Young Horses Research Articles

Impact of oral Phytozen® EQ supplementation on plasma cortisol and behavior responses of young horses exposed to stressful stimuli.

Calming supplements are common in the equine industry. This study tested the hypothesis that Phytozen® EQ, a blend of citrus botanical oils, magnesium, and yeast would reduce startle response as well as reduce behavioral and physiological signs of stress in young (1.5-6 years of age) horses (n = 14) when tied in isolation and when trailered in isolation. During the 59-day (d) trial, horses were assigned to either the control (CON; n=7) or treatment (PZEN; n=7) group that received 56 g of Phytozen® EQ daily. Horses underwent a 10 min isolation test on d 30 and a 15 min individual trailering test on day 52 or 55. For both tests, blood samples were obtained pre, immediately after, and 1-hour (h) post for analysis of plasma cortisol concentrations, which were analyzed by repeated measures ANOVA. On d 59, horses underwent a startle test, for which time to travel 3 m and total distance traveled were recorded. These data were analyzed using a T-test. During trailering, PZEN horses tended to have lower overall geometric mean (lower, upper 95% confidence interval) cortisol concentrations than CON (81 [67, 98] vs. 61 [48, 78] ng/mL; P = 0.071). For the startle test, PZEN horses tended to have longer geometric mean times to travel 3 m than CON horses (1.35 [0.39, 4.70] vs. 0.26 [0.07, 0.91 seconds [s], P = 0.064). Other data points were not different between treatments (P > 0.1). It is possible that this dietary supplement could have beneficial calming effects on horses undergoing trailering or in novel situations.

Colic due to dorsally located perirectal abscesses: A retrospective case series of six horses

SummaryPerirectal abscesses have only rarely been reported in horses. This case series describes the clinical signs, diagnosis, treatment and outcome of six horses with this condition. The equids affected were geldings or stallions of various breeds and aged from 4 months to 21 years (median age 10 months). Typical clinical signs included colic with tachycardia, tenesmus, dyschezia and abdominal distension. The diagnosis was obtained by transrectal palpation and ultrasonographic examination in five of six horses and by median laparotomy in one horse. All abscesses were located dorsally to the rectum. Rectal impaction and large colon tympany were common secondary findings. Microbiologic examination of abscess content revealed Streptococcus equi subspecies zooepidemicus in all horses. Abscesses were treated by a combination of medical and surgical approaches. One horse had to be euthanised due to complete obstruction of the pelvis by the abscess. In the other horses (5/6), the disorder was associated with an excellent prognosis. In terms of the total population of horses presenting with colic in the investigated period, the prevalence of cases with perirectal abscesses was 0.4%. Although uncommon, described cases indicate that perirectal abscesses should be considered as differential diagnosis for young and mature horses with colic signs.

Osteochondrosis in the central and third tarsal bones of young horses.

Recently, the central and third tarsal bones of 23 equine fetuses and foals were examined using micro-computed tomography. Radiological changes, including incomplete ossification and focal ossification defects interpreted as osteochondrosis, were detected in 16 of 23 cases. The geometry of the osteochondrosis defects suggested they were the result of vascular failure, but this requires histological confirmation. The study aim was to examine central and third tarsal bones from the 16 cases and to describe the tissues present, cartilage canals, and lesions, including suspected osteochondrosis lesions. Cases included 9 males and 7 females from 0 to 150 days of age, comprising 11 Icelandic horses, 2 standardbred horses, 2 warmblood riding horses, and 1 coldblooded trotting horse. Until 4 days of age, all aspects of the bones were covered by growth cartilage, but from 105 days, the dorsal and plantar aspects were covered by fibrous tissue undergoing intramembranous ossification. Cartilage canal vessels gradually decreased but were present in most cases up to 122 days and were absent in the next available case at 150 days. Radiological osteochondrosis defects were confirmed in histological sections from 3 cases and consisted of necrotic vessels surrounded by ischemic chondronecrosis (articular osteochondrosis) and areas of retained, morphologically viable hypertrophic chondrocytes (physeal osteochondrosis). The central and third tarsal bones formed by both endochondral and intramembranous ossification. The blood supply to the growth cartilage of the central and third tarsal bones regressed between 122 and 150 days of age. Radiological osteochondrosis defects represented vascular failure, with chondrocyte necrosis and retention, or a combination of articular and physeal osteochondrosis.

Perceived sidedness and correlation to vertical movement asymmetries in young warmblood horses.

