Weight carrying capacity of horses

Weight carrying capacity of horses, is a frequently discussed topic in the horse industry. Many people have their own opinions on the subject, but research on the weight carrying capacity of horses is limited, underscoring the need to establish this knowledge systematically. The Icelandic horse is a popular breed worldwide, as a riding and competition horse and is commonly used by tourist companies and in riding schools. Knowledge about how conformation is associated with weight carrying capacity is highly needed, both from a performance and a welfare perspective.

In a collaboration between the Swedish University of Agricultural Sciences (SLU) and Hólar University, researchers studied the physiological response (e.g. plasma lactate) to a ridden weight carrying exercise test in 16 Icelandic tourism horses/long-distance riding horses, during the falls 2021 and 2022. The exercise test was performed outside on an oval track in tölt (~5.7m/s which equals ~ 20 km/h speed), including 4 steps (each ~640 m), with increasing weight for each step. The weight carried was 20 %, 25 %, 30 % and 35 % of the horses’ body weight. After each step the horses stopped and the rider dismounted, a blood sample was taken for measuring plasma lactate and hematocrit, as well as measuring heart rate, breathing frequency, rectal temperature. The same body measurements were collected on the horses in the study as those measured at breed evaluation field tests for Icelandic horses. Also, the effect of back conformation (narrow vs. broad back) was investigated, using a flexible ruler.

The weight carrying capacity varied considerably among the horses. Four of them did not reach the lactate threshold (where a plasma lactate concentration of 4 mmol/L and the concentration starts to accumulate) during the test, despite carrying a rider equal to 35% of their own body weight. The horses that reached the lactate threshold (4 mmol/L) carried, on average, a rider weighing 31.4% of their own body weight, a higher ratio than observed in similar ridden exercise test with trained school-horses used for riding instruction at Hólar University. On average, those horses carried 23% of their body weight when their plasma lactate concentration reached 4 mmol/L. However, it should be borne in mind that the school-horses at Hólar did not have the same background in endurance training as the long-distance riding horses (See earlier research on school-horses: https://www.cambridge.org/core/journals/animal/article/effect-of-rider-weight-and-additional-weight-inicelandic-horses-in-tolt-part-i-physiological-responses/0234FDFE916E8FB9849119BC33E7FB78).

Long-distance riding horses with wider chests, “uphill” conformation and a straight backline had greater weight carrying capacity. Chest width and „uphill“ conformation have previously been postitively associated with higher scores for riding ability in a breed evaluation field tests, but this is the first time they have been linked to a horse‘s physiological response, indicating that these conformational traits positively influence the horse‘s weight carrying capacity.The researchers hypothesised that a wider back was associated with greater weight carrying capacity, however, this study did not support that, despite earlier research by the same group suggesting such a relationship.

These new insights into the relationship between physiological response (e.g., plasma lactate concentration) during ridden exercise test and specific conformational traits of Icelandic riding horses are of large importance for future research and a small step forward of answering the question what is affecting the weight carrying capacity in riding horses.

Here is a link to the new scientific article on weight carrying capacity: https://actavetscand.biomedcentral.com/articles/10.1186/s13028-025-00818-5

The researchers thank again the owners of the horses at Syðra-Skörðugil and Bergland in Skagafjörður for their important contribution of lending their horses in the study and for good collaboration.