I buy spring mix for my bunnies, usually an organic brand like Earthbound Farms, which is often on sale at the store. It’s important to check the date on the package. I only get ones that are at least 5 days away from their date. But even packages that are a week or more away from their use by date usually have some bad pieces in them right from the start, which causes problems. A couple of days after opening the bags or boxes, I sort through all of it and pull out any slimy or wilted pieces. I put all of the good pieces on a paper towel, to be returned to the package later. A lot of pieces will be fine themselves, but they have gotten brown liquid on them from the pieces near them that have gone bad. That liquid can be rinsed off so that you can save those good pieces. When all the pieces have been removed, with the bad ones discarded, and the good ones on a paper towel, I wipe out the box, then put the good pieces back in.