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Effect of Antibiotic Growth Promoters on Broiler Performance, IntestinalGrowth Parameters, and Quantitative Morphology1

R. D. Miles,*2 G. D. Butcher, P. R. Henry,* and R. C. Littell

*Department of Animal Sciences, Department of Large Animal Clinical Science,and Department of Statistics, University of Florida, Gainesville 32611

ABSTRACT The effects of addition of bacitracin meth-ylene disalicylate (BMD) or virginiamycin (VM) to a corn-soybean meal diet on broiler performance and gastroin-testinal tract (GIT) growth parameters and morphologywere studied at various ages during growthand finishing.Male and female birds were killed at 1, 3, 5, or 7 wk ofage for gross and histologic examination of the duodenumand ileum. Feeding either antibiotic increased BW anddecreased intestinal length and weight at all times com-pared with control birds. However, intestinal length and

weight decreases were greater in birds fed VM than BMD

Key words: antibiotics, intestinal tract, broiler, morphology

2006 Poultry Science 85:476485

INTRODUCTION

The establishment of a microbial population in the gas-trointestinal tract (GIT) of all warm-blooded animals soonafter birth is inevitable. Naqi et al. (1970) showed that a

bacterial population is present in the small intestine in theavian species within 24 h following hatch. The microflora,

capable of intense metabolic activity, have both beneficialand detrimental effects on the host animal. Apajalahti etal. (2004) recently reported that, based on new techniquesto determine characteristic features of the microbial DNAfrom GIT community samples, approximately 90% of the

bacteria in the chicken GIT represent previously unknownspecies; thus, gut microflora are a largely unexplored phe-nomenon, especially regarding the effects of antibioticgrowth promoters upon these largely unknown species. Ithas been hypothesized that gut microflora decrease nutri-ent absorption by increasing GIT thickness, the rate ofdigesta passage, and also increase nutrient requirementsof the host by increasing turnover of the gut mucosae and

by competing with the host for a portion of the dietaryenergy and protein (Ravindran et al., 1984; Apajalahti etal., 2004).

2006 Poultry Science Association, Inc.Received July 8, 2005.Accepted October 29, 2005.1Florida Agricultural Experiment Station Journal Series Number R-

10894.2Corresponding author: RDMiles@ufl.edu

476

at 1 and 3 wk of age. The only change found in theduodenum resulting from dietary treatment was an in-crease in the number of villi per unit length in birds givenVM but not BMD or control. In the ileum, the muscularismucosa was thinner in birds given VM than in those fedthe control diet. Chicks supplemented with VM had asmaller total villus area and shorter villus height andcrypt depth in the ileum than birds fed the control dietor BMD. Physical changes in the intestine of birds giveneither antibiotic growth promoter, although not the same,

resulted in improved performance.

All antibiotics control growth and proliferation of micro-organisms; however, all antibiotics do not accomplish thiscontrol by the same mechanism (Ferket, 2004). Conse-quently, antibiotics differ with regard to their ability toinfluence certain disease states or improve growth andfeed efficiency. Much of the work with antibiotic growthpromoters continues to be from the standpoint of studying

the effects on easily cultured bacterial populations such aslactobacili and Clostridium perfringes and poultry healthrather than resulting physical changes to the GIT (Georgeet al., 1982; Engberg et al., 2000; Sims et al., 2004). Severalresearchers have studied intestinal morphology in poultryduring the last decade but predominantly from the stand-point of normal development and notwith regard to effectsof antibiotics (Uni et al., 1995, 1998, 1999; Geyra et al.,2001).Krinke andJamroz (1996)studied organ morphologyafter feeding various amounts of avoparcine to broilers for70 d but not at earlier times, and they failed to provide astatistical analysis of their results. The objective of this

experiment was to study the effect of 2 antibiotic growthpromoters on broiler performance and GIT growth param-eters and morphology at various ages during growthand finishing.

