The conformation of the equine hoof is an important factor

Transcription

1 bs_bs_banner The feral horse foot. Part A: observational study of the effect of environment on the morphometrics of the feet of 100 Australian feral horses BA Hampson,* MA de Laat, PC Mills and CC Pollitt Aim To better understand the morphology of, and the effect of different travel patterns and varying substrate environments on, the feral horse foot to better manage the feet of domestic horses. Methods The left forefeet of 20 adult feral horses from each of five geographically separated populations in Australia (n = 100) were investigated. Populations were selected on the basis of substrate hardness under foot and the amount of travel typical for the population. Feet were radiographed and photographed and 40 morphometric measurements of each foot were obtained. Results Of the 40 parameters, 37 differed significantly (P < 0.05) among the populations, which suggested that substrate hardness and travel distance have an effect on foot morphology. Harder substrates and longer travel distances were associated with short hoof walls and minimal hoof wall flaring. Softer substrates and moderate travel distances were associated with long flared walls, similar to that of typical untrimmed feet of domestic horses. Conclusions The morphology of the feral horse foot appeared to be affected by the distance travelled and by the abrasive qualities and mechanical properties of the substrate under foot. There were marked differences in some conformation parameters between the feral horses in the current study and domestic horses in previous studies. Although the conformation of the feral horse foot may have some prescriptive value, concerns regarding abnormal foot anatomy warrant further investigation. Keywords brumbies; feet; horses; morphology; radiometrics Abbreviations CE, coronet extensor distance; DWA, dorsal hoof wall angle; PA, palmar angle; TDWT, thickness of the soft tissue dorsal to the dorsal border of the distal phalanx Aust Vet J 2013;91:14 22 doi: /j x The conformation of the equine hoof is an important factor affecting sporting performance, 1 3 duration of competitive life 4 and risk of injury, 2 including catastrophic musculoskeletal injury. 5 Foot conformation can be altered by human intervention, such as by hoof trimming, and the application of podiatric devices, such as horse shoes. 6,7 As early as 1899, various models of hoof trimming and balancing have been debated 8 and there is still no universal agreement on the optimal model of hoof conformation. 9 To address the supposedly deleterious effects of human intervention on the equine hoof, an interest in the wild horse hoof model has *Corresponding author. Australian Brumby Research Unit, School of Veterinary Science, The University of Queensland, St Lucia, Queensland 4072, Australia; b.hampson1@uq.edu.au emerged and it has been proposed as a realistic model for the equine foot, despite limited documentation and no detailed empirical investigations. It has also been proposed that the free-roaming lifestyle of the wild horse promotes ideal foot health because of the long distances travelled, varied natural diet and an absence of the purported harmful effects of domestication, including some traditional farriery practices. However, an investigation found poor foot conformation and a high incidence of foot pathology in feral horses of Kaimanawa, New Zealand, and suggested that the feral horse foot model may not be ideal unless the environment inhabited by the horse is also considered. The aim of the present study was to describe the range of foot parameters in several feral horse populations in Australia. A further aim was to investigate the effects of different environmental conditions, particularly the substrate characteristics, on feral horse feet. These data could then be used by the equine husbandry community to make more informed decisions on the value of the feral horse foot model in guiding domestic horse foot care practices. Materials and methods Subjects The bodies of 20 mature-age feral horses from five different locations in Australia were obtained (n = 100) following standard controlled culling operations. All (58 males, 42 females) were assessed by dentition as >4 years of age. The home range available to the horses varied from 70,000 to 800,000 ha. DNA analysis using hair from horses in four of the five populations confirmed that 93% of the genetic material was common between the populations. 15 Each habitat had sustained a feral horse population in excess of 2000 head for at least the past 100 years Horse populations The feral horse populations were selected on the characteristics of the substrate, landscape topography and the availability of food sources in relation to water sources in each habitat (Table 1). Substrate refers to the type of ground surface on which the horses were moving. Sand was considered a soft substrate, whereas a hard substrate consisted primarily of rock and a mixed substrate was a combination of soft and hard. The travel pattern of feral horses was determined by the spatial relationship between food and water in the habitat. 17,19 We predicted the general travel patterns of the horses in each habitat by identifying food and water sources in each habitat, and from information with global positioning systems data derived from feral horse tracking. 19,20 For the purpose of this study, we defined travel distances as moderate (5 10 km/day) or long (>10 km/day) (Table 1). A previous study had 14