The prevalence of vertical asymmetries is high in "owner-sound" warmblood riding horses, however the origin of these asymmetries is unknown. This study investigated correlations between vertical asymmetries and motor laterality. Young warmblood riding horses (N = 65), perceived as free from lameness were evaluated on three visits, each comprising objective gait analysis (inertial measurement units system) and a rider questionnaire on perceived sidedness of the horse. A subgroup (N = 40) of horses were also subjected to a forelimb protraction preference test intended as an assessment of motor laterality. We hypothesized associations between vertical asymmetry and motor laterality as well as rider-perceived sidedness. Vertical asymmetry was quantified as trial means of the stride-by-stride difference between the vertical displacement minima and maxima of the head (HDmin, HDmax) and pelvis (PDmin, PDmax). Laterality indexes, based on counts of which limb was protracted, and binomial tests were used to draw conclusions from the preference tests. In the three visits, 60-70% of horses exhibited vertical asymmetries exceeding clinically used thresholds for ≥1 parameter, and 22% of horses exhibited a side preference in the preference test as judged by binomial tests. Linear mixed models identified a weak but statistically significant correlation between perceived hindlimb weakness and higher PDmin values attributable to either of the hindlimbs (p = 0.023). No other statistically significant correlations to vertical asymmetry were seen for any of the questionnaire answers tested. Tests of correlation between the absolute values of laterality index and asymmetry parameters (HDmin, HDmax, PDmin, PDmax) identified a weak correlation (p = 0.049) with PDmax, but when accounting for the direction of asymmetry and motor laterality, no correlations were seen for either of the asymmetry parameters. No convincing evidence of associations between vertical asymmetries and motor laterality were seen and further studies investigating motor laterality and the origin of vertical asymmetries are needed.

Detection of Theileria equi‎ in Baghdad Racing Horses Using ‎Hematological and ‎Molecular Assay


 
 
 
 The aim of this study was to investigate the prevalence of Theileria equi‎ infection in racing ‎horses in ‎‎Baghdad governorate, utilizing clinical signs, microscopic examination, molecular ‎assays, and ‎hematological ‎tests to identify significant differences between infected and non-‎infected horses. Between ‎January and ‎December 2021, a total of 160 racing horses from three ‎locations in Baghdad governorate (Al-‎Amiriya ‎Equestrian Club, Arabian horses in Alzwraa ‎Zoo, and Iraqi Equestrian School) were randomly ‎selected for ‎examination. Microscopic ‎examination of blood smears revealed 11 positive samples for ‎Theileria equi‎ ‎‎(6.875%), while ‎the results of the polymerase chain reaction assay targeting the 18S ‎ribosomal RNA gene ‎‎confirmed 26 positives (16.25%). Sequenced amplicons and phylogenetic tree analysis ‎‎revealed a genetic ‎similarity of 93.03–‎100% and 94-100% site coverage compared to many ‎global ‎countries' isolates. ‎Interestingly, only 16 (61.5%) of the 26 infected horses showed ‎mild to subclinical ‎symptoms or were ‎carriers of the disease without tick infestation. ‎Hematological blood parameters ‎showed non-significant ‎differences between infected and ‎non-infected horses, except for a decrease in ‎packed cell volume (PCV) ‎and hemoglobin (Hb) ‎concentration, which caused anemia in 12 horses among ‎the 26 molecularly positive ‎cases ‎‎(46.15%). Of these cases, 6 horses (23.08%) had normocytic ‎normochromic anemia, while 3 ‎‎(11.54%) ‎cases each had normocytic hypochromic and microcytic ‎hypochromic anemia. ‎Notably, young horses (two ‎years old) were more susceptible to infection (odds ratio ‎‎15.4) ‎than those over six years old. Additionally, ‎sex and breed did not show any significant ‎correlation ‎with equine theileriosis. In conclusion, this study ‎detected Theileria equi‎ infection ‎in young racing horses in ‎Baghdad. Clinically, most infected cases showed ‎mild to ‎asymptomatic signs accompanied by anemia. ‎Molecular investigation revealed high genetic ‎‎similarity to isolates reported globally. These findings ‎highlight the importance of ‎implementing measures ‎to control and prevent the spread of Theileria equi‎ in ‎racing horses in ‎Baghdad and other regions. Further ‎studies are warranted to better understand the ‎‎epidemiology, pathogenesis, and risk factors associated ‎with equine theileriosis‎.
 
 
 

Subchondral lucencies of the medial femoral condyle in yearling and 2-year-old Thoroughbred sales horses: Prevalence, progression and associations with racing performance.

