Poult. Sci.
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Poult Sci 2008. 87:606-611. doi:10.3382/ps.2006-00458
© 2008 Poultry Science Association
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Willis, W. L.
Right arrow Articles by Reid, L.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Willis, W. L.
Right arrow Articles by Reid, L.

ENVIRONMENT, WELL-BEING, AND BEHAVIOR

Investigating the Effects of Dietary Probiotic Feeding Regimens on Broiler Chicken Production and Campylobacter jejuni Presence1

W. L. Willis2 and L. Reid

Department of Animal Sciences, North Carolina Agricultural and State University, Greensboro 27411

2 Corresponding author: willisw{at}ncat.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
An experiment was conducted to investigate the effect of restricted feeding of a commercially available probiotic diet on production/processing performance, Campylobacter jejuni prevalence, and organ weights in broiler chickens. Five hundred forty 1-d-old broiler chicks were randomly assigned to a control or a direct-fed microbial (PrimaLac, DFM) diet and subjected to ad libitum full-fed (A), restricted 8-h (R), or skip-a-day (S) feeding regimens. Each of the 6 treatments was replicated 3 times with 15 male and 15 female chicks per pen for 49 d. Significant (P < 0.05) differences between BW in the control and DFM groups with regard to feed type were found at d 7 (A), female d 21 (R) male and females, and d 49 (A and S) male and females. Body weights of males in the control group were significantly higher than the DFM (A) and differed by regimens (A>R>S) at d 49, whereas weights of females did not differ in regimens A and S. Body weight in the control females of regimen R was significantly higher than those in regimens A and S. Carcass yield was significantly higher for males in the control regimen A, 78.1 vs. 74.6% for the DFM regimen A; however, females did not differ significantly in this regimen, but did so in regimen S with 72.6 vs. 69.0%. The gizzard weights were significantly higher for broilers exposed to S and R regimens when compared with the A regimen. The prevalence of C. jejuni in the DFM-treated broilers regimen R was lower (33 vs. 60% positive) for the control group at 21 d. The weekly BW throughout the study reflected many variations, but broiler chickens receiving the control feed on regimen A performed better than those receiving the DFM feed. From the present results, it was concluded that supplementation of DFM reduced the presence of C. jejuni but had no significant effect on the growth performance of broilers; however, there were some significant trends regarding sex, feed, and feeding methods on the performance results.

Key Words: probiotic • broiler chicken • feeding regimen • Campylobacter jejuni • sex


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
The microbial populations in the gastrointestinal tracts of poultry play a key role in normal digestive processes and in maintaining the animal’s health. Changes in diet can substantially affect these bacteria and their health-promoting effects. Dietary changes, however, are often desirable to improve product outcomes such as weight, flavor, and body part proportions. Consequently, to counter reducing effects resulting from dietary change, means for preserving and promoting essential microbial populations in the gut are needed. One promising approach is to use probiotic feed supplements.

Probiotics beneficially affect the host animal by improving its intestinal microbial balance. These bacteria are defined as live microorganisms that are host originated, nonpathogenic, resistant to gastric acid and bile. They have a high affinity for attachment to the mucosal wall, produce inhibitory compounds, and adjust to immune responses (Patterson and Burkholder, 2003). They produce beneficial changes in gut flora by manufacturing acids that inhibit the growth of harmful bacteria (Sun, 2005).

Studies on the beneficial impact on poultry performance have indicated that probiotic supplementation can have positive effects. Kabir et al. (2004), for example, conducted a 6-wk growth performance study with broilers and found that live weight gain and carcass yields were significantly higher in broilers fed probiotic supplementation. They also found significant differences among spleen and bursa weights. Davis and Anderson (2002) reported that PrimaLac as a direct-fed microbiotic improved egg size and lowered feed cost in laying hens. Lan et al. (2003) found higher (P < 0.01) weight gains in broilers subjected to 2 probiotic species. However, Karaoglu and Durdag (2005) used Saccharomyces cerevisiae as a dietary probiotic to assess performance and found no overall weight gain difference.

