I use Roundup to control grass and weeds like I'm sure many of you do. I normally mix up a batch in a 10 gal sprayer tank. This most recent batch I made up is about a month old. When I used it the other day, it seemed to have no effect on the vegetation I used it on. So my question is does Roundup degrade once it has been mixed and sits in a tank after a few weeks? I always make sure it is well stirred up after setting. The premixed stuff you buy in hand held spray bottles seems to last a whole season. So what's the deal?