Subchondral lucencies (SCLs) in the distal aspect of the medial femoral condyle (MFC) of young Thoroughbred horses are a source of controversy on presale radiographs. There is limited scientific evidence regarding the risk of progression and impact on future racing performance. To (1) identify the prevalence of MFC SCLs on sales repository radiographs in yearling and 2-year-old Thoroughbreds; (2) identify any association between grade of MFC SCL and future racing performance and (3) monitor changes in MFC SCL grades between yearling and 2-year-old sales. Prospective cohort study. Radiographs were obtained with consignor permission from a 2016 yearling sale and five 2017 2-year-old sales. Stifle radiographs were evaluated and MFC SCLs graded on a scale of 0-3. Axial MFC lucencies were recorded separately. Maximum MFC grades per horse were analysed for associations with racing performance outcomes, adjusted for sex, to the end of the horses' 4-year-old racing year. Analysis was via logistic, negative binomial or linear regression as appropriate, with the threshold for significance set at α = 0.05. Radiographs from 2508 yearlings (5016 stifles) and 436 2-year-olds (872 stifles) were included in the study. MFC SCLs of grades 1-3 were observed in 242 (9.65%) yearlings and 49 (11.2%) 2-year-olds. Bilateral MFC SCLs of grades 1-3 were observed in 54 (2.2%) yearlings and 12 (2.8%) 2-year-olds. Grade 1 MFC SCLs in yearlings either remained unchanged (14/31), progressed to a grade 2 (6/31) or resolved (11/31) by the 2-year-old sale. Grade 2 MFC SCLs in yearlings remained unchanged (6/10), progressed to a grade 3 (2/10) or improved to a grade 1 (2/10). Yearlings with a grade 3 MFC SCL had a 78% probability of starting a race (95% confidence interval [CI] 58.2-89.6%), compared with an 84% probability of racing for grade 0 yearlings (95% CI 82.7-85.8%). Six of the seven yearlings with axial MFC lucencies raced. This study may underestimate the prevalence of severe lesions in the general yearling population of U.S. Thoroughbreds. However, the convenience sample used is representative of the population of interest at sales. The study design could not address exclusions prior to sale. Grade 1 MFC SCLs are the most common type seen in yearling and 2-year-old sales horses. The majority of yearling grade 1 MFC SCLs resolved or remained static by 2-year-old sales. It was also possible for grade 2 and 3 MFC SCLs to improve one grade between sales. Fewer sales yearlings with a grade 3 MFC SCL raced, but in those that did race there was no evidence of worse performance compared to unaffected peers. Axial MFC lucencies did not affect racing performance.

Productive traits and biological features of young horses of Yakut breed in different areas of breeding

The Republic of Sakha (Yakutia) is one of the regions of Russia where drove breeding of horses has long been developed. In the modern realities of horse breeding, to further increase the number of drove breeding of horses and improve their productive, reproductive and adaptive traits, one of the most important techniques is to increase the genetic diversity of animals. The purpose of the research was livestock characteristics, the study of productive traits and biological characteristics of young horses of Yakut breed of the Arctic zone (Momsky ulus) in a comparative aspect with young horses of the Middle Taiga zone (Amginsky ulus). So, in terms of pre-slaughter weight and slaughter yield, the foals of the Momsky ulus had superiority. At the age of 6 months, they had a higher preslaughter weight by 9,3 kg (P ≥ 0,99), and a heavier carcass by 10,09 kg (P ≥ 0,99), compared with their herdmates of the Amginsky ulus. According to the slaughter yield, the foals of the Momsky ulus also had superiority. In the foals of the Momsky ulus at the age of 6 months the slaughter yield was 56,5 %, which was higher in comparison with the young animals of the Amginsky ulus by 2,5 abs.% (P ≥ 0,95). Probably, the higher pre-slaughter weight of foals in the Momsky ulus was due to more productive pasture lands with a higher nutritional value of plants. Chemical analysis of horse meat showed that the meat of foals of the Arctic zone (Momsky ulus) has a lower fat content. This allows you to get meat with a lower energy value, a suffi ciently high protein content and is a higher-quality product, which is probably due to the composition of the feed of this zone. Breeding of Yakut horses in the Arctic zone of Yakutia is more attractive from the point of view of the growth and development of young animals, obtaining better quality horse meat.

Proximate factors affecting mortality and maternal abandonment of young free-roaming feral horse foals

The burgeoning population of feral horses in the American west is due to high population growth, resulting from low adult mortality and high foal survival. In two populations of feral horses in western Utah, USA only 15 foals died (5%; mean age <1 month) over a 4-year period. Seven additional foals (age <70 days) were observed separated from their dam, with no return to suckling or associating with the dam (i.e., abandoned). Factors affecting fate of foals were examined by comparing dead and separated foals with siblings (n = 19 dams, n = 32 siblings). Foals becoming separated or dying were observed in all years of the study, were unrelated to horse density, environmental effects, or gather events. There was no effect of dam body condition, parity, or age on foal survival or separation, and no effect of length of time the dam was in a group, whether the foal was born into the same group as conceived, and number of group changes made by the dam while pregnant. Dams of foals that died or were separated were more likely to change groups within 2 months after the foal was first seen, mostly after foal death or separation. Separated foals were near their dam less often, but there was no difference in frequency of social interactions. Separation of foals and dams is a natural occurrence in feral horses and survival likelihood for these foals is high – all separated foals that remained on the range in this study survived.

