资源| 天堂√8在线中文| 国产精品美女特级片免费视频播放器 | 精品国产一区二区三区四区第35| 一个人免费在线观看的高清视频| 午夜激情福利司机影院| 国产aⅴ精品一区二区三区波| 国产亚洲精品久久久久久毛片| 欧美黑人欧美精品刺激| 久久香蕉国产精品| bbb黄色大片| 在线观看免费视频日本深夜| 久久天堂一区二区三区四区| 制服诱惑二区| 超碰成人久久| 亚洲片人在线观看| 黄频高清免费视频| 国产在线精品亚洲第一网站| 妹子高潮喷水视频| 国产av不卡久久| 欧美中文综合在线视频| 老司机靠b影院| 国产精品自产拍在线观看55亚洲| 性欧美人与动物交配| 国产黄片美女视频| 国产精品自产拍在线观看55亚洲| 日韩欧美在线二视频| 精品人妻1区二区| 满18在线观看网站| 制服人妻中文乱码| 丁香欧美五月| 色精品久久人妻99蜜桃| 超碰成人久久| 久久狼人影院| 中国美女看黄片| 女警被强在线播放| 91九色精品人成在线观看| 自线自在国产av| 香蕉丝袜av| 精品一区二区三区四区五区乱码| 亚洲精品国产一区二区精华液| 免费无遮挡裸体视频| 午夜免费成人在线视频| 美女高潮喷水抽搐中文字幕| 欧美日韩福利视频一区二区| 亚洲 欧美一区二区三区| 18禁观看日本| 露出奶头的视频| 99久久国产精品久久久| 搞女人的毛片| 久久久久久久午夜电影| 少妇的丰满在线观看| 曰老女人黄片| 免费在线观看完整版高清| 精品一区二区三区四区五区乱码| 亚洲国产欧美网| 伊人久久大香线蕉亚洲五| 草草在线视频免费看| 一进一出好大好爽视频| 国产男靠女视频免费网站| 男人舔女人的私密视频| 亚洲专区中文字幕在线| 99国产综合亚洲精品| 亚洲免费av在线视频| 国产精品乱码一区二三区的特点| 日韩欧美一区视频在线观看| 一本久久中文字幕| 精品国产乱子伦一区二区三区| 日韩欧美国产一区二区入口| 国产av不卡久久| 很黄的视频免费| 婷婷亚洲欧美| 长腿黑丝高跟| 久久这里只有精品19| 久久久久免费精品人妻一区二区 | 国产男靠女视频免费网站| 精品久久久久久久毛片微露脸| 99riav亚洲国产免费| 最近最新中文字幕大全电影3 | 国产一区二区在线av高清观看| 国产区一区二久久| 国产成人欧美| 在线观看免费日韩欧美大片| 午夜免费鲁丝| 美女午夜性视频免费| 日日干狠狠操夜夜爽| 男女那种视频在线观看| 日本免费一区二区三区高清不卡| 欧美成人性av电影在线观看| 久久人妻av系列| 久久国产精品影院| 99精品欧美一区二区三区四区| 啪啪无遮挡十八禁网站| 亚洲一区高清亚洲精品| av电影中文网址| 国产一区在线观看成人免费| 欧美zozozo另类| 日韩欧美国产在线观看| www.自偷自拍.com| 欧美成人一区二区免费高清观看 | 国产视频一区二区在线看| 波多野结衣高清作品| 99久久精品国产亚洲精品| 女性被躁到高潮视频| 他把我摸到了高潮在线观看| 一进一出抽搐动态| 成人永久免费在线观看视频| 亚洲国产高清在线一区二区三 | 啦啦啦免费观看视频1| 丝袜人妻中文字幕| 久久 成人 亚洲| 亚洲欧美日韩无卡精品| 国产av又大| 我的亚洲天堂| 中文字幕人成人乱码亚洲影| 成人国语在线视频| 国产一级毛片七仙女欲春2 | 成人精品一区二区免费| www日本黄色视频网| 久99久视频精品免费| 亚洲国产欧美日韩在线播放| 黄频高清免费视频| 久久 成人 亚洲| 日韩三级视频一区二区三区| 色av中文字幕| 亚洲av成人不卡在线观看播放网| 香蕉国产在线看| 色老头精品视频在线观看| 天天添夜夜摸| 国产亚洲精品av在线| 精品乱码久久久久久99久播| 精品一区二区三区四区五区乱码| 少妇粗大呻吟视频| 最近最新免费中文字幕在线| 美国免费a级毛片| 国产一区二区三区在线臀色熟女| 久久午夜综合久久蜜桃| 久久人人精品亚洲av| 青草久久国产| 亚洲最大成人中文| 女生性感内裤真人,穿戴方法视频| 久久久久国产一级毛片高清牌| 一区二区三区激情视频| 欧美日韩亚洲综合一区二区三区_| 老司机福利观看| 久久精品人妻少妇| 69av精品久久久久久| 久久伊人香网站| 午夜福利在线在线| 18禁裸乳无遮挡免费网站照片 | 久久青草综合色| 精品久久久久久,| 成人手机av| 国产精品98久久久久久宅男小说| 欧美黑人欧美精品刺激| 国产精品久久电影中文字幕| 欧美一区二区精品小视频在线| 久久久久久久午夜电影| 亚洲欧美激情综合另类| 少妇熟女aⅴ在线视频| 午夜免费观看网址| 欧美丝袜亚洲另类 | 18禁美女被吸乳视频| 亚洲精品av麻豆狂野| 亚洲美女黄片视频| 18禁裸乳无遮挡免费网站照片 | 婷婷亚洲欧美| 男女午夜视频在线观看| 一本久久中文字幕| 好看av亚洲va欧美ⅴa在| 精品电影一区二区在线| 欧美成人一区二区免费高清观看 | 99国产精品一区二区三区| av在线天堂中文字幕| 久久国产精品人妻蜜桃| 99精品欧美一区二区三区四区| 高清在线国产一区| 国产蜜桃级精品一区二区三区| 99久久国产精品久久久| 老汉色∧v一级毛片| 欧美成人性av电影在线观看| 一级毛片高清免费大全| 亚洲性夜色夜夜综合| 亚洲av电影不卡..在线观看| 色在线成人网| 成人亚洲精品av一区二区| 欧美日本亚洲视频在线播放| 99精品久久久久人妻精品| 国产成人欧美| 国产成人影院久久av| 老司机深夜福利视频在线观看| 狠狠狠狠99中文字幕| 99久久99久久久精品蜜桃| 十八禁网站免费在线| 男男h啪啪无遮挡| 亚洲,欧美精品.| 丰满人妻熟妇乱又伦精品不卡| 国产在线观看jvid| 欧美日韩亚洲国产一区二区在线观看| av天堂在线播放| 给我免费播放毛片高清在线观看| 久久久久久人人人人人| 成人手机av| 亚洲精品一卡2卡三卡4卡5卡| 欧美成人一区二区免费高清观看 | 真人做人爱边吃奶动态| 波多野结衣av一区二区av| 热99re8久久精品国产| 亚洲精品中文字幕在线视频| 日韩大码丰满熟妇| 色哟哟哟哟哟哟| tocl精华| 在线观看免费视频日本深夜| 男女视频在线观看网站免费 | 免费看日本二区| 午夜影院日韩av| 在线看三级毛片|