I've never seen Oxford ragwort growing in pasture. It simply can't compete in dense grassy swards. I'm not arguing against its toxicity, but I simply can't imagine it getting into hay or silage except via extreme wanton carelessness. Common ragwort, on the other hand, needs be be removed before haymaking or silage-making. Senecio is a very large genus (it includes common groundsel) and the level of toxicity among the species is widely varied.