Prevalence and risk factors for laminitis within the Norwegian pony breed Nordlandshest/Lyngshest

BackgroundLaminitis is a systemic condition resulting in debilitating pain and structural changes within the feet, and hence has major welfare implications. Causes include endocrine and systemic inflammatory conditions. Ponies are frequently affected, and observations in the field suggest that occurrence of laminitis is also common in Norwegian breeds. The aim of this study was to estimate the prevalence and risk factors for laminitis within the Norwegian pony breed Nordlandshest/Lyngshest.ResultsThe study was a cross-sectional study based on questionnaires sent to members of the Norwegian Nordlandshest/Lyngshest breed association. Questionnaires were received for 504 animals, of which 464 records were eligible and included in analyses. The population comprised 71 stallions, 156 geldings, and 237 mares, with an age between 1 and 40 years (median and interquartile ranges: 12 (6–18) years). The estimated 3-year period prevalence of laminitis was 8.4% (95% confidence interval (CI95): 6.0–11.3%), whereas lifetime prevalence was 12.5% (CI95: 9.6–15.9%). Mares had a significantly higher period- and lifetime prevalence of laminitis than male horses, and horses 10 years and older had a significantly higher prevalence than younger horses. The lifetime prevalence of laminitis was 3.2% in horses 9 years and younger, whereas from 17.3–20.5% in older horses. Multivariable logistic regression analysis identified age, sex, and regional adiposity as significantly (P < 0.05) associated with the 3-year period outcome of laminitis: Horses older than 9 years had a three-fold increase in the likelihood of having laminitis compared to the younger horses (Odds Ratio (OR)10–14 years = 3.37 (CI95 = 1.19–9.50), OR15-19 years = 3.06 (CI95 = 1.04–9.05), and OR>20 years = 2.70 (CI95 = 0.90–8.02). Mares were more than twice as likely (OR = 2.44 (CI95 = 1.17–5.12) to have laminitis compared to male horses, and horses with regional adiposity had increased odds (OR = 2.35 (CI95 = 1.15–4.82) of laminitis compared to horses without regional adiposity.ConclusionsLaminitis appears to be a considerable welfare issue in the Norwegian pony breed Nordlandshest/Lyngshest. The identified risk factors age, sex, and regional adiposity highlight the need for improved owner education and awareness of strategies to reduce laminitis risk.

PREVALENCE OF GASTROINTESTINAL TRACT PATHOLOGY AND HELICOBACTERIOSIS IN HORSES

This article presents information on the prevalence of helicobacteriosis in horses of different age groups. It has been found that gastrointestinal tract (GIT) pathology is widely prevalent in horses, ranging from 50% to 80%. The main conditions among GIT pathologies in horses are gastrophylosis, erosions, ulcers, and helicobacteriosis. Helico-bacteriosis is registered in 66.7% to 100% of the investigated adult population and in 20% to 66.7% of the young horses. Stomach examinations using endoscopy and video gastroscopy provide a clear picture of the mucous membrane's condition in different parts of the stomach and allow for the collection of biomaterials for additional research. The percentage of animals affected by helicobacteriosis increases with age. Animals suf-fering from helicobacteriosis experience significantly reduced productivity, delayed growth and development in young horses, leading to premature culling. It is incorrect to only test suspicious animals with indications for helicobacteriosis, as not all carriers of Helicobacter pylori show symptoms of the disease. The use of the drug Domosedan at a dose of 0.5 μg per kilogram of body weight, or Combistress at a rate of 0.5 cm3 per 100 kg of body weight, effectively calmed the an-imals, fully relaxed the GIT, and facilitated successful endoscopy and video gastroscopy examinations.

Successful combination treatment of bilateral guttural pouch mycosis with salpingopharyngeal fistulation and potassium iodide

SummaryA 9‐month‐old Thoroughbred colt was presented to the University College Dublin Veterinary Hospital for investigation of suspected neck pain. Clinical signs included severe dullness, bilateral mucopurulent nasal discharge, ptosis of the right eyelid, reduced range of motion of the neck and ataxia. Endoscopy of the medial compartment of both guttural pouches showed fungal‐like plaques, from which Aspergillus was cultured. Laser salpingopharyngeal fistulation was performed under standing sedation and the colt received treatment with oral potassium iodide. The colt developed bacteraemia post‐operatively which resolved after antimicrobial treatment. Serial endoscopic evaluation of the guttural pouches showed complete resolution of fungal plaques 4 months post‐operatively. Minimal neurologic deficits remained 6 months post‐operatively. This report highlights the importance of considering guttural pouch mycosis in young horses and those presenting with reduced range of neck motion and neck pain. It offers support for the use of salpingopharyngostomy as an effective treatment method for guttural pouch mycosis and documents its use in the case of bilateral disease confirmed by fungal culture. It also documents the recovery of a horse from GPM with associated severe neurological signs.