The use of probiotic bacteria to exclude the colonization of pathogens in the gastrointestinal tract of poultry has also been studied with great interest. Campylobacter jejuni is one of the most common bacterial causes of foodborne illness, and a few studies have shown that probiotics may be able to reduce the amount of bacteria in chickens. Morishita et al. (1997) found that chickens receiving a mixture of Lactobacillus acidophilus and Streptococcus faecium early in life were colonized significantly less with C. jejuni than chickens in the control group. Chang and Chen (2000) reported significant in vitro inhibition of C. jejuni by a mixture of 4 Lactobacillus species.

Thus, although probiotics have been shown to be a potential tool for reducing disease-causing foodborne bacteria in laboratory experiments, their possible use with restricted feeding regimens has not been investigated. This important period in poultry production is critical because changes in diet can make the animal more vulnerable to infection. Therefore, this study was undertaken to investigate various feeding regimens of probiotic diets on the production performance of broiler chickens and C. jejuni colonization.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Experimental Design and Husbandry

Five hundred forty day-of-hatch broiler chicks (Ross x Ross) male and female (270 each) were obtained from a local commercial hatchery. Chicks were vaccinated for Newcastle, infectious bronchitis, and Marek’s disease at the hatchery. The chicks were divided into groups of 15 males and 15 females, weighed, and randomly assigned to 1.524 m x 3.657 m floor pens with 2 hanging tube feeders and 1 suspended drinker. Lighting and water were continuous throughout the trial.

The chicks were divided equally (males and females) into control and direct-fed microbial (DFM) groups. These groups were then further divided into 3 feeding regimens: ad libitum (A), restricted 8-h (R), and skip-a-day (S). Ad libitum feeding provided the broilers with 24-h access to feed for the entire trial; restricted 8-h broilers were allowed feed from 0800 to 1600 h; and the skip-a-day feeding broilers were allowed feed 24 h. Feed was withheld for 24 h on alternating days. Each condition was replicated 3 times with the regimens starting on d 8 of production. The chicks were initially started at 35°C, and then the temperature was gradually decreased to 25°C by the end of 3 wk. The study was conducted over a 49-d period.

Probiotics

Probiotics were provided through microbial cultures (PrimaLac, Star Labs, St. Joseph, MO). An analysis of the culture mix indicated a minimum presence of 1.04 x 108 cfu per g (Lactobacillus acidophilus, Lactobacilles casei, Bifid-obacterium thermophilus, Enterococcus faecium). The starter/grower diets were formulated and marketed by a commercial feed manufacturer. These diets were fed continually throughout the 49-d trial. Nutrient concentrations met or exceeded minimum requirements according to the National Research Council (1994).

Feed Consumption/Conversion, Mortality, BW, Carcass Yield, Organ Weight

Mortality was recorded daily and BW weekly, and feed consumption/conversion was assessed at 3-wk intervals (21 and 42 d). Carcass yield percentages were determined at 49 d of age by pooling 3 males and 3 females from each treatment group/replicate. The pooled birds were weighed, stunned, bled out, scalded, and defeathered using a rotary drum picker. All internal organs were removed and pooled weighed. Fat pads were obtained and weighed in the same manner.

Campylobacter jejuni Isolation

Campylobacter jejuni presence was assessed on d 21 and 42. Cloaca swabs were randomly taken from 5 broilers per replicate at d 21 and 6 at d 42, placed in an ice cooler, and transported to the laboratory within 3 h for analysis. Samples were plated onto BBL agar plates (BBL-Agar, Oxford, MD), sealed into plastic bags, and filled with microaerophilic gas (5% oxygen, 10% carbon dioxide, and 85% nitrogen). The plates were incubated for 48 h, and analyzed for the presence of C. jejuni. Confirmation was conducted by performing microscopic analysis and latex agglutination testing.