2 Table 1. Substrate type and daily travel distances for five populations of feral horses in Australia Population Latitude ( ) Longitude ( ) Substrate a Travel distance b Contribution c Babbiloora, QLD Mixed Moderate 11% King s, NT Hard Long 11% Mussellbrook, QLD Hard Moderate 14% Cliffdale, QLD Soft Moderate 47% Palparara, QLD Soft Long 17% a Mixed is a combination of hard and soft (sandy) substrates. b Moderate, 5 10 km/day; long, >10 km/day. c The contribution of each population to the relationship between morphometric parameters yielding a significant difference (P < 0.05) between populations is shown. Horses from Cliffdale accounted for almost half of the interactions with other populations. NT, Northern territory; QLD, Queensland. shown that available forage at these same geographical sites had significantly varied levels of protein and that water-soluble and ethanolsoluble carbohydrate levels in all locations were above those expected for even high-quality forage. Some macro- and micro-element levels were grossly excessive while others were grossly deficient, with no evidence of ill health among the horses. 21 Foot collection The left forelimb of each horse was disarticulated at the radiocarpal joint, immediately packed in ice and frozen within 8 h of slaughter. The limbs were thawed to room temperature for analysis within 2 weeks of collection. Radiometric assessment The hairline was clipped to expose the coronet. A lead-impregnated rubber strip was placed circumferentially around the distal hairline on the lateral coronet to identify the coronet band on the radiographs. Another strip was placed from the most distal hairline on the coronet, down the midline dead centre of the hoof wall and passed under the distal wall and sole, ending at the frog tip sole junction. Each limb was loaded into a custom-built hydraulic loading device designed to apply a nominated downward vertical force of 1300 newtons through a hydraulic ram to simulate the stance phase. The starting position for loading was with the third metacarpal positioned vertically and the loading arm positioned over the centre of the foot. The load was applied through the proximal carpus by a cup attached to the distal end of the hydraulic ram. A 100-mm long, stainless steel rod was placed at ground level directly beneath the central long axis of the foot for measurement calibration. A lateromedial radiograph was taken with a portable X-ray machine positioned in a fixed location on the hydraulic device platform. The focal length was 700 mm for all radiographs. Radiographs were electronically scanned to allow digital measurement (Figure 1a c). The 6 angular and 12 length measurements (Table 2) were taken from the lateromedial foot radiographs using digital image analysing software (ImageJ v1.38, National Institutes of Health, Bethesda, MD, USA). Photometric assessment Following the radiometric assessment, feet were photographed while positioned on a custom-built photographic jig. A Nikon D100 digital camera fitted with a 55-mm Micro Nikkor lens and Nikon SB-800DX Speedlight flash was screw-mounted to the proximal end of the jig, 700 mm from the foot. The camera was aligned to the centre of each foot and a 100-mm ruler was placed in the measurement plane to provide image calibration. The feet were photographed from the dorsal, medial, lateral, solar and caudal views. Feet were then sectioned in the sagittal plane along the centre axis with a band-saw and the medial portion photographed (Figure 2a e). ImageJ software was used to obtain 8 angular and 13 length measurements from each photograph (Table 3). Additionally, a subjective assessment of the shape of the dorsal hoof wall was made. Prior to obtaining the measurements, an internal validity study was performed. All 40 parameters were measured three times by the same author (B.A.H.) on five randomly selected hooves, with a coefficient of variation of 4.13%. Hooves were measured in random order for both the internal validity and main studies and the operator was blinded by coding foot numbers and electronically removing identifying markers. Randomisation was realised by drawing a number from a container. Statistical analysis and ethical considerations The project was approved by The University of Queensland Animal Ethics Committee (AEC-PCA), monitoring compliance with the Animal Welfare