Conditioned medium of mesenchymal stem cells: application in mares with endometrial fibrosis

The use of stem cells to promote tissue regeneration has increased in recent years mainly due to their role in growth, remodeling, and repair processes, increasing the number of progenitor cells in the affected tissue. Conditioned medium, a by-product containing metabolites, growth factors, and extracellular matrix proteins is secreted by cultured stem cells. This conditioned medium has been used as a cell-free therapy in regenerative medicine. The objective of the present study was to evaluate the effect of mesenchymal stem cell conditioned medium on endometrial fibrosis in mares through histopathological evaluation. The selected mares had an average age of 18.61 ± 5.68 years and a history of no foaling in the last two breeding seasons. They were grouped according to the degree of uterine fibrosis described by Kenney in 1978: Grade II (n=4), Grade III (n=2). All mares underwent evaluation of vulvar conformation, complete gynecological examination, and ultrasonographic evaluation of the reproductive tract, as well as uterine cytological, bacteriological, and histopathological examinations. The conditioned medium was made from the culture of stem cells derived from the adipose tissue of the lumbarregion of a young horse. During diestrus, using an artificial insemination pipette, 20 mL of conditioned medium was infused into the uterus of each mare, divided into 10 mL per uterine horn. After applying the medium to a horn, 5 mL of sterile 0.9% NaCl solution was administered to ensure that the conditioned medium was infused. Twenty-one days after treatment with the conditioned medium, the six mares were submitted to a new collection of uterine biopsies for histopathological evaluation. In this second evaluation, the same changes characteristic of endometrosis were observed in the histopathology, such as nests with glandular dilation and the presence of areas of fibrosis. These findings suggest that, although no improvement was observed in the histopathological characteristics of endometrial biopsies, the use of the conditioned medium of stem cells did not induce an inflammatory reaction following its intrauterine use, suggesting its safety for further studies in equine medicine. In conclusion, the application of 20 mL of conditioned medium from stem cells in the uterine lumen of six mares with endometrosis grades II and III did not induce endometrial regeneration in the evaluated period.

Evaluation of the appearance of osteochondrosis lesions by two radiographic examinations in sport horses aged from 12 to 36 months.

Osteochondrosis is a developmental orthopedic disease characterized by a defect of enchondral ossification. This pathological condition develops and evolves during growth and is influenced by various factors, in particular genetic and environmental. However, little research has been conducted on the dynamic of this condition in horses after the age of 12 months. The retrospective study presented here investigates changes in osteochondrosis lesions through two standardized radiographic examinations carried out on young Walloon sport horses after one year of age (mean age at first and second examination was 407 (±41) and 680 (±117) days respectively). Each examination, analyzed independently by three veterinarians, included latero-medial views of the fetlocks, hocks, stifles, plantarolateral-dorsomedial hocks view and additional radiograph if the operator deemed it necessary. Each joint site was graded as healthy, osteochondrosis (OC) or osteochondrosis dissecans (OCD) affected. A group of 58 horses was studied, among them 20 presented one or more osteochondrosis lesions for a total of 36 lesions present during at least one examination. In this population, 4 animals (6.9%) presented osteochondrosis during only one examination (2 at the first examination and 2 at the second one). Moreover, it was possible to demonstrate the appearance, disappearance and more generally the evolution of 9/36 lesions (25%) within the different joints. The results of the study suggest that, although substantial main limitations, osteochondrosis lesions can evolve after the age of 12 months in sport horses. Knowing this is useful in helping to decide the appropriate radiographic diagnosis timing and management.

119 The effect of parasitic load on the equine microbiome, hematological parameters, and overall body condition

Strongyles are nematode parasites that affect the large intestine and cecum of the horse, with symptoms ranging from loss of condition to colic. Thirty horses from the University of Connecticut campus were used to study the effect of gastrointestinal strongyles on the equine gut microbiome, immune response, body condition, and skin and hair coat conditions. This study included young (n = 16) and adult (n = 14) horses each fall and spring from Fall 2020 to Spring 2022, with young horses 1 to 5 years old and adult horses 6 to 19 years old. Fecal flotation analysis was performed at the beginning of each fall and spring season to classify horses as susceptible to parasites if they had 300 or more strongyle eggs per gram (EPG) or resistant to parasites if they had less than 300 strongyle EPG. Fecal and blood samples were collected each fall and spring to assess gut microbial composition, complete cell counts, and differential cell counts. Body condition scores and body weights were assessed twice per season. Dermatological conditions were recorded once monthly. A split plot design with age as the main effect and fecal egg count (FEC) as the split plot effect was used. Adult and young horses that were susceptible to parasites had significantly increased percentages oflymphocytes and neutrophils and decreased percentages of monocytes and eosinophils compared with horses that were resistant to parasites in Fall 2021 and Spring 2022 (P < 0.05). In adult horses, linear correlation analysis revealed that as parasite load increased, body weight decreased (r = −0.29) and complete cell count decreased (r = −0.29), although these trends were not observed in young horses. Ten percent of the variation seen in microbial community structure was due to phenotype. Linear correlation analysis revealed some changes in skin and hair condition, such as an increase in skin moistness (r = 0.32), tailhead moistness (r = 0.36), and hair thickness (r = 0.44) as parasite load increased and a decrease in hair luster (r = −0.32) as parasite load increased. The effects of strongyle infection on dermatology are still unclear as some of the effects seen contradict previous literature and warrant further investigation. Strongyle load may affect body weight in adult horses. Strongyle infection may affect immune response, but have a minimal effect on microbial composition.

