cow health: abscess
You'll see this sometimes on a cow: a lump the size of a golf ball. It may come up on her jaw, sometimes her neck or ribs -- other places, too. It can come up overnight. The first time you see that happen, maybe you worry.
Maybe you worry the second time, too.
Eventually, though, you've seen it enough times that you know a few things. Like, it's not life-threatening. It's not making your cow miserable (she may not even feel it). It does not need immediate care -- in fact, it probably doesn't need any care. It will probably go away on its own.
Book-wisdom says an abscess happens when your cow gets a puncture wound -- like from a thorn, or a sharp bit of hay -- and bacteria move in. An encapsulated infection is the result -- an infected cyst. Either your cow's immune system kicks in and the abscess goes away, or her immune system kicks in and the thing ruptures.
You can cut an abscess open and drain it and if you like taking over from nature, go ahead, but (according to our vet and to our own experience) if the thing isn't just about to rupture on its own, it's not the kind of abscess that benefits from being cut. And if it was going to rupture anyway, there's no profit in making a big X cut just because you can.
This is Old Man Experience speaking, not the Merck manual. Maybe things will be different for you and your cow.