Can I keep left over diluted Roundup Concentrate product in my tank sprayer until next time I use it?

Do not mix more than can be used within 5 days after which a gradual loss of activity will occur. Only mix what you require and if necessary under estimate the amount needed. If you do have any left over, it can be disposed of by emptying the contents onto some bare soil that is not near a drain, water point or pond. You should rinse the sprayer thoroughly and dispose of the rinsings in the same way.