Act (2001) and The code of practice for the care and use of animals for scientific purposes (approval no. SVS/393/07/AHF). The photometric and radiometric data are presented as mean SD, except for the coronet band dorsal hoof wall angle (CB-DWA) measurement, which is presented as median (range). Differences among the five populations for each parameter were analysed with one-way analysis of variance (ANOVA). Tukey s test was used for pair-wise comparison of the means between populations. The CB-DWA data were not normally distributed and therefore analysed with a Kruskal-Wallis ANOVA on ranks. Medial and lateral wall angles were compared within each population using linear regression. Statistical significance was accepted as P< Statistical analyses were performed using R version (R Project for Statistical Computing: 15

3 Results Of the 18 radiometric parameters (Table 2), there were statistical differences (P < 0.05) among the populations for all but one parameter, which was the height of the distal tip of the distal phalanx above the ground surface (P > 0.05). Similarly, of the 22 photometric parameters (Table 3), all but two parameters (ML C/A and HA) differed (P < 0.05) among populations. There were 100 statistically significant relationships illustrating a difference (P < 0.05) among populations. Horses from Cliffdale accounted for almost half of the interactions with other horse populations (Table 1). The DWA and palmar angle (PA) of the distal phalanx differed slightly among populations (Table 3). The range of mean photographic DWA across locations was to (Table 3). Radiometric analysis for all horses combined (n = 100) showed an overall mean DWA of The shape of the solar surface of the foot of the high travel, softsubstrate population was consistently square at the toe and differed significantly (P< 0.05) from all other populations (Table 3). The squared toe also appeared in 20% of the horses in the King s population. The toe length measured radiometrically was greatest ( mm) in the Cliffdale group and was significantly different (P < 0.05) from the King s group, with a toe length of mm. The amount of dorsal wall flaring was minimal in the two hardsubstrate populations ( and ) and these differed (P < 0.05) from the Cliffdale population ( ). Medial and lateral wall angles were significantly different (P < 0.05) in all but one population. The combined mean medial and lateral wall flare angles for the soft-substrate horses was 20 and only 8 for the hard-substrate horses. Comparative means ( SD) for the total thickness of the dorsal hoof wall assessed radiometrically (DW total) ranged from mm in the soft-substrate habitats to /2.2 mm in the hardsubstrate habitats (Figure 3) and these differences were significant (P < 0.05). A further breakdown of these parameters revealed an effect of population (hard vs soft substrate) on the thickness of both the stratum medium and stratum internum. Figure 1. Visual representation of the radiometric parameters of 100 Australian feral horses. CBA, coronet band angle; CB-DWA, coronet band dorsal hoof wall angle; CE, coronet extensor distance; DFAX, hoof wall angle; DPAX, distal phalanx angle; HH, heel height; PA, palmar angle; PCL, palmar cortex length; XCD, cup depth; XFT P3T, Frog tip P3 tip; XHLTIP, hoof wall thickness tip; XHLTOP, hoof wall thickness-top; XSD, sole depth; XTL, toe length. The range of means for individual populations for the vertical distance between the proximal limit of the extensor process of the distal phalanx and the proximal hoof wall, or the coronet extensor (CE) distance, was from to mm, with a range of values from 2.3 to 14.6 mm (Figure 4). The CE distance for the Cliffdale population was lowest and significantly different from that of the other four populations (P < 0.05). Discussion The current study investigated the detailed foot morphology of 100 feral horses from five geographically separated locations in Australia. Of the 40 morphological parameters measured, 37 varied among the populations. However, the DWA and PA of the distal phalanx were very similar among populations and varied little between horses, 16

4 Table 2. Description and mean SD of hoof radiometric measurements in 100 feral horses from five different locations in Australia Measurement Description Location b Hoof wall thickness-top* Coronet extensor distance* Toe length* Sole (S) depth* Cup (C) depth* Sole depth + cup depth Frog tip P3 tip* Heel height (HH)* Total soft tissue thickness, shortest distance between the dorsal surface of the hoof wall and the dorsal cortex of the distal phalanx, assessed below the extensor process of the distal phalanx Vertical distance between the distal margin of the coronet band and the proximal margin of the extensor process Oblique length of toe from the distal tip of the distal phalanx to the break-over point on the distal hoof wall Vertical distance between the tip of the distal phalanx and the external