Statistical Analysis

Data were analyzed with the ANOVA procedure from the Statistical Analysis System (SAS Institute Inc., 1990). Statements of statistical significance were based on P < 0.05. The least significant difference test was used for means pairs testing.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Weekly Weights, Mortalities, Carcass Yield Percentage, Feed Conversion, Organ Weights, and Campylobacter jejuni Presence

Mortality was not significantly (P < 0.05) affected by probiotic treatment or feeding regimens over the 6-wk period (data not shown). Mean values of weekly BW are shown in Table 1Go. At d 7, average weights were significantly (P < 0.05) different by sex in the R regimen of the control group and in all the feeding regimens of the DFM group. Weights of males in regimens A and R of the control group were significantly higher than in regimen S, whereas weights of females in regimens R and S of the control group were significantly lower than in regimen A. At d 14, BW of regimen R in the DFM group were significantly different by sex (male > female). In the control and DFM groups at d 21, BW of all individuals were significantly different by regimens (A>R>S). Of special note in regimen R, BW was significantly different between the control and DFM group. At d 28, BW of regimen A was significantly higher than those of regimens R and S in the control and DFM group. Body weights of all regimens in the control and DFM groups at 35 d was not significantly different between males and females except for regimen A in the DFM group. In males of the control group, BW in regimens A and R were significantly higher than those in regimen S. In females of the control group, weights were significantly different by regimens (A>R>S). In males and females of the DFM group, BW were significantly different by regimens (A>R>S). The BW of all regimens in the control and DFM groups at d 42 were not significantly different by sex, but BW were significantly different by regimens (A>R>S). In the control and DFM groups, BW were significantly different by regimens (A>R>S). At 49 d of age, BW of all regimens in the control and DFM groups did not differ significantly between males and females except for regimen A in the control group. Weights of males in the control group were significantly higher than the DFM (A) and differed by regimens, whereas females did not differ in regimen A and S control groups. The BW of the control females in regimen R were significantly higher than those in regimens A and S. In the experimental DFM groups, BW in regimen S was significantly lower than in regimens A and R for males and females.


View this table:
[in this window]
[in a new window]

  		Table 1. Average weekly weights of male and female broilers subjected to different feeding regimens (g/bird)
 
Overall, d 21 regimen R, and d 49 regimens A and S showed significant differences between the control and experimental group regarding feed type. The weekly BW during this experiment supports the notion that the efficacy of probiotic application depends on many factors such as environmental and stress factors, diet, administration (method, level, frequency), farm sanitation, undefined microorganisms, and bird age (Ewing and Cole, 1994; Patterson and Burkholder, 2003). Yet, others (Stavric and Kornegay, 1995) reported that it is difficult to assess earlier studies because experimental procedures were not clearly defined, their viability of the microorganisms was not verified, and the data were not statistically analyzed. The results from this study suggest that sex and feeding regimens in some cases can be added to the many factors that influence the efficacy of some probiotic application. There are documented growth-promoting effects of probiotics in broiler diets (Aarestrup et al., 2000). Recently, a study by Mountzouris et al. (2007) reported that probiotic treatment displayed a growth-promoting effect comparable with the avilamycin treatment. In the present study, the growth-promoting effect on BW of the DFM at d 49 in the A regimen males was not comparable with the control feed, whereas the females responded significantly better on the DFM feed. In another study conducted by Chichlowski et al. (2007), comparable BW with DFM and the control diet in a 21-d trial were reported. Although this study lacked some positive effects on BW with DFM, it is generally accepted that many of the factors previously discussed influenced the results of this experiment. Moreover, most trials with probiotics reported thus far have been experimentally different with various numbers and strains of microorganisms.

Carcass Yield Percentage

Carcass yield percentages are presented in Table 2Go. Significant (P < 0.05) higher carcass yield percentages were noted in males fed the control diet in regimen A (78.1%) when compared with DFM in the same regimen (74.6%), whereas it did not differ with females in this regimen. Regimen S with the DFM-fed females had significantly higher carcass yield of 72.6 vs. 69.0% when compared with the control-fed females. Kabir et al. (2004) reported different results with broilers on probiotic supplements that yielded significantly (P < 0.05) higher carcass yields when compared with the control. Both male and female broilers on the R and S regimens had comparable carcass yield results.


