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DENTISTRY Relationship Between Equine Cheek Tooth Occlusal Morphology, Apparent Digestibility, and Ingesta Particle Size James L. Carmalt, MA, VetMB, MVetSc, Diplomate ABVP, Diplomate ACVS; and Andrew Allen, DVM, MVetSc, PhD In some herbivorous species, dental morphological variables have been significantly correlated with fecal particle size. This study does not support this finding in the horse. Further research is needed to determine the role of equine dentition in subsequent gastrointestinal feed processing. Authors’ addresses: Department of Large Animal Clinical Sciences (Carmalt) and Department of Pathology (Allen), Western College of Veterinary Medicine, 52 Campus Drive, University of Saskatchewan, Saskatoon SK S7N 5B4, Canada; e-mail: [email protected]. © 2008 AAEP. 1. Introduction Floating (rasping or filing) is the most common equine dental procedure performed.1 Some veterinary texts suggest that weight loss, ptyalism, and colic, among other medical abnormalities, can be caused by dental disease.2– 4 Published clinical trial data, barring two abstracts,5,6 refute these statements and currently, whereas the majority of published evidence suggests that disease of the hard dental tissues does not lead to either weight loss or colic,7,8 a single retrospective study claims that dental disease was the cause of weight loss.9 There is a significant body of research into the effect of chewing in the ruminant and other herbivorous species.10 The morphology of teeth and their role in chewing effectiveness has not been widely studied. In human studies, molar occlusal surface area has been used, but in herbivorous species in which the anisognathic jaw results in lateral translocation of the mandible during mastication, the NOTES 386 2008 Ⲑ Vol. 54 Ⲑ AAEP PROCEEDINGS enamel ridge perimeter distance may be more useful.11–15 We conducted a prospective, observational study to investigate the effect of various equine cheek tooth occlusal morphological variables and oral pathology score (OPS) on subsequent ingesta particle size reduction and apparent feed digestibility in the horse. 2. Materials and Methods Seventeen registered horses (six Clydesdales, three Percherons, and eight Quarter horses) of known age were used. They had been maintained solely on one of three different hays, which was fed ad libitum for ⱖ14 days before death. They were euthanized using an intravenous mixture of pentobarbitala and embutarateb administered as a bolus into the left jugular vein. Heads were disarticulated post-mortem at the atlanto-occipital joint, and the mandibles were separated from the crania by sectioning through the DENTISTRY vertical ramus of the mandibles using a band saw. Teeth and oral mucosa were cleaned in running water and dried. The cheek teeth were examined, and the presence of malocclusions, missing teeth, diastemata, gingivitis, or periodontal disease (defined as gingival recession with hyperemia and feed packing) was also noted. Based on these findings, an oral pathology score (OPS) was assigned to the mouth, as previously described.16 Briefly, each abnormality was assigned a single point, and the OPS was the sum of all abnormalities found. No weighting was given to any one malocclusion or soft tissue abnormality. Maxillary and mandibular cheek teeth were photographed using a digital camerac with a scale positioned at the level of the occlusal surface to account for any size distortion of the image. A computerbased photographic programd linked to a personal computer with an attached graphic pade was used to trace the outlines of the teeth and their internal morphology. By this method, a black and white replica of the occlusal surface was created (Fig. 1). Morphological variables were measured and calculated from these images for the maxillary and mandibular arcades (Table 1). The maxillary and mandibular measurements were combined to give a total value representative of the complete amount of enamel and surface area within the mouth of each animal. Gastrointestinal samples were collected within 30 min of death. A 1-l representative sample of the agitated contents of the stomach was obtained in addition to entire fecal