With the price of wire, T-posts, corner posts, etc. you may be looking at quite a bit of money, relatively speaking, to rebuild or substantially repair that 20 acre trap, not to mention the time required. If you're lucky the 20 acres is a perfect square, the most desirable design from the standpoint of fencing cost. And of course with the bulls running all year right across the fence line there's no real guarantee that they won't jump the fence to do their normal duty with your heifer anyway --- it has certainly been known to happen! Seems like a lot of work and expense for one heifer (or is it more than just the one?), if that's what you're really talking about at this time. If I were you I think I might be tempted to save myself a day or more of hard work, and take that fencing money plus money from the sale of the heifer and put it towards the purchase of another bred cow.