Question
I want to kill the stumps of willow and birch in grased land what chemical would you suggest

Answer
Use the herbicide Roundup. There are brands with the chemical Glyphosate (active ingredient in Roundup)in them and these are just as effect at the SAME strength as Roundup (read the label and compare strengths).It can be painted on freshly cut stumps or sprayed on the foliage. the plant takes the herbicide up and down into the roots system and kills the whole tree. Beware Roundup will kill any green plant so make sure you spray only what you want dead.