In "Do Horses Expect Humans to Solve Their Problems" (Lesimple et al. 2012), the researchers "hypothesized that because domestic animals are so attentive and dependant [sic] to humans’ actions for
resources, the counter effect may be a decline of self sufﬁciency, such as individual task solving."
The experiments challenged horses to open a box containing their food ration and "more than half of the horses that showed exploration behaviors toward the experimenter failed to open the chest (N D 18, 60%), while only a third of those that did not (N D 5, 31.2%) failed (Fisher’s exact test, p D 0.04)."
Were I continuing their experiment, I'd test upper level horses, including a set described by their owners as intelligent and inquisitive. I suspect the choice of riding school horses used for students "from beginners to moderate level" played a part in so many horses failing to open the box. Typically, riding schools chose docile, patient horses for beginner/intermediate rather than horses with great curiosity or keen intelligence.
I could certainly be wrong about horses in general, but, whether filled with food nor not, few boxes go unopened around here. My horses like me, but they love a good puzzle, and they adore toys.