Conflicting research and anecdotal evidence have created disagreement among equestrians as to whether two-year-old horses should be trained and raced. The objective of this literature review is to evaluate epidemiological studies, as well as physiological data on equine bone, articular cartilage, and tendons to better determine the impact of training and racing two-year-old horses. The evaluation of numerous studies on the topic provides evidence that a horse which is trained or raced as a two-year-old has a lower risk of injury and better adapted tissues for the rigors of racing. Unfortunately, the current prolific use of pain-mitigating substances in the racing industry does place horses, including young cohorts, at greater risk of injury, and should be used with caution.

Studies have attempted to demonstrate the benefits of silicon on bone health using a wide range of Si amounts—provided in the diet or through supplementation—and several different animal species. Previous studies in humans have also demonstrated a positive correlation between Si intake and bone health measures. The aim of the current review is to determine the effective levels of Si intake or supplementation that influence bone health to better inform future study designs and guidelines. Articles were identified using one of two search terms: “silicon AND bone” or “sodium zeolite A AND bone”. Articles were included if the article was a controlled research study on the effect of Si on bone health and/or mineral metabolism and was in English. Articles were excluded if the article included human subjects, was in vitro, or studied silica grafts for bone injuries. Silicon type, group name, Si intake from diet, Si supplementation amount, animal, and age at the start were extracted when available. Dietary Si intake, Si supplementation amount, and the amount of Si standardized on a kg BW basis were calculated and presented as overall mean ± standard deviations, medians, minimums, and maximums. Studies that left out animal weights, amount of food or water consumed, or nutrient profiles of the basal diet were excluded from these calculations. Standardized Si intakes ranged from 0.003 to 863 mg/kg BW, at times vastly exceeding current human Si intake recommendations (25 mg/d). The lack of data provided by the literature made definitively determining an effective threshold of supplementation for skeletal health difficult. However, it appears that Si consistently positively influences bone and mineral metabolism by around 139 mg Si/kg BW/d, which is likely unfeasible to attain in humans and large animal species. Future studies should examine this proposed threshold more directly and standardize supplemental or dietary Si intakes to kg BW for better study replication and translation.

Laterality can be observed as side biases in locomotory behaviour which, in the horse, manifest inter alia as forelimb preferences, most notably in the gallop. The current study investigated possible leading-leg preferences at the population and individual level in Thoroughbred racehorses (n = 2095) making halt-to-gallop transitions. Videos of flat races in the UK (n = 350) were studied to record, for each horse, the lead-leg preference of the initial stride into gallop from the starting stalls. Races from clockwise (C) and anti-clockwise (AC) tracks were chosen alternately at random to ensure equal representation. Course direction, horse age and sex, position relative to the inside rail and finishing position were also noted. On C courses, the left/right ratio was 1.15, which represents a significant bias to the left (z = –2.29, p = 0.022), while on AC courses it was 0.92 (z = 0.51, p = 0.610). In both course directions, there was no significant difference between winning horses that led with the left leading leg versus the right (C courses, z = –1.32, p = 0.19 and AC courses, z = –0.74, p = 0.46). Of the 2,095 horses studied 51.26% led with their L fore and 48.74% with their R, with no statistically significant difference (z = -1.16, p = 0.25). Therefore, there was no evidence of a population level motor laterality. Additionally, 22 male and 22 female horses were randomly chosen for repeated measures of leading leg preference. A laterality index was calculated for each of the 44 horses studied using the repeated measures: 22 exhibited right laterality (of which two were statistically significant) and 21 exhibited left laterality (eight being statistically significant); one horse was ambilateral. Using these data, left lateralized horses were more strongly lateralized on an individual level than the right lateralized horses (t = 2.28, p = 0.03, DF = 34) and mares were more left lateralized than males (t = 2.4, p = 0.03, DF = 19).

Iron overload disorder afflicts captive rhinoceros but has not been documented in the wild. The specific cause for the disorder has not been identified but is likely associated with diet and management. Compared with wild counterparts, captive rhinoceros eat diets containing more iron, have greater fat stores, and exercise less. It has been suggested that the problem may be linked to development of insulin resistance in the captive population. Given that controlled experiments with sufficient numbers of rhinoceros are logistically not possible, an equine model was used to look for a relationship between iron status and insulin resistance; the nutritional requirements of horses are used as a guide for rhinoceros, because they have similar gastrointestinal tracts. Sixteen horses were tested to determine blood insulin responses to an oral drench of dextrose (0.25 g/kg bodyweight) and a meal of pelleted corn (1.5 g/kg bodyweight). Fasting blood samples were taken 30 and 0 min before administration. Further blood samples were taken every 30 min for 4 hr after administration to determine peak insulin and total area under the insulin curve (AUC). Fasting samples were tested for serum ferritin concentrations. Correlations were determined between ferritin and peak insulin concentrations and insulin AUC after administration of oral dextrose and pelleted corn. The strongest correlation was between ferritin and insulin AUC after dextrose administration (r = 0.61; P = 0.01) followed by AUC after feeding a meal of pelleted corn (r = 0.60; P = 0.01), with the correlation for peak insulin being 0.53 (P = 0.03) after dextrose administration and 0.56 (P = 0.02) after pelleted corn. When evaluating responses by gender, a significant correlation existed only for females, influenced by one insulin resistant individual. These data suggest a potential link between insulin resistance and body stores of iron and also suggest that approaches to reduce the susceptibility to insulin resistance should be incorporated into management of captive browsing rhinoceros.