marker on the distal sole plane Vertical distance between the sole marker and the ground surface Vertical height of the tip of the distal phalanx above the ground surface Horizontal distance from the frog tip to the tip of the distal phalanx Vertical distance between the distal margin of the distal phalanx and the ground measured at the line of the navicular notch Palparara Cliffdale Mussellbrook King s Babbiloora HH S+ C* Distal phalanx angle* Distal phalanx axis; caudal angle formed between a line along the dorsal cortex of the distal phalanx and a line along the bearing surface Hoof wall angle* Hoof axis; caudal angle formed between a line along the dorsal surface of the hoof wall and a line along the bearing surface of the hoof wall Distal phalanx rotation* Rotation of the distal phalanx; # DPAX-HFAX PA* CBA* CB-DWA a * ( ) ( ) ( ) ( ) ( ) *Significant differences (P < 0.05) between locations for this variable. Group comparisons within each parameter are shown with individual symbols (Palparara:, Ciffdale:, Mussellbrook:, King s:, Babbiloora: #). a CB-DWA data are presented as median (range) and analysis performed using ranks. b Palparra, Cliffdale, Mussellbrook & Babbiloora in Queensland; King s in the Northern Territory. CBA, coronet band angle; CB-DWA, coronet band dorsal hoof wall angle; PA, palmar angle. 17

5 Figure 2. Visual representation of the photometric parameters of 100 Australian feral horses. CD, cup depth; DFA, dorsal flare angle; DRH, Brumby roll height; DWA, dorsal hoof wall angle; DWHT, hoof horn thickness; DWL, toe length; DWST, dorsal wall soft tissue thickness; DW total, total thickness of the dorsal hoof wall; FL, frog length; FROG G, frog to ground height; FROG W, frog width; FW, foot width; HA, heel angle; HEEL W, heel width; LFA, lateral flare angle; LWA, lateral wall angle; MFA, medial flare angle; MWA, medial wall angle; M/L CA, mediolateral balance; QR, quarter relief; W-SOLE, width of sole. 18

6 Table 3. Description of and mean SD hoof morphometric measurements in 100 feral horses from five different locations in Australia Measurement Description Location a Medial wall angle* Medial flare angle* Lateral wall angle* Lateral flare angle* Mediolateral balance* Foot length* Foot width* Heel width* Angle of the proximal 1/3 of the medial wall, excluding wall flare, with the ground surface Angle between the proximal 1/3 of the hoof wall and the distal hoof wall angulation Angle of the proximal 1/3 of the lateral wall, excluding wall flare, with the ground surface Angle between the proximal 1/3 of the hoof wall and the distal angulation Angulation of the coronet band with the ground surface measured from the dorsal view Length of ground-bearing surface of the foot when on a flat surface Length of the widest point of the solar surface of the foot Width of the heels taken at the most caudal abaxial weight-bearing points of the heels Palparara Cliffdale Mussellbrook King s Babbiloora # # # # Frog width* Width of the widest point of the frog Dorsal wall shape* 1 = round; 2 = slightly squared; 3 = squared Frog to ground *height Toe angle* Heel angle Dorsal flare angle* Toe length* Brumby roll height* Quarter relief* Wall to sole height* Cup depth* Minimum height of the frog above the flat ground surface Angle between the dorsal hoof wall and the ground surface measured from the lateral view PA of the most caudal hoof wall tubule visible at the heel measured from the lateral view Angle between proximal 1/3 of the dorsal wall and distal wall flare at toe. Measured from the lateral view Distance from the hairline at the midline dead centre to the most distal point of the bearing surface Distance from the ground surface to the most distal extent of the external hoof wall at the midline dead centre Maximum height of the quarter arch from the ground surface Vertical difference between the distal hoof wall and the distal sole plane at the white line at the midline dead centre. A positive number suggests that the hoof wall extends beyond the sole Distance between the sole distal margin and the ground surface at the level of the distal tip of the distal phalanx # # # Dorsal wall soft Width of the stratum internum tissue thickness* HT* Width of the strata medium and externum Hoof wall thickness* Distance from the dorsal surface of the distal phalanx to the outer hoof wall at the tip of the distal phalanx *Significant differences (P < 0.05) between locations for this variable. Group comparisons within each parameter are shown with individual symbols (Palparara:, Ciffdale:, Mussellbrook:, King s:, Babbiloora: #). a Palparra, Cliffdale, Mussellbrook & Babbiloora in Queensland; King s in the Northern Territory. HT, hoof horn thickness; PA, palmar angle. 19

