try a solution of glyphosate and verdone or banlene. two parts glyphosate and one part verdone and equal parts water. mix with a tablespoon of sulphate of ammonia and a teaspoon of wash up liquid. I found this cure very satisfactory. Mares tail is a very difficult weed to eradicate and it helps to rub the plant vigourously to break the natural wax coating before applying solution . it can take up to five years to fully eradicate this weed with this proceedure but stick with it and you will suceed , good luck !