You are right, cut/dried ragwort results in both more concentrated toxins and also loses its bitterness which would otherwise discourage animals from eating it. So you did exactly the right thing by removing the toppings, if people arent going to remove the toppings they are better leaving the ragwort or (better) pulling up the whole plant. By topping the field you will encourage shorter sweeter grass which the sheep might prefer too! If you drop a coin in the grass and cant see it, it might be too long for sheep.
Ragwort is toxic to sheep but proportionately to body weight they have to eat a lot more of it than cattle and horses for it to have serious consequences. So for most sheep, the end comes by other means long before ragwort kills them. Sheep (like cattle and horses) will normally avoid eating the plant too, altho they will eat the young, less bitter and less toxic rosettes in spring if stocked at fairly high density, which can be useful in ragwort control. It is unusual for them to choose to eat the flowering stalks tho, is it dry where you are, has it started to wilt a bit, that might make it more palatable?
Sprays have a role too but it depends on the situation; we inherited 30 years worth of rampant ragwort on 40 acres, but cant spray because our water and that of our neighbours comes directly from the fields we would be spraying. As a result we use a mix of topping, pulling and sheep to try to make headway, we will also be ploughing and reseeding some fields in the future.