7 Figure 3. Mean SD total dorsal wall thickness (TDWT) from feral horses from five different locations in Australia: Palparara (n = 20), Cliffdale (n = 20), Mussellbrook (n = 20), King s (n = 20) and Babbiloora (n = 20). TDWT consists of a soft tissue (DWSTT) and hoof (DWHT) component that was measured separately for each horse. TDWT in the Cliffdale horses was less (P < 0.05) than in all other populations. TDWT also differed (P < 0.05) between the Babbiloora and King s populations. DWSTT was lower (P < 0.05) in the Cliffdale population when compared with the Mussellbrook, King s and Babbiloora populations. DWHT was lower (P < 0.05) in the Cliffdale and Babbiloora populations compared with King s and the Cliffdale and Mussellbrook populations also differed (P < 0.05) from each other. Figure 4. Mean SD length (mm) from the coronet band to the extensor process (CE) in feral horses from five different locations in Australia: Palparara (n = 20), Cliffdale (n = 20), Mussellbrook (n = 20), King s (n = 20) and Babbiloora (n = 20). *The CE distance in the Cliffdale population was lower (P < 0.05) than in the other four groups of horses. which suggested that these parameters may be important to the biomechanical function of the foot. By considering the influence of substrate and movement separately, it could be concluded that both factors affect foot morphology. Horses travelling moderate distances in soft substrate had significantly different feet shape from horses Figure 5. Mean SD palmar angle (PA) measurements ( ) in feral horses from five different locations in Australia: Palparara (n = 20), Cliffdale (n = 20), Mussellbrook (n = 20), King s (n = 20) and Babbiloora (n = 20). *The horses from the Cliffdale area had a larger (P < 0.05) PA than did the other four groups of horses. travelling long distances in soft substrate. Similarly, there were significant differences in the foot morphology of horses travelling long distances over hard versus soft substrates. The most significant differences existed between the two environmental extremes. It appears that the foot morphology of feral horses is primarily determined by the interaction of the foot with the surface environment. Although differences in the DWA of the five populations were statistically significant, there was little variation (3.5 ) in functional terms in the population means. The combined mean for all populations (n = 100) assessed by digital photography (unloaded) was compared with assessed by radiometric analysis (loaded). The range and variation were smaller than in a previous study, 3 suggesting that it may have useful prescriptive value in determining the natural hoof balance. In a study of 95 racing Thoroughbred horses, 5 there was a significant effect of DWA on catastrophic musculoskeletal injury. In that study, injured horses had a lower DWA than non-injured horses ( ). A reduction of only 2 was considered sufficient to have a biomechanically injurious effect on horses. The DWA of Thoroughbred racehorses in two comparative studies 6,7 was determined by farrier intervention and was considerably lower than that of the feral horses in the current study.although a low DWA may be advantageous in performance, 1 thereisevidencethat there are consequences, such as a higher injury rate, with such a conformation. The conformation of the feral horse foot may offer a guide to best practice for overall soundness. It has been suggested that the normal PA of the forefoot should be Strasser suggested that the natural orientation of the solar surface of the distal phalanx was ground parallel and claimed observations of wild horses and the lack of foot pain in healthy domestic horses with this PA as evidence supporting this conformation. 13 However, ground parallel or negative PA was linked to foot pathology in the Kaimanawa feral horses. 14 The low range of values in the current study ( , Figure 5) suggests that the PA of horses living in a 20

8 natural environment is close to 6. The small variation among the horses from each environment in the current study suggests that this parameter is functionally important. The shape of the solar surface of the Palparara horses feet was consistently square at the toe and differed from the other four populations. Previous observational study of wild horses concluded that the natural hoof wall was squared at the toe, promoting early break-over that was beneficial to locomotion. 