83 Effect of a Saccharomyces cerevisiae fermentation product on intake and digestion in young horses undergoing exercise training

Young growing horses subjected to exercise have greater dietary requirements than horses at maintenance necessitating the delivery of a more nutrient dense diet. Inclusion of a grain concentrate in the total daily ration achieves this goal but also may put the young horse at risk of digestive disorders and reduced forage utilization. As such, the objective of this study was to determine if a dietary S. cerevisiae fermentation product (SCFP; TruEquine C, Diamond V) could influence nutrient utilization in young horses being introduced to grain concentrate feed and if the response was dose dependent. Thirty Quarter horse yearlings (561 ± 15.8 d of age and 371 ± 24.4 kg BW) were adapted to a 14% CP grain concentrate feed (Nutrena SafeChoice Original formulated without microbial ingredients), fed at 1% of BW as fed to meet or slightly exceed nutrient requirements for yearling horses undergoing light to moderate work and allowed ad-libitum access to Coastal bermudagrass (Cynodon dactylon) hay (67.8% NDF) and fresh water. Horses were stratified by age, BW, and sex and randomly assigned to one of 3 levels of SCFP: 0, 46, or 92 mg/kg BW divided evenly and top-dressed onto the grain concentrate meal at 12 h intervals. After 14 d of adaptation, intake and total fecal production were measured overa 4-d period. Horses were housed in individual 3.7 × 3.7 m stalls and allowed 1 h of exercise in individual 3.7 × 7.4 m runs daily during the collection period. All statistical analyses were performed in SAS 9.4 using PROC MIXED with the model containing treatment and horse included as the random variable. There was no significant treatment difference for any measure of intake (P ≥ 0.33); forage DM intake averaged 6.14 kg/d, total DM intake averaged 9.60 kg/d, and total digestible organic matter intake averaged 4.97 kg/d. There was a tendency for improved DM digestibility (P = 0.09) with supplementation (56.3 and 56.8% for 46 and 92 mg/kg BW, respectively) compared with no supplementation (54.2%). Supplementation of SCFP resulted in no significant differences (P ≥ 0.15) in organic matter (OM) or NDF digestibility. There were no differences in any indicator of nutrient digestibility between the low and high level of SCFP supplementation. An observed tendency for improved DM digestibility with SCFP supplementation suggests further research is warranted.

71 Lameness responses to a single exercise bout following dietary supplementation of Saccharomyces cerevisiae fermentation product in young horses

Yearling Quarter Horses (mean ± SEM; 561 ± 3 d; 373.3 ± 5 kg BW; 13 fillies, 14 colts) were used in a randomized complete block to test the hypothesis that dietary supplementation of a postbiotic Saccharomyces cerevisiae fermentation product (SCFP) mitigates lameness following a single exercise bout in young horses. For the 76-d trial, yearlings were fed a basal diet of 1% BW/d (as-fed) grain-based horse feed formulated without added microbials (SafeChoice® Original, Nutrena) twice daily plus ad libitum Coastal bermudagrass (Cynodon datylon) hay, and water. Horses were grouped by age, sex, and BW and randomly assigned to 1) concentrate only (CON; n = 10), 2) concentrate top dressed with 46 mg/kg SCFP (TRT 1; n = 8), or 3) concentrate top dressed with 92 mg/kg SCFP (TRT 2; n = 9). Objective lameness assessments were obtained by a wireless Inertial Sensor System (ISS) followed by a subjective evaluation performed by a licensed veterinarian. All horses underwent a baseline lameness assessment on d 0, followed by 2 lameness assessments on d 76 (pre-exercise, and post-exercise). Objective lameness evaluations were performed at the trot for 2 30 m, back-and-forth passes, wearing the wireless ISS lameness analysis system and evaluated in triplicate per assessment. For the subjective lameness analysis, horses were additionallytrotted in a circle in both directions within a 16 × 14 m area and evaluated by an individual licensed veterinarian using the American Association of Equine Practitioners (AAEP) lameness scale. Data were analyzed using PROC GLM in SAS v9.4 with diet and time included as fixed effects. When analyzing AAEP lameness evaluations, no differences were detected for dietary treatment or diet × time interactions related to primary (P = 0.55; P = 0.98), secondary (P = 0.43; P = 0.78), or tertiary (P = 0.77; P = 0.77) limbs. Additionally, no dietary treatment effects or diet × time interactions were detected using the wireless ISS for front Q score (P = 0.19; P = 0.92) or hind pushoff Q score (P = 0.53; P = 0.27). Furthermore, there was no effect of dietary treatment on hind impact Q score (P = 0.68). However, there tended to be a diet × time interaction (P = 0.06) whereby horses on TRT 2 tended to have a lower hind impact Q score. Additionally, post-exercise front Q scores increased over time (P ≤ 0.01) compared with both baseline and pre-exercise values regardless of dietary treatment. These results indicate the intensity of exercise may not have been sufficient to lead to dietary differences related to gait asymmetry.

