Behavioral changes in thoroughbred horses following pre-training: insights from novel object tests and activity monitoring under field conditions.

Racehorse training starts with pre-training, which is conducted at a very early age and remains controversial from an animal welfare perspective. This study investigated how a 12-week pre-training program influenced the behavior of young Thoroughbred horses. Horses were assessed before (A) and after (B) pre-training, which progressed from habituation and handling to ridden exercise and canter work on a racetrack. Behavioral responses were evaluated using standardized novel object tests and continuous 24-h stall monitoring via inertial measurement unit sensors. Novel object test behavior was video-recorded and analyzed using a validated ethogram comprising 34 behavioral parameters. Following pre-training, horses showed significantly increased locomotor activity during novel object tests, including longer durations of trot (A: 16.1 ± 6.7 s; B: 41.6 ± 6.7 s; p < 0.01), gallop (A: 1.9 ± 1.6 s; B: 11.4 ± 1.6 s; p < 0.001), and bucking (A: 1.0 ± 2.8 s; B: 22.2 ± 2.8 s; p < 0.001). Tactile exploration also increased, reflected in more frequent muzzle contact with the object (A: 2.2 ± 0.8; B: 6.0 ± 2.0; p < 0.05), despite a reduction in the total number of approaches (A: 8.1 ± 0.9; B: 6.3 ± 0.7; p < 0.05), suggesting altered patterns of interaction with the novel object following pre-training. Stall behavior correlated with responses in the novel object test, supporting the consistency of individual behavioral tendencies across contexts. Principal component analysis revealed four main profiles: highly active/offensive behavior, exploratory engagement, vigilance/defensive behavior, and calm observation. Notably, inter-individual variability between individuals increased after pre-training. The shift toward increased locomotion and tactile exploration suggests adaptive behavioral modulation in response to early training. While some horses transitioned toward more active and confident profiles, others remained stable, indicating that certain traits may be inherent whereas others are experience-dependent. These findings highlight the importance of multidimensional behavioral assessment and demonstrate that integrating experimental and continuous monitoring approaches can improve the interpretation of novel object test responses and support evidence-based equine welfare assessment.