11 This observation prompted the introduction of a square toe horseshoe based on the wild horse foot model, but in our study the squared toe only appeared consistently in the Palparara population and was apparently caused by excessive wear from the hoof being dragged through deep sand in the swing phase of gait. The squared toe also appeared in 20% of the horses in the King s population, but was probably related to the horses digging in the sand to reach subsurface water (B.A.H., pers. obs.). This foot shape was also encountered in horses travelling extremely long distances (n = 3), typical of young bachelor stallions searching and competing for mares. We conclude that the typical shape of the feral horse foot is round at the toe, as is the case for the domestic horse. The toe length was predictably greater in soft-substrate feet than in the hard-substrate feet. Greater wear of the distal hoof wall, by its increased interaction with the harder substrate environment, was thought to cause this difference. The depth of the cup in the solar surface of the foot was greatest in the soft-substrate populations and reduced in the hard- and mixed-substrate populations. This finding may be the result of decreased wear on the sole because the hoof wall was longer peripheral to the margins of the distal phalanx or it may be also because of the higher position of the distal phalanx within the hoof capsule (smaller CE distance) in the soft-substrate group. Feet of horses from hard-substrate environments had more upright hoof walls with minimal hoof wall flaring as compared with softsubstrate feet. A more upright hoof wall may provide protection against wall flaring and may be an important foot health factor in the hard-substrate populations. Long travel distances over a hard substrate produced a foot type with symmetrical medial and lateral wall angles with minimal wall flaring, whereas moderate travel over a soft substrate produced feet that were long and flared with less upright lateral hoof wall angles. An important finding of the current study was the effect of substrate on TDWT. Linford determined the mean SD radiographic TDWT in 41 sound racing Thoroughbreds to be mm adjacent to the extensor process of the distal phalanx and mm adjacent to the distal tip of the distal phalanx. 3 Comparative means ( SD) obtained from the current study range from mm to mm proximally and mm to mm distally. It should be noted that the larger difference between the proximal and distal TDWT measured in feral horses was attributed to abrasive wearing of the outer hoof wall tubules, effectively reducing the distal measurement. These data represent a maximum difference of the means between Linford s Thoroughbreds and our feral horse populations of 34.4% proximally and 30% distally. A further derivative of this parameter is the radiometric ratio of the hoof wall thickness and the palmar cortex length of the distal phalanx. Linford reported the mean SD ratio in Thoroughbreds to be % proximally and % distally. 3 The corresponding means ( SD) from the current study range from % to % proximally and % and % distally. Such thickening has been found to result from thickening of the lamellar layer 23 and has been linked to laminitis. 3,23 25 In the study of foot pathology in feral horses in New Zealand, 14 11/20 Kaimanawa left forefeet had a value higher than 28.1%, a value that was linked to a high prevalence of clinical laminitis in that population. Pollitt suggested this ratio should be close to 25% in the sound foot 25 and Linford determined that a thickness ratio greater than 28.1% exceeded the 95% confidence limits and should be considered abnormal. 3 Therefore, all five feral horse populations in the current study would be identified as abnormal, with a strong suspicion of a high prevalence of laminitis in four of the five populations. However, there was little gross evidence of laminitis in any of the current feral horse populations. The increased hoof wall depth in the feral horse populations suggests an effect of long travel distances and hard substrates on the architecture of the suspensory apparatus of the distal phalanx and the outer hoof wall. It is possible that the increased depth of the hoof wall in these horses is a tissue adaptation in response to long travel on hard substrates. There may also be genetic selection for horses with a thicker hoof wall in this environment. Hampson et al. found evidence of natural selection in a variation in lamellar architecture in the same groups of hard-substrate horses. 26 A thicker hoof capsule may dissipate load over a larger volume of tissue. However, it is also possible that a thicker stratum internum and stratum medium represents a pathological consequence of a harsh lifestyle, similar to changes seen in traumatic laminitis. 3 Histopathology studies are required to confirm the presence of laminitis. The length of the hoof wall distal to the sole plane is of interest to hoof care providers because it is an important guideline in the process of trimming a hoof. At the dorsal midline dead centre, the length of distal hoof wall ranged from a mean ( SD) of mm beyond the adjacent sole plane to mm proximal to the adjacent sole plane. This morphology supports the previous observation that feral horses, particularly from a hard-substrate habitat, bear substantial weight on the peripheral sole as well as the hoof wall. 27 Further research is required to determine whether repetitive peripheral sole loading over hard substrates is detrimental to foot health or whether the foot is able to adjust to this loading pattern without consequence. However, the present study results provide sufficient evidence of possible pathology in hard-substrate feet to caution the use of this aspect of the foot model to guide trimming practices. Some investigators have concluded that the CE distance is an important parameter of foot health, particularly in respect to laminitis. 