MATERIALS AND METHODS

At 1 d of age, 10 male and 10 female Ross Ross chickswere assigned randomly to each of 72 floor pens to adensity of 0.7 ft2/bird, creating 24 replicates of 3 experi-

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Table 2. Effect of antibiotics and age on BW of broilers1

BW, g

Control Bacitracin methylene disalicylate Virginiamycin

Age, wk Male Female Male Female Male Female

1 156 4.61 156 5.23 170 2.99 165 2.93 163 5.57 164 3.393 749 16.9 640 19.2 770 19.2 699 13.3 769 18.9 666 16.25 1,752 40.8 1,414 31.4 1,746 36.5 1,479 26.9 1,801 35.4 1,492 27.27 2,705 61.1 2,175 46.4 2,752 54.0 2,306 41.0 2,738 37.5 2,269 57.7

1Each value represents the mean SE of 16 birds except for wk 7 virginiamycin females = 15. Main effect ofdiet (P < 0.05). Main effects of sex and week (P < 0.001). Sex week interaction (P < 0.001).

fed the control diet at 7 wk of age. Body weight averaged2.48 0.024 kg for birds given the control diet, 2.53 0.023kg for those given BMD, and 2.54 0.022 kg for thosegiven VM. Chickens fed VM had better (P < 0.05) feedconversion (1.95 0.015 kg/kg)than birds given the control(1.99 0.022 kg/kg), but those fed BMD (1.97 0.020 kg/kg) were not different from either of the other 2 groups(data not shown).

There was also an effect of antibiotic treatment (P < 0.05)on BW of broilers used for the sampling procedures (Table2). Across all time periods and both sexes, BW averaged1.22, 1.26, and 1.26 kg for birds given control, BMD, andVM, respectively. There were also main effects of time andsex as well as their interaction (P < 0.001) on BW in whichthe males grew progressively larger than the females asthey became older.

There was a diet week interaction (P < 0.01) on GITweight per unit BW (Table 3) as well as main effects (P 0.05) in this percentage.

There were main effects of sex (P < 0.01) and week(P < 0.0001) on villus height in the duodenum (Table 6).Duodenal villus height averaged 1,753 m for males and1,650 m for females across all times and dietary treat-

Table 7. Effect of antibiotics and age on characteristics in the ileum of broilers 1

Muscularis Lamina Lamina propria,interna Muscularis mucosae propria core Villus area, % of

Time, wk Treatment thickness, m thickness, m area, m2 m2 villus area

1 Control 125 18 21.2 1.1 22,257 1,173 40,203 1,479 55.3 1.9BMD2 143 14 22.8 1.8 23,027 2,145 38,798 3,003 59.4 5.0Virginiamycin 93 8 18.7 1.4 23,382 1,275 39,892 1,597 58.4 1.6

3 Control 157 15 25.1 1.8 34,234 2,703 59,785 3,532 56.7 2.0BMD 131 9 21.9 2.1 34,855 2,537 61,640 3,422 56.1 1.7Virginiamycin 141 11 22.6 1.3 33,507 3,989 53,556 3,477 65.6 11.4

5 Control 169 18 25.0 1.9 41,407 2,043 74,237 4,231 56.1 1.4BMD 132 12 22.2 1.1 38,569 2,616 63,851 2,798 60.1 2.0

Virginiamycin 130

11 21.5

1.3 29,603

1,724 52,244

2,422 56.5

1.77 Control 204 13 30.8 1.4 34,723 1,324 59,149 1,745 58.8 1.7BMD 174 16 26.7 1.8 40,058 2,350 67,676 4,480 59.6 1.7Virginiamycin 168 10 26.8 1.0 36,183 1,870 61,177 2,912 59.0 0.7

ANOVA Probability

Treatment (T) 0.0663 0.0429 NS 0.0135 NSSex (S) NS3 NS NS NS NST S 0.0583 NS NS NS NSWeek (W) 0.0001 0.0001 0.0001 0.0001 NST W NS NS 0.0500 0.0023 NSS W NS NS NS 0.0394 NST S W NS NS NS NS NS

1Each value represents the mean SE of 5 males and 5 females.2BMD = Bacitracin methylene disalicylate.3NS = not significant (P > 0.10).