balls from the small colon/ rectum. Both samples were frozen immediately at ⫺20°C. Feed and gastrointestinal samples were submitted to a commercial laboratoryf for digestibility analysis, including dry matter, crude protein, aciddetergent fiber, neutral detergent fiber, energy, and acid insoluble ash content (AIA). Apparent digestibility was calculated using the acid insoluble ash method.17–19 Mean forage particle size was measured using a purpose-built forage particle separator in accordance with American Society of Agricultural Engineers (ASAE) Standards 1998 (R2007).20 Fecal particle size analysis was similarly undertaken, in accordance with ASAE Standards 1998.21 Statistical analysis was performed using a Windows-based computer package.g A one-way analysis of variance (ANOVA) was used to determine whether there were significant differences in the apparent digestibilities of ingested hays and whether there were hay group effects on hay (preingestion), stomach, and fecal particle size, independent of dental variables. A linear regression was used to examine the effect of oral pathology score on stomach and fecal particle size before and after controlling for feed group. The level of significance was set at p ⬍ 0.05. A Spearman rank correlation was used to examine the effect of dental variables on feed digestibilities. Interpretation was as follows: Fig. 1. (A) A graphic representation of the mesial and distal infundibular surface area and enamel ridge perimeter distance in maxillary teeth in a maxillary specimen. (B) A graphic representation of the outer enamel ridge perimeter distance and inner surface area of the maxillary cheek teeth in a maxillary specimen. (C) A graphic representation of the outer surface area of the maxillary cheek teeth in a maxillary specimen. (D) A graphic representation of the enamel ridge perimeter distance and inner surface area of the mandibular cheek teeth in a mandibular specimen. (E) A graphic representation of the outer surface area of the mandibular cheek teeth in a mandibular specimen. AAEP PROCEEDINGS Ⲑ Vol. 54 Ⲑ 2008 387 DENTISTRY Table 1. Explanations of Measured and Calculated Maxillary, Mandibular, and Total Horse Cheek Tooth Occlusal Variables Data Variable Max-TSA Max-ISA Max-OSA Max-TERPD Max-IERPD Max-OERPD Man-TSA Man-ISA Man-OSA Man-TERPD Horse-SA Horse-ISA Horse-OSA Horse-ERPD Horse-IERPD Horse-OERPD Explanation Maxillary total surface area Maxillary inner surface area ⫽ area of the tooth bounded by the OERPD (Incl. infundibulae) Maxillary outer surface area ⫽ area of external cementum (OSA ⫽ TSA-innerSA) Maxillary total enamel ridge perimeter distance (TERPD ⫽ IERPD ⫹ OERPD) Maxillary infundibular enamel ridge perimeter distance ⫽ mesial and distal Maxillary outer enamel ridge perimeter distance Mandibular total surface area Mandibular inner surface area ⫽ area of the tooth bounded by the ERPD Mandibular outer surface area ⫽ area of external cementum (OSA ⫽ TSA-innerSA) Mandibular enamel ridge perimeter distance Max-TSA ⫹ Man-TSA Max-ISA ⫹ Man-ISA Max-OSA ⫹ Man-OSA Max-TERPD ⫹ Man-TERPD Max-IERPD Max-OERPD ⫹ Man OERPD ⬍ 0.1, trivial correlation; 0.1 ⱖ ⬎ 0.3, slight correlation; 0.3 ⱖ ⬎ 0.5, moderate correlation; 0.5 ⱖ ⬎ 0.7, substantial or large correlation; ⬎ 0.7, very large correlation.22,23 3. Results The mean age of the horses was 7.6 ⫾ 6.31 (SD) yr (range, 2–21.2 yr). There was no significant effect of body size (0, standard horses; 1, large horses) on dental morphology. Horses had a mean OPS of 5.75 ⫾ 5.07 (range, 0 –17). There was no statistical increase in OPS with age. Nutrient analysis indicated that there was no significant difference in apparent digestibility between the three hays fed to the horses (p ⬎ 0.05). Despite a significant difference in mean particle size of the ingested hay (p ⬍ 0.0001), there was no significant difference in ingesta particle size within the stomach. Additionally, particle size of the feces (1.36 ⫾ 0.16 mm; range, 1.15–1.72 mm) was not significantly different from that of the stomach (1.29 ⫾ 0.26 mm; range, 0.69 –1.76 mm). There was no significant correlation between OPS and particle size of stomach contents or feces. There was also no significant correlation between OPS and the difference in feed and stomach particle size, even when controlling for diet. There was no correlation between any of the dental variables and apparent feed digestibilities. 