I'm not entirely sure what is meant by sour. It seems to mean more than weedy (though weedy it certainly was). I think it means that the donkeys have fouled it, and being fatstidious animals (unlike cows) they then refuse to eat it.
The field has now been topped - mown with the mower at a high setting, with the cuttings left to melt back in. This will encourage tasty fresh growth, after which my compost heap will become less appealing.