ments. The villus height was shorter (P < 0.05) in chicksat 1 wk of age (1,377 m) than at 3 (1,858 m), 5 (1,830m), or 7 wk (1,741 m). There was a treatment sexinteraction (P < 0.05) in which the females given eitherantibiotic had shorter (P < 0.05) villi than either sex fedcontrol or the males given BMD. There was also a 3-wayinteraction (P < 0.01) on duodenal villus height. At 1 or 7wk, the females given all diets tended (P < 0.10) to have

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Table 8. Effect of antibiotics and age on characteristics in the ileum of broilers 1

Depth of VillusVillus height, Number of crypts of width at Mitotic

Time, wk Treatment m villi/1,000 m Lieberkuhn, m midvillus, m index,2 %

1 Control 657 19 10.6 0.62 136 7.9 87.9 3.5 2.92 0.18BMD3 692 47 10.6 0.44 169 12.8 88.3 2.4 3.16 0.18Virginiamycin 599 21 10.1 0.36 150 6.7 90.0 2.6 2.97 0.19

3 Control 1,019 38 9.4 0.34 157 5.5 93.9 2.8 2.87 0.12BMD 1,004 50 9.3 0.41 152 5.0 100.3 2.7 3.03 0.21Virginiamycin 922 33 9.5 0.48 153 8.1 97.8 3.0 2.74 0.18

5 Control 1,107 54 9.4 0.33 173 9.5 102.9 2.2 2.61 0.18BMD 1,118 45 10.4 0.54 148 9.9 96.5 3.0 2.91 0.14Virginiamycin 1,086 46 11.2 0.55 142 8.3 83.1 3.5 2.91 0.13

7 Control 1,397 48 10.4 0.56 178 11.8 93.3 3.1 2.30 0.10BMD 1,377 67 9.5 0.26 191 10.8 96.9 3.6 2.14 0.08Virginiamycin 1,225 50 10.1 0.52 141 6.5 96.2 6.4 2.48 0.12

ANOVA Probability

Treatment (T) 0.0229 NS 0.0229 NS NSSex (S) NS4 0.0712 NS NS NST S NS 0.0212 NS NS 0.0681Week (W) 0.0001 0.0245 0.0205 0.0112 0.0001T W NS NS 0.0011 0.0077 NSS W NS NS NS NS NST S W NS NS 0.0242 NS 0.0974

1Each value represents the mean SE of 5 males and 5 females.2

Mitotic index was calculated as the percentage of cells in the crypts of Lieberkuhn undergoing mitosis at the moment of fixation.3BMD = bacitracin methylene disalicylate.4NS = not significant (P > 0.10).

shorter villi than the males on the same diets. At 7 wk ofage, females (1,494 m) given VM had shorter (P < 0.05)villi than their male (1,910m) counterparts as did females(1,610 m) given BMD at 5 wk compared with their male(2,082 m) counterparts. Generally, at 3 or 5 wk of agemales given the control diet or VM tended (P < 0.10) tohave taller villi than females on the same diet and of similarage, whereas at that time males given BMD had shorter

villi than their female counterparts.There were effects of week (P < 0.001) and treatment (P

< 0.05) on the number of villi per 1000 m of duodenum(Table 6). Birds given VM had more (P < 0.05) villi (8.47/1,000 m) than those given control (7.71/1,000 m) orBMD (7.91/1,000 m). At 1 wk, chicks had more (P < 0.05)villi than at older ages (8.76, 7.84, 8.13, and 7.39/1,000 mat 1, 3, 5, and 7 wk, respectively).

The depth of the crypts of Lieberkuhn in duodenumwas greater (P < 0.0001) at 7 wk of age than it was in anyyounger birds (Table 6). There was also a main effect ofweek (P< 0.0001) on the villus width at midvillus in duode-num (Table 6). Villus width averaged 109 m for chicks

at 1 wk of age, which was less (P < 0.05) than that at 3(118 m) or 5 wk (117 m), which was, in turn, less thanthat in birds at 7 wk (128 m). There was no difference (P> 0.10) in duodenal mitotic index (Table 6).