4. Discussion This study did not find any correlation between dental morphological variables and feed digestibility. There was also no correlation between mean particle size of the stomach contents and feed digestibility, a finding that is in agreement with the digestibility studies in red deer.12 As horses age, the amount of residual crown is reduced, until it equals that of the exposed crown. Further attrition will result in loss of occlusion and 388 2008 Ⲑ Vol. 54 Ⲑ AAEP PROCEEDINGS eventually tooth loss. Despite this loss, we did not see clinically apparent weight loss associated with the loss of occlusion (in most cases), nor did we see evidence of insufficient mastication or poor apparent feed digestibility. Similarly, although there are many influences on growth of young horses from weaning to 4 yr of age, there is no clinically apparent saltatory increase in weight or condition associated with the eruption of more cheek teeth and the associated increase in enamel or surface area for mastication. The findings of this study may corroborate these observations in that there was no difference in apparent feed digestibility between immature horses with incomplete dentition and mature horses with a complete set of 24 cheek teeth. The results of our study indicated that, despite being fed one of three different hays (with attendant variability in feed particle length), there was surprisingly little variation in the size of particles removed from the stomach. Interestingly, fecal particle size was not significantly different from that of the stomach, indicating that, despite the digestive process, there was no further size reduction as the ingesta passed down the gastrointestinal tract. This would suggest that mastication is the most important determinant of feed particle size reduction in the horse, as in the ruminant.10 More research is needed to determine the role of equine dentition on the subsequent gastrointestinal processing of ingested feed. There was no evidence that dental abnormalities in our sample of clinically normal horses contributed to mal-digestion or weight loss, as shown by one of the horses in this study who was 30 yr old. His OPS was 17, included in which were missing teeth, diastemata, and advanced periodontal disease. This horse did not have statistically larger stomach or fecal particle sizes than other study horses, nor was there a reduction in apparent digestibility of feed in this horse. DENTISTRY There is obviously a critical point at which oral pain related to dental disease is sufficient to limit voluntary feed intake. Tamzai9 published a study on horses with chronic weight loss. In this case series, a significant proportion could be linked to dental disease, which, when treated, resulted in the resolution of the weight problem. Discussions with the author have shown that this series included six cases of dental overgrowth impinging on soft tissue, two severe wavemouths, one tooth fracture, and two apical infections.h It is possible that these abnormalities were such that oral pain was the limiting factor. It may be that feed intake contributes relatively more to forage feeding value than digestibility per se and hence effects of poor dentition reducing voluntary feed intake have more of an impact on animal performance or growth than does a reduction in digestibility. In addition to this, there must be a minimum amount of tooth necessary for efficient mastication. Below this limit, the reserve capacity of the gastrointestinal tract is exceeded, and mal-digestion or weight loss results. This morphological limit is currently unknown. References and Footnotes 1. Scrutchfield WL. Dental prophylaxis. In: Baker GJ, Easley J, eds. Equine dentistry. Philadelphia: WB Saunders, 1999;185–205. 2. Baker GJ. Diseases of the teeth. In: Colohan PT, Mayhew IG, Merritt AM, et al., eds. Equine medicine and surgery, 4th ed. Goleta: American Veterinary, 1991;550 –570. 3. Knottenbelt DC. The systemic effects of dental disease. In: Baker GJ, Easley J, eds. Equine dentistry. Philadelphia: WB Saunders, 1999;127–138. 