Well, alfalfa lasts for several years, but it gets more sparse with age. I think a field is good for about 10 years (don't quote me on that!). But when it starts looking sad, you can't just throw out new seed. You have to plow the whole thing under and start again, otherwise you're just wasting some rather expensive seed. Stuff sets some amazing roots, too. Our neighbor re-did his field across the road from us a couple of years ago and some of the seed found it's way into some of my planters. Thought I was going to end up in China before I found the ends of those roots to get them out!