There was a main effect of week (P < 0.0001) on thethickness of muscularis interna in ileum (Table 7). As thechicks matured, this tissue became thicker and averaged120, 143, 144, and 182 m at 1, 3, 5, and 7 wk, respectively.Birds given antibiotics tended (P < 0.10) to have a thinnerlayer of muscularis interna than chicks given control (164,145, and 133 m for control, BMD and VM-fed birds, re-spectively).

Muscularis mucosae of ileum was also thicker (P < 0.05)in birds given control (25.5 m) than in those given VM(22.4 m; Table 7). Chicks given BMD (23.4 m) wereintermediate. There was also an effect of week (P < 0.0001)on the thickness of muscularis mucosae of ileum (Table7). The layer averaged 20.9, 23.2, 22.9, and 28.1 m at 1,3, 5, and 7 wk, respectively, with the value for 7 wk greater(P < 0.05) than those at earlier ages.

The lamina propria core area (Table 7) was smaller (P< 0.05) in chicks at 1 wk (22,889 m2) than at 3 (34,199m2), 5 (36,526 m2), or 7 wk (36,988 m2). There was alsoa treatment by week interaction (P < 0.05) on ileum laminapropria core area. At 1, 3, or 7 wk, values were similar forall dietary treatments; however, at 5 wk chicks given con-trol or BMD had greater (P < 0.05) core area of laminapropria than birds given VM.

There were main effects of treatment (P < 0.05) and week(P < 0.0001) on the total villus area of ileum (Table 7).Chicks given VM (51,717 m2) had a smaller (P < 0.05)total villus area than those given the control diet (58,344m2) or BMD (57,991 m2). The total villus area was

smaller (P < 0.05) in chicks at 1 wk (39,631 m2) than at 3(58,327 m2), 5 (63,444 m2), or 7 wk (62,668 m2). Therewas also a treatment by week interaction (P < 0.01) inwhich dietary treatments resulted in similar villus area at1 and 7 wk, but birds given VM had smaller ( P < 0.05)villusarea at 3 or 5 wk than birds on the other 2 treatments.There was also a week by sex interaction (P < 0.05) on totalvillus area in which males displayed a quadratic responseto time whereas the females showed a cubic response. Thevillus area peaked in males at 5 wk and then declined at7 wk. In females the villus area reached a plateau from 3to 5 wk then increased again. As with duodenum, when

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the percentage of villus area that is comprised of laminapropria was calculated for ileum, there was no difference(P > 0.10).

Main effects of treatment (P < 0.05) and week (P < 0.0001)on villus height were found in the ileum (Table 8). Villusheight increased linearly with age from 649 m at 1 wkto 982, 1,103, and 1,333 m at 3, 5, and 7 wk, respectively.Each of these means differed (P < 0.05) from the other 3

means. Birds given VM (958 m) had shorter (P 0.05) tracts than females (178 10.9 g force).

DISCUSSIONWith the recent technological developments in digital

imaging and cell labeling, there have been a number ofpapers describing maturation of the intestinal tract of poul-try, including quantitative measurements of morphology,during the first 10 to 14 d of life (Uni et al., 1995, 1998,1999; Noy and Sklan, 1995; Noy et al., 2001; Geyra et al.,2001; Batal and Parsons, 2002); however, few, if any, havestudied the effects of antibiotic growth promoters or gen-der, or followed broiler chicks to market age.

It could be speculated that males grow more rapidlybecause the intestinal tract, and hence, absorptive capacity,

develops more rapidly. There was a 100 g difference inBW by 3 wk of age, which widened to a difference ofalmost 500 g at 7 wk. The BW did not differ between sexesat 1 wk but did at 3 to 7 wk. Males overall had the lighterweight tracts than females due, in part, to thinner muscu-laris mucosae in the duodenum by 2m. Males had longervilli in duodenum by an average of 100 m, which wouldincrease surface area for absorption regardless of the dietfed, but this was not found in the ileum. Older females(from 5 to 7 wk), however, had shorter GIT (on a per unitof BW basis) than the males and, thus, less tissue mass tomaintain, but also less surface area availablefor absorption.

