is there anything which kills horses tail

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The advice that I have been given is that you do not need the certificates to use the product on your own land/garden/allotment plot.

It is illegal to sell it and then use it without a licence.

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5.   No person shall sell, supply or otherwise market to the end-user a pesticide approved for agricultural use unless that person

(a)   has obtained a certificate of competence recognised by the Ministers, or

(b)   sells or supplies that pesticide under the direct supervision of a person who holds such a certificate.

6.   (1)   In paragraphs 4 and 5 above, "pesticide approved for agricultural use" means a pesticide (other than a pesticide with methyl bromide or chloropicrin as one of its active ingredients) approved for one or more of the following uses:-

(a)   agriculture and horticulture (including amenity horticulture);

(b)   forestry;

(c)   in or near water other than for amateur, public hygiene or anti-fouling uses;

(d)   industrial herbicides, including weedkillers for use on land not intended for the production of any crop.

from CONSENTS GIVEN UNDER THE CONTROL OF PESTICIDES REGULATIONS 1986 ( found by clicking the links on the Progreen site I linked above)