It does not seem to be so much the actual cover as the scent of the animals; for a man of course can be seen over sheep, and under the legs of cattle.

But the breath and odour of sheep or cows prevent the game from scenting him, and, what is equally effective, the cattle, to which they are accustomed, throw them off their guard.
The cart-horses in the fields do not answer so well: if you try to use one for stalking, unless he knows you he will sheer off and set up a clumsy gallop, being afraid of capture and a return to work.

But cows will feed steadily in front, and a flock of sheep, very slowly driven, move on with a gentle 'tinkle, tinkle.' Wild creatures show no fear of what they are accustomed to, and the use of which they understand.
If a solitary hurdle be set up in a meadow as a hiding-place from behind which to shoot the rabbits of a burrow, not one will come out within gun-shot that evening.

They know-that it is something strange, the use of which they do not understand and therefore avoid.

When I first began to shoot, the difficulty was to judge the distances, and to know how far a rabbit was from a favourite hiding-place.