6,28 The present study reported a range of means for CE distance, from to mm, with a range of values from 2.3 to 14.6 mm. The lower mean was for the Cliffdale horses and it was different from each of the other four populations. The reason for the difference is unclear, but is presumably because of either an unidentified environmental factor or an internal derangement of the suspensory apparatus of the distal phalanx in some populations. If the suspensory apparatus of the distal phalanx was stretched because of the pathology of chronic laminitis, then the distal phalanx would sink within the hoof capsule. 6,28 The unusually large TDWT in the hard-substrate horses in the present study suggests that lamellar stretching and pathological inner wall anatomy are responsible for a lower position of the distal phalanx 21

9 in the four populations. The observation that TDWT was significantly less in the soft-substrate horses, with a smaller CE, supports this argument. Study limitations One limitation of the current study was the inability to completely control all external variables that may have affected foot morphology. This was a difficult task in an observational study across five wilderness environments. Caution should be exercised when comparing uncontrolled populations of horses. The effect of diet on the foot morphology observed in this study is unknown, but the high carbohydrate levels in the diet of feral horses may account for some of the abnormal morphologies observed. 21 Some caution should be exercised in interpreting data taken from frozen and thawed limbs. Although complex studies of equine foot morphology have been performed on frozen and thawed limbs, 14,29,30 the effects of the freezing and thawing process on the morphology of the foot have not been investigated. This study presents evidence of an effect of the environment on the conformation of the feral horse foot. We were unable to determine whether the foot type seen in a particular environment was the result of natural selection or simply a consequence of the environmental influences on the foot. We have described potentially deleterious consequences of the feral horse habitat, particularly hard-substrate habitats. The wild horse foot has been proposed as the ideal model, 10,12,13 despite a lack of detailed empirical investigation. The present study suggests that no single feral horse foot model exists. Instead, foot morphology results from its interaction with the environment inhabited by the horse. We found five morphometric foot models from five different environments. Further investigation of feral horse feet is warranted to determine the extent of pathology before the effect of the environment can be fully established. There is currently no clear evidence to support the use of the feral horse foot as a model for foot care over the model currently used by the majority of veterinarians and hoof care providers. However, there were some consistent morphometric parameters among all the feral horses in the present study and these may be important when considering the natural form of the equine foot. Acknowledgments This research was funded by a grant from the Rural Industries Research and Development Corporation, Australia, and from internal funding of the Australian Brumby Research Unit, School of Veterinary Science, The University of Queensland. References 1. Cust A, Anderson G, Whitton R, Davies H. The association between hoof conformation and performance in the racingthoroughbred in Macau. In: Proceedings of the 6th International Conference on Equine Locomotion, June, 2008, Cabourg, France, 2008: Kobluk CN, Robinson RA, Gordon BJ et al. The effect of conformation and shoeing: a cohort study of 95 Thoroughbred racehorses. AAEP Proc 1990;35: Linford RL. Qualitative and morphometric radiographic findings in the distal phalanx and digital soft tissues of sound Thoroughbred racehorses. Am J Vet Res 1993;54: Ducro BJ, Gorissen B, Eldik P, Back W. Influence of foot conformation on duration of competitive life in a Dutch Warmblood horse population. Equine Vet J 2009;41: Kane AJ, Stover SM, Gardner IA et al. Hoof size, shape, and balance as possible risk factors for catastrophic musculoskeletal injury of Thoroughbred racehorses. Am J Vet Res 1998;59: Kummer M, Geyer H, Imboden I, Auer J, Lischer C. The effect of hoof trimming on radiographic measurements of the front feet of normal Warmblood horses. Vet J 2006;172: van Heel MCV, van Weeren PR, Back W. Compensation for changes in hoof conformation between shoeing sessions through the adaptation of angular kinematics of the distal segments of the limbs of horses. 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