How to get rid of cows?

I’ve had a nagging feel for a while that my grass was disappearing but thought it was just in my head. The other day I noticed a large poop pile but couldn’t identify which animal it belonged to until today when my husband saw a heard of 30 cows invading our field. The field is about 3 acres in size with a dense forest surrounding the east and northern sides, where the cows enter from, a rock wall on the western side and my home on the southern side. How do I keep them off my land? Money is tight at the moment but any and all suggestions are welcome.