118 Relationship between age, gender, height, body condition score, and body weight in Thoroughbred yearlings raised in Kentucky

Forty years ago, Henneke and coworkers developed a body condition score (BCS) system for horses that is widely used today. This system applies a score of 1 to an emaciated horse and a score of 9 to an extremely obese horse. The system was originally designed to measure BCS in Quarter Horse broodmares, but it has subsequently been adapted to measure body condition in horses of all breeds and life stages. The NRC (2007) suggests that weight gain of 16 to 20 kg will increase the body condition of a 500-kg horse from a 4 to a 5. There is little information regarding the relationship between BCS and weight gain in young horses. This study evaluated the relationship between age, gender, wither height (HT), BCS, and body weight (BW) in a large group of yearlings raised in central Kentucky over a 17-year period (2002–2018). Measurements were taken from colts (n = 2,048) and fillies (n = 2,032) during August of their yearling year. BCS was scored in 0.5 increments. Differences in measurements between genders were determined using an unpaired t-test. Colts (age 515.1 ± 34.7 d, BW 461.7 ± 34.7 kg, HT 155.3 ± 3.5 cm, BCS 5.88 ± 0.34; mean ± SD) and fillies (age 516.6± 34.0 d, BW 446.8 ± 32.4 kg, HT 153.7 ± 3.4 cm, BCS 5.94 ± 0.34; mean ± SD) were not different in age, but colts were heavier (P < 0.01) and taller (P < 0.01). Fillies had a higher BCS (P < 0.01). Over 95% of the yearlings had BCS of 5.5 to 6.5 with > 60% having a BCS of 6.0. Within each gender, multiple linear regression analysis using age, year of birth, HT, and BCS as independent variables and BW as the dependent variable resulted in regression coefficients for BCS of 15.1 ± 1.5 kg and 20.1 ± 1.4 kg (±SE) for colts and fillies, respectively. This suggests that weight gains from 15 to 20 kg will increase the body condition score of 450-kg yearlings from 5.5 to 6.5. The majority (>80%) of the yearlings measured in this study were being prepped for yearling sales where BCS of 6.0 is considered ideal. An understanding of how much BW gain is needed to affect BCS should be useful in the sales-prepping process.

Influence of Mechanistic Target of Rapamycin (mTOR)-Regulated Anabolic Pathways on Equine Skeletal Muscle Health.

Skeletal muscle is a highly dynamic organ that is essential for locomotion as well as endocrine regulation in all populations of horses. However, despite the importance of adequate muscle development and maintenance, the mechanisms underlying protein anabolism in horses on different diets, exercise programs, and at different life stages remain obscure. Mechanistic target of rapamycin (mTOR) is a key component of the protein synthesis pathway and is regulated by biological factors such as insulin and amino acid availability. Providing a diet ample in vital amino acids, such as leucine and glutamine, is essential in activating sensory pathways that recruit mTOR to the lysosome and assist in the translation of important downstream targets. When the diet is well balanced, mitochondrial biogenesis and protein synthesis are activated in response to increased exercise bouts in the performing athlete. It is important to note that the mTOR kinase pathways are multifaceted and very complex, with several binding partners and targets that lead to specific functions in protein turnover of the cell, and ultimately, the capacity to maintain or grow muscle mass. Further, these pathways are likely altered across the lifespan, with an emphasis of growth in young horses while decreases in musculature with aged horses appears to be attributable to degradation or other regulators of protein synthesis rather than alterations in the mTOR pathway. Previous work has begun to pinpoint ways in which the mTOR pathway is influenced by diet, exercise, and age; however, future research is warranted to quantify the functional outcomes related to changes in mTOR. Promisingly, this could provide direction on appropriate management techniques to support skeletal muscle growth and maximize athletic potential in differing equine populations.