Jukes et al. (1956) noted that males had generally longer,

heavier tracts than females. No interactions between weekand sex were detected for most of the other morphologicalmeasurements in both sections of the GIT. The depth of thecrypts and mitotic index did not differ between genders,indicating a similar rate of tissue renewal.

The antibiotic growth promoters used in this experimentwere chosen because, although they both control lacticacid-producing bacteria as well as other gram-positive mi-croorganisms, they do not share a similar mode of action.Thus, it would be possible for them to have different effectson the GIT morphology. Bacitracin methylene disalicylateis a branched, cyclic decapeptide compound that interfereswith cell membrane function, suppresses cell wall forma-tion by preventing the formation of peptidoglycan strands,and inhibits protein synthesis (Kahn et al., 2005). Virgin-iamycin is a streptogramin combination that is a fermenta-tion product of Streptomyces virginiae, which inhibitsprotein synthesis by blocking ribosomal activity (Cocito,1979). The 2 major molecules forming this antibiotic (M1and S1) function synergistically by binding to 2 separateloci on the bacterial ribosome (Cocito, 1979).

Growth promoting antibiotics do not sterilize the GIT;they control certain populations of microorganisms to the

betterment of the host, as described recently by Ferket(2004). The various benefits of feeding antibiotic growth

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promoters, including VM and BMD, have been reviewedwidely (Henderickx, 1981; Mygind-Rasmussen, 1984;Miles, 1985; Lindsey, 1985; Bafundo et al., 2003; Ferket,2004) and will not be repeated herein. The benefits ob-served on chick growth and feed conversion in the presentstudy were similar to those reported widely in the reviews

just mentioned and were consistent across all ages. Thetreatment week and 3-way interactions on BW were

not significant, indicating no difference in the effect ofantibiotics at different ages or either gender despite the factthat percentage of increase in BW was somewhat higher at1 wk of age (7.0% for BMD and 5.1% for VM) than at the3 older ages, which averaged 3.7 and 3.3% for BMD andVM, respectively. These growth promoters are known toinhibit normal early microbial proliferation, and hencecompetition for needed nutrients during the gut matura-tion process, which normally takes approximately 6 to 9d in chicks (Geyra et al., 2001).

Some benefits of feeding antibiotics reported previously,such as sparing energy from tissue maintenance that can

be used instead for growth, or improving the absorption

of various nutrients, could result in some anticipatedchanges in GIT morphology that would include decreasedcell proliferation, thinner mucosae, less lamina propria,and an increase in absorptive surface area. In the presentexperiment there was a greater effect of antibiotic growthpromoters in the ileum than in the duodenum. Stutz etal. (1983) supplemented broiler chicks with 55 ppm of

bacitracin and reported a 9% decrease in weight of theduodenum and a 41% decrease in the ileum. When Cu-montmorillonite was fed to chicks, villus height increased

by 12% in the duodenum and 14% in the jejunum andileum (Xia et al., 2004). Iji et al. (2001) reported age (1, 7,14, and 21 d) and location (duodenum, jejunum, and ileum)

effects as well as their interaction on crypt depth, villusheight, and villus surface area of chicks. Hence, in thepresent trial measurements in the 2 locations were ana-lyzed separately. Krinke and Jamroz (1996) reported re-duced duodenal cell proliferation in chicks fed avoparcin,

but these authors did not examine other sections of the GIT.The thinning effect of antibiotics on the GIT of poultry

has been known for many years (Coates et al., 1955; Jukeset al., 1956) and was observed with both of the growthpromoters used in the present experiment. Early on Draper(1958) demonstrated that the decrease in intestinal weightof chicks fed penicillin resulted in improved nutrient use.Stutz and Lawton (1984) noted decreased weight of smallintestine in broilers in numerous experiments with variousantibiotic growth promoters, including bacitracin and VM,