View this table:
[in this window]
[in a new window]

  		Table 2. Carcass yield percentage of male and female broilers at 49 d of age
 
Feed Conversion

Feed conversions were assessed on d 21 and 42 for the broiler chickens (Table 3Go). There were no significant differences observed between the control and DFM feed groups; however, there were significant differences (P < 0.05) found among feeding regimens. Broilers on the A and R regimens had similar results on d 21 (1.52 kg; 1.54 kg, respectively). The S regimen resulted in a more efficient conversion (1.44 kg). Feed conversion assessment at d 42 showed significant differences (P < 0.05) between feed conversions within feeding regimens. The R feeding regimens (1.55) were the most efficient feed conversions followed by S (1.65) and A (1.85), respectively. Broilers on the A and R regimens had greater consumption in feed from d 21 to 42, which produced noteworthy increases in those feed conversions. The broilers on the R feeding schedule remained constant, therefore, yielding more efficient feed conversion (1.55 kg), which was similar to reports by Ballay et al. (1992), Dozier et al. (2002), and Oyedeji and Atteh (2005). The result that probiotic supplementation was associated with female broilers utilizing their feed more efficiently (resulting in weight gain) is consistent with findings reported by Kabir et al. (2004).


View this table:
[in this window]
[in a new window]

  		Table 3. Feed conversion of male and female broilers at 21 and 42 d of age
 
Relative Organ and Tissue Weights

Mean values for relative organ weights are found in Table 4Go. There were no significant differences among organ weights relative to feed types; however, variations were found in the feeding regimens. Significant differences (P < 0.05) were found between the gizzards and fat pads of male and female broilers. Male and female broilers on R and S regimens produced larger gizzards than male and female broilers on the A regimen. Broilers on the R and S regimens, in the absence of feed, probably consumed litter while scratching for loose feed. This resulted in an increased gizzard size due to the presence of indigestible material and greater density of muscle tissue. These findings are in agreement with those of Oyedeji and Atteh (2005), which showed broilers on R and S regimens had similar BW and produced significantly (P < 0.05) larger gizzards than those on the A regimen. Fat pad weights did not differ significantly among the control feed in regimen A and the DFM. There were not any noteworthy differences for discussion between fat pads in regimens A, B, and S, or sex, although some significant variations were observed in the treatments. The sizes of the bursas did not differ significantly (P < 0.05) by treatment group. The size of the bursa, in part, corresponds with the relative health condition of the broiler itself. Healthy broilers will produce larger bursas than sick, weak, or stressed ones (Bennett, 2001). Kabir et al. (2004) found a significant (P < 0.05) increase in spleen weights of vaccinated broilers where nonvaccinated broilers produced significantly (P < 0.05) larger bursas. Our results did not reflect larger bursas from DFM supplementation. This was probably due to a different mixture of organisms utilized. All broilers in this study were presumed to be healthy and not unduly stressed in the experiment based on the bursa size.


View this table:
[in this window]
[in a new window]

  		Table 4. Relative organ and tissue weights of male and female broilers at 49 d of age
 
Campylobacter jejuni Prevalence

Campylobacter jejuni prevalence is found in Table 5Go. On d 21, 20% of broilers were positive on the control feed regimen A for C. jejuni, whereas the DFM R and control-fed broilers had a 33 and 60% occurrence, respectively. For the other feeds, the S regimen had a prevalence of 27% followed by the A feeding regimen of 20%. By d 42, all control A-fed broilers in each regimen had a 56% positive presence of C. jejuni, whereas all DFM-fed broilers had a lower percentage (44%). The DFM-fed broilers showed trends of a reduction of C. jejuni presence that were similar to results found in studies performed by Chang and Chen (2000) and Morishita et al., (1997). The percentage of positive C. jejuni broilers on DFM feed in the present study was lower than those found in a study by Willis et al. (2000). There were some indications that feed restriction influenced the presence of C. jejuni at 21-d sampling. By using the competitive exclusion approach, some probiotics (Owings et al., 1989; Jin et al., 1998; Line et al., 1998; Nisbit, 1998; Netherwood et al., 1999; Fritts et al., 2000) and prebiotics (Chambers et al., 1997; Fukata et al., 1999) have shown reduction in colonization and shedding of Salmonella and Campylobacter. Although the DFM group had a lower level of Campylobacter, they were not pathogen free. This suggests that DFM alone was not sufficient to reduce their presence to acceptable levels. This provides an opportunity for additional research to enhance the impact of probiotics on foodborne pathogens in broiler chickens.


