An ancient group of plants, and interesting. Was used once as an aid to scrubbing pots and pans because it fixes silica in the stems and 'leaves' making it slightly abrasive. Very deep rooted, so the only quick way is to kill it. The plant tends to 'shed' any spray, so break or bruise the foliage first (I used to strim it), to allow any weedkiller to 'stick', and use a glyphosate based one. It doesn't need to be a nasty weedkiller which would mean you can't plant soon after, however, it may well need a second application. The good thing - this time of year is ideal, on a hot and sunny day. The plants will be thirsting for moisture and if you give it to them laced with killer they'll take it up much better with deadly effect.

From my very long experience glyphosate is not very effective against marestail . I've tried the bruising thing and it helps a bit but i sprayed a large patch weekly for 6 months and it kept coming back

There is an effective weed killer which i understand farmers use

Lot's more about it here

http://www.flickr.com/photos/31559373@N00/4757456972/in/photolist-8fpddd

A, you're probably right, which is why I added the bit about a second application. I may have been lucky when I did it. Trouble with the agricultural strength stuff is its non-availability, (unless you know someone!). My worst problem was through tarmac so I could spray copiously.

I've just had my Gardeners'World Magazine delivered today that I Subscribe to & in it, in the Forum there was someone asking......
"Can I defeat Mare's Tale"
The answer was.... Mare's Tail equals heavy duty weed matting. You'll need to exclude light for two or three years.
Well, That was one of the answers.
There & 10 answers to this on gardenersworld.com/forum
when I opened it I put in....Can I defeat Mare's Tale

Best of Luck :o))

Thanks to all for help re mares tail as alot of it is in the veggie patch i think will just have to keep pulling it up