but not with other antimicrobial agents such as 3-nitro-4-hydroxy-phenylarsonic acid, arsanilic acid, furazolidone,or sulfathiazole. Stutz et al. (1983) also reported a decreasein the GIT length per unit of BW of all 3 sections of smallintestine when 55 ppm of zinc bacitracin was fed to broilerchicks for 14 d. Dafwang et al. (1985) reported decreasedintestinal weights in chicks given various antibiotics, in-cluding VM, in several experiments. Henry et al. (1986,1987) reported that VM decreased intestinal tract weightof broiler chicks at 3 wk of age an average of 0.6 g/100 g

of BW in 3 experiments. The decrease was attributed tothinning of the intestinal wall through visual observation,although this was not measured. Fethiere and Miles (1987)also reported an average decreased intestinal tract weightof 0.35 g/100 g of BW in chicks given VM. Franti et al.(1972) noted a general decrease in intestinal tract lengthwhen zinc bacitracin or oxytetracycline was fed to broilersat levels to promote growth rather than therapeutic levels

for various periods of time up to 60 d of age. In the presentexperiment, the intestinal length was also shorter in birdsgiven VM than in those given the control diet.

Although the tracts were measurably thinner and lighterin weight in chicks given the antibiotics in the presentstudy, these tracts were not, however, weaker and thusmore susceptible to breakage and contamination of car-casses during processing. As reported by Bilgili and Hess(1996), the males in the present study had stronger tractsthan the females.

Numerous anatomical features determine the total ab-sorptive surface area of the GIT. Tract length, villus heightand width, and the number of villi per unit area all contrib-

ute to absorptive capacity along with the structure of themicrovilli, which was not studied in this trial. Solomonand Tullett (1988) also noted taller, narrower, and regularlyshaped villi in the ileum at 3 wk of age but provided noquantitative units of measurement. They concluded thatthese villi provided greater surface area for absorption.In the present experiment, the villi were shorter in theduodenum and ileum of birds given VM but the structureof the microvilli was not examined by electron microscopyto further define surface area. Shamoto and Yamauchi(2000) reported villus height in White Leghorn males fedconventional grower mash for approximately 49 d to besimilar to that found in the present experiment at the same

age. Xu et al. (2003) fed various amounts of fructooligosac-charide to broilers for 49 d, and Xia et al. (2004) studiedthe effects of Cu-montmorillonite in broilers for 49 d. Bothof these authors reported villus heights in duodenum andileum that were less than one-half of the size of those foundin the present study at a similar age. These authors alsoreported crypt depths more than twice that in the presentstudy for birds given the control or antibiotic-treated diets.Changes in GIT morphology such as deeper crypts andshorter villi have been associated with the presence oftoxins (Yason et al., 1987). Mathlouthi et al. (2002) foundshorter, narrower villi in ileum of birds fed rye-based com-pared with corn-based diets. The ileal villi of their corn-fed chicks averaged 70% of the height found in 21-d-oldchicks in the present study. Mathlouthi et al. (2002), how-ever, had no antibiotics or coccidiostats in any of their diets.

Contrary to the results of Solomon and Tullett (1988),the villus height in the ileum was shorter in birds givenVM and tended to be shorter in the duodenum. In theduodenum, however, there were more villi per unit area,which would also increase absorptive surface area. Solo-mon and Tullett (1988) did not have the large number ofreplications used in the present experiment. Gordon andBruckner-Kardoss (1961b) reported a 30% decrease in theabsorptive surface area in the lower intestine of germ-

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free vs. conventional adult rats. The difference was not aspronounced in the duodenum.

Decreased lamina propria and reduced mucosal surfacearea have been reported in germ-free vs. conventional ratsand chicks (Gordon and Bruckner-Kardoss, 1961b). Gor-don and Bruckner-Kardoss (1961a) also reported that feed-ing penicillin created a suppression in lamina propriasimilar to that in germ-free chicks. Prior to this, Jukes

et al. (1956) reported a decrease in the thickness of themuscularis mucosae and lamina propria, as well as shortervilli in the duodenum of chicks fed penicillin for 4 wkcompared with control birds. In the present experiment,the percentage of the villus area containing the laminapropria in the duodenum or ileum did not differ signifi-cantly when either VM or BMD were fed, although therewas a numerical decrease of VM-fed and increase of BMD-fed birds in duodenum at 1 wk of age compared with