10 Extra-label bisphosphonate effects on bone turnover biomarkers in juvenile, exercising horses

Bisphosphonates, such as clodronate disodium (CD), modulate bone metabolism and, though their extra-label use in young horses is common, impacts on skeletal development and longevity are unknown. The objective of the current study was to determine the effects of CD use on systemic biomarkers of bone turnover in juvenile, exercising horses. We hypothesized that bone resorption would decrease, and bone formation would increase as a result of CD administration and repeat treatments would have a greater effect. To test this, 32 juvenile Quarter Horses were used in a 168-d trial. Horses were stratified by age (500 ± 13 d), BW (336 ± 26 kg), sex (n = 16 geldings; n = 16 fillies), and initial bone optical density and randomly allocated to one of 4 treatment groups receiving either 1.8 mg/kg BW CD (OSPHOS®) or isovolumetric saline (placebo). Investigators were blinded to treatments which included a control (CON; n = 8), single-dose CD on d 84 (1X; n = 8), 2-doses CD on d 0 and 84 (2X; n = 8), and 4-doses CD on d 0, 42, 84, and 126 (4X; n = 8). Horses were housed individually in stalls (3.6 × 7.3 m), fed 1% BW/d concentrate, and allowed ad libitum access to Coastal bermudagrass hay and water. Horses exercised 5 d/wk using a free stall exerciser with a phase-based progressive workload; phase I (d 0–84) simulated sales preparation and phase II (d 85–168) mimicked an early training program. Blood was collectedon d 0, 42, 84, 126, and 168, before treatment injections on days of administration. Serum was analyzed for receptor activator of nuclear factor kB ligand (RANKL), tartrate resistant acid phosphatase 5b (TRAP5b), c-terminal crosslinks of type I collagen (CTX-1) and bone-specific alkaline phosphatase (BAP) via commercial ELISA or EIA kits. All data were analyzed using PROC MIXED of SAS with a baseline covariate for BAP and BAP:TRAP5b. No differences were seen in osteoclastic differentiation signal RANKL (P ≥ 0.24). In contrast, a treatment × time interaction was noted for osteoclastic TRAP5b (P = 0.03); specifically, TRAP5b decreased in 4X from d 0 to 126 before returning to baseline at d 168 and decreased in 2X at d 84 compared with baseline, while CON and 1X increased or remained the same over time. Concentrations of CTX-1, a type I collagen degradation marker, increased over time (P < 0.01) in all treatments. Likewise, osteoblastic BAP (P < 0.01) and BAP:TRAP5b (P = 0.04) increased from d 42 to 84 and remained elevated until d 168. These results indicate that multiple doses of CD (2X and 4X) over 168 d reduced activity of osteoclasts despite having no measurable effect on bone formation or type I collagen degradation.

97 Effect of psyllium husk supplementation on equine fecal nutrient composition and in vitro fermentation

Ingestion of sand by equids housed in areas with sandy soils can cause serious and potentially life-threatening issues. Feeding psyllium husk (Plantago ovata) is one of the most common treatments used within both the domestic horse and zoological communities for mitigating sand accumulation in the large intestine of hindgut fermenting species. The degree to which the large intestine microbiome adapts to addition of psyllium in the diet, thereby reducing its efficacy, remains unknown. The aim of this study was to identify the degree to which psyllium husk (PSY) degradation changed over a 14-d supplementation period, both in vivo and using an in vitro batch culture system. We hypothesized that the hindgut microbiome would adapt to PSY supplementation over time, altering fermentation patterns. Young Quarter Horse geldings (437 ± 48 kg, 20 ± 1 mo of age; mean ± SD) were randomly assigned to receive either PSY supplementation (n = 6) or no supplementation (control; n = 2) in a repeated measures design. Horses assigned to treatment received PSY mixed into their morning concentrate ration at a rate of 0.31g DM/kg BW on d 1–14. Fecal samples were collected from all horses at the time of the morning feeding on d 0–14. Fecal samples were analyzed for dry matter (DM), neutral detergentfiber (NDF), acid detergent fiber (ADF), and hemicellulose (HEM; calculated as NDF-ADF). Fecal samples collected on d 0, 7, and 14 from horses receiving PSY were used to create fecal inocula representing each individual horse to determine in vitro dry matter degradability (DMD) of PSY when included as a substrate for in vitro fermentation. Gas production was measured repeatedly over a 48-h period, while DMD was measured at 12, 24, and 48 h. Fecal composition data were analyzed as a linear mixed effects model, with main effects and interaction of treatment and day. In vitro gas production and DMD were analyzed with main effects and interaction of day and hour of fermentation. Fecal DM (P = 0.93) and HEM (P = 0.82) were not different on any day of the study. In vitro, gas production was greater (P < 0.05) for inocula created from d 7 and d 14 feces compared with d 0. In vitro DMD was also greater (P < 0.05) at 24 and 48 h on d 7 and 14 compared with d 0. Findings indicate that microbial degradation of PSY may increase as treatment duration is lengthened, which could reduce the potential efficacy of PSY over time.