Loading the improvised bow from the hotbar works only for sharpened sticks.
Improvised Arrows, Composite Arrows and Bone Arrows can be added to the quickbar but pressing them does nothing, they can be "chambered" one at a time by dragging them onto the bow while in the hand slot and clicking the "chamber" option.
Combining arrows gives the option "reload".
Sharpened sticks also point directly vertical once they're stuck in an object, the others seem to stick out at the correct angle.
There is also no way to remove the arrow once it is "chambered".