birds given the control diet. This decreasing trend wasobserved for both antibiotics at 3 wk of age but not when

birds were older. There was an interaction between treat-ment and time on the area of villi in duodenum and ileum

and the area of lamina propria in the ileum.In the present experiment, the muscularis interna and

muscularis mucosae from the ileum was almost consis-tently thinner in birds given antibiotics and correlated withintestinal weight, whereas that in the duodenum showedan interaction with time that was not very well correlatedwith the weights. Krinke and Jamroz (1996) measuredthickness of the intestinal wall from the muscularis muco-sae tothe tip ofvillus inthe duodenumat 70d and reportedsimilar thickness in broilers given a control diet or thosesupplemented with 3 levels of avoparsine.

The crypt is the area where stem cells divide to allowreplacement of endocytes in the villi. A deeper crypt and

greater mitotic index are indicative of faster tissue turnoverto permit renewal of the villus as needed in response tonormal sloughing or inflammation from pathogens andtheir toxins. More energy would be required for fasterturnover, and hence, less available for growth of other

body tissues. Greater tissue mass in the GIT, to protect thebody from entry of pathogens, would also require moreenergy/protein to maintain. The changes that occur inthe GIT as the result of normal microflora are generallyconsidered to represent a mild state of inflammation (Wal-ton, 1988). The walls of the intestine thicken due to anincrease in the inflammatory cells of the lamina propria.These villi renew their surface layer frequently, which re-sults in almost 20% of the protein surface being shed dailyinto the lumen of the intestine. Addition of a growth pro-moter to the diet should decrease microbial insult and,thus, the amount of inflammation in the GIT (Walton,1988). There is a reduction in the number of inflammatorycells below the surface epithelium. Less of the protein sur-face is shed into the lumen. Visual inspection of the GIT ofpoults infected with an innoculum from turkeys diagnosedwith malabsorption syndrome revealed thin walls distalto the duodenum that contained liquid, gas-filled digesta(Sell, 1990). Jejunal tissue showed the villi were shorterand the crypts deeper in infected birds. Krinke and Jamroz

(1996) reported decreased intestinal cell proliferation asmeasured by bromodeoxyuridine labeling in broilers given7.5, 10, or 15 ppm of avoparcine for 70 d.

Birds given either antibiotic in the present experimenthad deeper crypts in the ileum indicative of faster tissuedevelopment at 1 wk of age. At 3 wk of age the cryptdepth was similar among all the treatments, but thereafter,chicks given VM had less crypt depth, generally indicative

of less inflammationin the tract. Birds given BMD followedthis trend up to 5 wk of age but then had a sudden increasein ileal crypt depth even beyond that of birds given thecontrol diet. In the duodenum a nonsignificant trend wasobserved in which birds given VM tended to have shortercrypts than birds given the control or BMD diets. Xia etal. (2004) reported increases in villus height and villusheight:crypt depth ratio in the small intestine of chickssupplemented with Cu-montmorillonite, probably as a re-sult of fewer E. coli and clostridium. The depth of the cryptsthey reported, however, were more than twice that foundin the present study at a similar age (7 wk). Values forcrypt depth in duodenum and ileum at 1 wk of age in the

present study were similar to those reported by Uni et al.(1998) for broilers of a similar age as were those in theileum at 3 wk compared with chicks fed corn-soy diets byMathlouthi et al. (2002).

There is no simple explanation for the exceedingly com-plex relationship among host, normal microbes, and theincoming nutrients needed by both. Results of this experi-ment have verified that although both of the antibioticgrowth promoters control some segments of the microbialpopulation, they also result in differing effects on GITmorphology. It is most likely a balance among these effectsproviding the extra nutrients for greater growth to occur.

ACKNOWLEDGMENTS

The authors wish to thank SmithKline Animal HealthProducts for providing virginiamycin andfunds in supportof this research and C. Beck, C. W. Comer, and D. M. Jankyfor technical support.

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