View this table:
[in this window]
[in a new window]

  		Table 5. Campylobacter jejuni prevalence (%) in male and female broilers
 


   FOOTNOTES
 
1 Research was supported by Evans-Allen funding through the USDA/Cooperative State Research, Education and Extension Service, 1400 Independence Ave., S.W. Washington, DC 20250, United States Department of Agriculture Cooperative State Research Service. Back

Received for publication December 21, 2006. Accepted for publication January 15, 2008.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Aarestrup, F. M., F. Bager, and J. S. Anderson. 2000. Association between the use of avilamycin for growth promotion and the occurrence of resistance among Enterococcus faecium from broilers: Epidemiological study and changes over time. Microb. Drug Resist. 6:71–75.[Web of Science][Medline]

Ballay, M., E. A. Dunnington, W. B. Gross, and P. B. Siegel. 1992. Restricted feeding and broiler performance. Poult. Sci. 71(Suppl. 1):4040. (Abstr.)

Bennett, C. 2001. Manitoba Agriculture Food and Rural Initiatives. http://www.gov.mb.ca/agriculture/livestock Accessed January 2006.

Chambers, J. R., J. L. Spencer, and H. W. Modler. 1997. The influence of complex carbohydrates on Salmonella typhimurium colonization, pH, and destiny of broiler ceca. Poult. Sci. 76:445–451.[Abstract/Free Full Text]

Chang, M. H., and T. C. Chen. 2000. Reduction of Campylobacter jejuni in a simulated chicken digestive tract by lactobacilli cultures. J. Food Prot. 63:1594–1597.[Web of Science][Medline]

Chichlowski, M., J. Croom, B. W. McBride, L. Daniwl, G. Davis, and M. D. Koci. 2007. Direct-fed microbial PrimaLac and Salinomycin modulate whole-body and intestinal oxygen consumption and intestinal mucosal cytokine production in the broiler chick. Poult. Sci. 86:1100–1106.[Abstract/Free Full Text]

Davis, G. S., and K. E. Anderson. 2002. The Effects of feeding the direct-fed microbial, PrimaLac, on growth parameters and egg production in Single Comb White Leghorn hens. Poult. Sci. 81:755–759.[Abstract/Free Full Text]

Dozier, W. A., R. J. Lien, J. B. Hess, S. F. Bilgili, R. W. Gorden, C. P. Laster, and S. L. Vieira. 2002. Effects of early skip a day feed removal on broiler live performances and carcass yield. J. Appl. Poult. Res. 11:297–303. (Abstr.)[Abstract/Free Full Text]

Ewing, W. N., and D. J. A. Cole. 1994. The living gut: An introduction to micro-organisms in nutrition: A review. Context Graphics, Dungannon, UK.

Fritts, C. A., J. H. Kersey, M. A. Motl, E. C. Kroger, F. Yan, J. Si, Q. Jiang, M. M. Campos, A. L. Waldroup, and P. W. Waldroup. 2000. Bacillus subtilis C-3102 (Calsporin) improves live performance and microbiological status of broiler chickens. J. Appl. Poult. Res. 9:149–155.[Abstract/Free Full Text]

Fukata, T., K. Sassai, T. Miyamoto, and E. Baba. 1999. Inhibitory effects of competitive exclusion and fructooligosaccharide, singly and in combination, on Salmonella colonization of chicks. J. Food Prot. 62:229–233.[Web of Science][Medline]

Jin, L. Z., Y. W. Ho, N. Abdullah, and S. Jalaludin. 1998. Growth performance, intestinal microbial populations, and serum cholesterol of broilers fed dies containing Lactobacillus cultures. Poult. Sci. 77:1259–1263.[Abstract/Free Full Text]

Kabir, S. M. L., M. M. Rahman, M. B. Rahman, and S. U. Ahmed. 2004. The dynamic of probiotics on growth performance and immune response in broilers. Int. J. Poult. Sci. 3:361–364.

