Can a Horse Learn While Under the Influence of a Tranquilizer (acepromazine maleate)?

Abstract

Although tranquilizers commonly are used as “training aids,” little is known about how this practice affects efficiency of learning in the horse. Forty mature horses were used to compare learning performance of tranquilized and non-tranquilized horses on spatial and discrimination learning tests. The spatial test was a one-choice “T” maze in which only one side was rewarded with feed, and the discrimination test was a choice between a white bucket and a black bucket in which only one color was rewarded with feed. On the test day horses were deprived of their usual morning concentrate ration until testing was completed as motivation to perform the tests. Horses were randomly assigned to either a control group, receiving 3ml of 0.9% saline, IM, or a tranquilized group, receiving 0.044 mg/kg acepromazine maleate IM. Horses were fitted with heart rate monitors during testing to establish that the horse had calmed down from the injection, and their heart rate had returned to resting value. Each horse then performed both learning tests on the same day. The horses’ selections during the test were recorded and analyzed using a t-test. Mean heart rates for control and tranquilized groups (49.8+1.4 and 53.8+1.5 respectively), in the discrimination test, were not significantly different (P=0.06). Likewise, during the spatial test no difference was detected (P=0.36) between heart rates for control and tranquilized horses (55.1+4.0 and 51.3+1.5, respectively). Significant heart rate differences between the control and tranquilized horses, were not expected and the heart rate was used to verify that all horses began testing in a calm state. Mean percent correct responses for tranquilized and control horses in the spatial test (80.0+3.4 and 72.5+3.6, respectively) did not differ (P>0.13). Similarly, the discrimination test detected no difference (P>0.43) in mean percent correct responses for tranquilized and control horses (69.8+2.0 and 67.6+2.0, respectively). Results indicate tranquilized horses had similar learning performance on simple spatial and discrimination tests as those that were not tranquilized. Tranquilization makes the horse more tractable without significantly affecting learning performance. Thus, it is a tool that may be utilized effectively by less skilled horse handlers on fractious horses to perform aversive procedures, e.g. trailer loading or clipping, while allowing the horse to learn to tolerate these procedures.