4. Baker GJ. Oral diseases of the horse. In: Harvey CE, ed. Veterinary dentistry. Philadelphia: WB Saunders, 1985;203–216. 5. Gatta D, Krusic L, Casini L, et al. Influence of corrected teeth on digestibility of two types of diets in pregnant mares, in Proceedings. 14th Symposium of the Equine Nutrition and Physiology Society 1995;326 –331. 6. Krusic L, Easley J, Pagan J. Influence of corrected teeth on daily food consumption and glucose availability in horses, in Proceedings. 1st Symposium on Horse Diseases 1995;53– 68. 7. Ralston SL, Foster DL, Divers T, et al. Effect of dental correction on feed digestibility in horses. Equine Vet J 2001; 33:390 –393. 8. Carmalt J, Townsend HGG, Janzen E, et al. Effect of dental floating on weight gain, body condition score, feed digestibility and fecal particle size in pregnant mares. J Am Vet Med Assoc 2004;225:1889 –1893. 9. Tamzali Y. Chronic weight loss syndrome in the horse: a 60 case retrospective study. Equine Vet Edu 2006;18:289 –296. 10. Pérez-Barberia FJ, Gordon IJ. Factors affecting food comminution during chewing in ruminants: a review. Biol J Linn Soc 1998;63:233–256. 11. Gross J, Demment M, Alkon P, et al. Feeding and chewing behaviors’ of Nubian ibex- compensation for sex-related differences in body-size. Funct Ecol 1995;9:385–393. 12. Pérez-Barberia FJ, Gordon IJ. The influence of molar occlusal surface area on the voluntary intake, digestion, chewing behaviour and diet selection of red deer (Cervus elaphus). J Zool Lond 1998;245:307–316. 13. Gipps J, Sanson G. Mastication and digestion in Pseudocheirus. In: Smith AP, Hume ID, eds. Possums and gliders. Sydney: Australian Mammal Society. 1984;237–246. 14. Lanyon JM, Sanson GD. Koala (Phascolarctos cinereus) dentition and nutrition. I. Morphology and occlusion of cheek teeth. J Zool Lond 1986;209:155–168. 15. Lanyon JM, Sanson GD. Koala (Phascolarctos cinereus) dentition and nutrition. II. Implications of tooth wear in nutrition. J Zool Lond 1986;209:169 –181. 16. Carmalt JL, Carmalt KP. Equine dentistry: what do we really know? J Vet Dent 2004;21:134 –5. 17. Frape D, Tuck M, Sutcliffe N, et al. The use of inert markers in the measurement of the digestibility of cubed concentrates and of hay given in several proportions to the pony, horse, and white rhinoceros (Diceros simus). Comp Biochem Physiol 1982;72:77– 83. 18. Sutton E, Bowland J, McCarthy J. Studies with horses comparing 4N-HCl insoluble ash as an index material with total fecal collection in the determination of apparent digestibilities. Can J Anim Sci 1977;57:543–549. 19. Cuddeford D, Hughes D. A comparison between chromiummordanted hay and acid-insoluble ash to determine apparent digestibility of a chaffed, molassed hay/straw mixture. Equine Vet J 1990;22:122–125. 20. American Society of Agricultural Engineers. ASAE S424.1 method of determining and expressing particle size of chopped forage materials by screening. In: ASAE standards, 45th ed. St. Joseph, MI: American Society of Agricultural Engineers, 1998;547–550. 21. American Society of Agricultural Engineers. ASAE S319.3 method of determining and expressing fineness of feed materials by sieving. In: ASAE standards, 45th ed. St. Joseph, MI: American Society of Agricultural Engineers, 1998;547–550. 22. Cohen J. Statistical power analysis for the behavioral sciences, 2nd ed. Hillside NJ: Lawrence Erlbaum, 1988. 23. Hopkins WG. A new view of statistics. Internet Society for Sport Science. Available at: www.sportsci.org/resource/ stats. Accessed on September 1, 2007. a Euthanyl Forte, Bimeda-MTC, Animal Health Inc, 420 Beaverdale Road, Cambridge, ON, N3C 2W4 Canada. b T-61, Intervet Canada Ltd, 250 Water Street, Whitby, ON, L1N 9T5 Canada. c Sony Cybershot DSC-P71, Sony Electronics Inc, Toronto, ON, Canada. d Adobe Photoshop, version 7, Adobe Systems, 345 Park Avenue, San Jose, CA 95110. e Graphire, version 3, Wacom Technology Co, 1311 SE Cardinal Ct., Vancouver, WA 98683. f Enviro-Test Laboratories, 819 58th Street East, Saskatoon, SK, Canada. g Statistics, version 7 for Windows, Analytical Software Inc, 2105 Miller Landing Road, Tallahassee, FL. h Tamzali Y. Personal communication, 2008. AAEP PROCEEDINGS Ⲑ Vol. 54 Ⲑ 2008 389