a phenomenon that has apparently gone unnoticed by herdsmen
How many centuries of herdsmen's writings do they have to base that on?
It's actually well-known as a general tendency. But the slope plays a part too. On a steep south-facing slope, grazing animals won't position themselves with their heads downslope, because it's kind of inconvenient for browsing purposes. Er, and might end in serious accident, as in rolling arse over tit down the slope.
Well, sitting on a horse who's grazing on the flat. I don't have the teeth for grass.
