Not necessary, because even if the other half of the grass contains no fungus, that doesn’t undermine the reasoning. The cows could learn to eat the other side of grass that has no fungus. (If you’re attracted to this answer, you might be making the mistaken assumption that cows must eat grass with fungus or that they prefer to eat grass with fungus.)

Necessary, because if it were not true — if NO cows that have been made ill by the fungus are capable of sensing the fungus in the grass — then we have no reason to think the cows will learn to avoid the grass that has the fungus. (The premise triggers if the cows can sense a substance that has made them ill. If they can’t sense the fungus...then we have no reason to think the premise will trigger.)

Not necessary, because even if the fungus can be found on other kinds of grass, we’re talking about grazing land that’s entirely fescue grass. The argument doesn’t involve a patch of a different kind of grass.

Not necessary, because even if other animals have become ill on the grass, the argument concerns only what cows will do. How other animals are affected doesn’t influence whether cows will be affected by the fungus or will be able to sense the fungus.

Not necessary, because even if the fungus can be destroyed without destroying the grass itself, nothing in the argument is based on the ability or inability to destroy the fungus. The fungus is already present. What matters is whether the cows will sense the fungus when they eat it and are harmed by it.