When a horse is grazing something interesting frequently happens. It appear to be grazing in one place, motionless. But a few minutes later it is somewhere else, still grazing. Groups of horses exhibit the same behavior, seldom where I last saw them. So I studied this a bit and here is what I found.
The horse is grazing, head down, but it takes a step or two every 3 to 8 seconds. Thus in a sense the horse is in constant motion, even though it is also constantly grazing. If it takes, say, a step every 6 seconds on average it will cover a lot of distance. Ten steps a minute and 600 steps an hour. Sometimes my little herd covers a fifteen acre field in an afternoon. They seem motionless because it takes less than a second to take the step, so they are only moving a small fraction of the time. But moving they are, sometimes for hours at a time.
One result of this pattern of constant motion while grazing is that the grass is taken down more or less uniformly in the field. This minimizes killing the grass due to over grazing, because the horse does not stand there eating all the grass it can reach before moving on. Rather it eats a little then moves on. This benefit might even be the source of the instinct that produces the behavior. After all the horse is expending considerable effort to move this way while eating, so there should be a good reason for it.
There are several other patterns of behavior which modify this uniformity of grazing, but that is a separate topic.