The Frequency of Errors in Determining Age Based on Selected Features of the Incisors of Icelandic Horses

SIMPLE SUMMARY: Methods of assessing the age of horses based on the appearance of teeth, although used for many years, seem to suffer from relatively large errors. However, this method can play an auxiliary role in identifying horses of unknown origin. Furthermore, this method is a useful tool for owners, breeders, or veterinary surgeons to make decisions regarding purchase, insurance, or treatment and provides information on the specific characteristics of horses of different breeds. This study aimed to assess the suitability of selected features of the incisors for the determination of the age of Icelandic horses. Determining the age of Icelandic horses based on the appearance of teeth only matched the real age in more than one-third of the 126 individuals assessed. The average percentage of errors made in the assessment of age based on the eruption and growth of deciduous incisors was significantly smaller compared to determining age based on replacement of deciduous to permanent incisors, the disappearance of cups, and/or changes in the shape of the grinding surfaces. It is likely that characteristic changes occurring in Icelandic horses’ incisors may be related to the specific course of development processes of this breed. ABSTRACT: The structure and changes occurring to horses’ teeth during ontogeny are not only used to assess the degree of somatic maturity but also the development of universal patterns and is therefore used to determine the age of horses. Research shows that methods of assessing the age of horses based on the appearance of teeth tend to suffer from relatively large errors. This is probably influenced by the results of intensive selection and being kept in living conditions that differ substantially from their natural environment. This study aimed to assess the suitability of selected features of the incisors to determine the age of Icelandic horses. One hundred and twenty-six Icelandic horses (78 mares and 48 stallions) of different ages (range: 0–24 years; groups: 0–2 years, >2–5 years, >5–11 years, and >11 years) were examined by an experienced horse person who was blinded to the actual age of the horse and did not know which age group horses were in. Age was determined by the inspection of each horse’s teeth and was compared to the actual age of the horse recorded in the breeding documentation, and the percentage of mistakes made regarding the age group was calculated. The estimated age did not match the real age in 36.5% of the horses. The age was more often underestimated (19.0%) by, on average, 0.9 ± 1.0 years than overestimated (17.5%) by, on average, 1.3 ± 1.4 years. Within age groups, the least number of errors in determining age were made in young horses aged 0–2 years, when the eruption and growing of the deciduous incisors and the disappearance of the cups was considered. The average percentage of errors in this group (2.1%) was significantly lower (p < 0.01) than for older horses, whose age was estimated based on the exchange of deciduous to permanent teeth (55.8%), disappearance of the cups (68.0%), and shape changes on the grinding surface (40.0%). Significantly more frequent underestimation of age based on replacing deciduous for permanent incisors and significantly more frequent overestimation of age on the basis of the disappearance of the cup may indicate that Icelandic horses up to 5 years of age are characterized by a slower rate of growth than horses of other breeds, especially warmblood horses. These results suggest that patterns used to determine the real age of horses based on changes occurring on the incisors should be modified in order to consider the specificity of the course of growth and maturation processes of horses of various types and breeds.