Karaoglu, M., and H. Durdag. 2005. The influence of dietary probiotic (Saccharomyes cerevisiae) supplementation and different slaughter age on the performance, slaughter and carcass properties of broilers. Int. J. Poult. Sci. 4:309–316.

Lan, P. T., T. Bignle, and Y. Benno. 2003. Impact of two lactobacillus strains feeding on fecal lactobacilli and weight gains in chicken. J. Gen. Appl. Microbial. 49:29–36.[CrossRef]

Line, E. J., J. S. Bailey, N. A. Cox, N. J. Stern, and T. Tompkins. 1998. Effect of yeast-supplemented feed on Salmonella and Campylobacter populations in broilers. Poult. Sci. 77:405–410.[Abstract/Free Full Text]

Morishita, T. Y., P. P. Aye, B. S. Harr, C. W. Cobby, and J. R. Clifford. 1997. Evaluation of an avian specific probiotic to reduce the colonization and shedding of Campylobacter jejuni in broilers. Avian Dis. 41:850–855.[CrossRef][Web of Science][Medline]

Mountzouris, K. C., P. Tsirtsikos, E. Kalamara, S. Nitsch, G. Schatzmayr, and K. Fegeros. 2007. Evaluation of the efficacy of probiotic containing Lactobacillus, Bifidobacterium, Enterococcus, and Pediococcus strains in promoting broiler performance and modulating cecal microflora composition and metabolic activities. Poult. Sci. 86:309–317.[Abstract/Free Full Text]

National Research Council. 1994. Nutrient Requirement of Poultry. 8th ed. Natl. Acad. Press, Washington, DC.

Netherwood, T., H. J. Gilbert, D. S. Parker, and A. G. O’Donnell. 1999. Probiotics shown to change bacterial community structure in the avian gastrointestinal tract. Appl. Environ. Microbiol. 65:5134–5138.[Abstract/Free Full Text]

Nisbit, D. J. 1998. Use of competitive exclusion in food animals. J. Am. Vet. Med. Assoc. 213:1744–1746.[Web of Science][Medline]

Owings, W. J., D. L. Reynolds, R. J. Hasiak, and P. R. Ferket. 1989. Influence of dietary supplements with Streptococcus faecium M-74 on broiler body weight, feed conversion, carcass characteristics, and intestinal microbial colonization. Poult. Sci. 69:1257–1264.[Web of Science]

Oyedeji, J. O., and J. O. Atteh. 2005. Response of broilers to feeding manipulations. Int. J. Poult. Sci. 42:91–95.

Patterson, J. A., and K. M. Burkholder. 2003. Application of prebiotics and probiotics in poultry production. Poult. Sci. 82:627–631.[Abstract/Free Full Text]

SAS Institute Inc. 1990. SAS User’s Guide. Statistics. SAS Inst., Cary, NC.

Stavric, S., and E. T. Kornegay. 1995. Microbial probiotics for pigs and poultry. Pages 205–231 in Biotechnology in Animal Feeds and Animal Feeding. R. J. Wallace and A. Chesson, ed. VCH, New York, NY.

Sun, X. 2005. Broiler performance and intestinal alterations when fed drug-free diets. Thesis. Virginia Tech, Blacksburg.

Willis, W. L., C. Murray, and C. Talbott. 2000. Effect of delayed placement on the incidence of Campylobacter jejuni in broiler chickens. Poult. Sci. 79:1392–1395.[Abstract/Free Full Text]





This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Willis, W. L.
Right arrow Articles by Reid, L.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Willis, W. L.
Right arrow Articles by Reid, L.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS