If your leaves are still on, then now is almost the perfect time to do it!

I have not used the Tordon RTU that esshup is describing. Great for your usage if that is already dyed.

I looked at your pictures again, but there is no good scale for my eye. I think they are thick enough, that I would cut them with a chainsaw about 8-10" above the ground? Do a comfortable working height for you and keep your chainsaw out of the dirt so it stays sharp longer.

The good think about clumps, is that you probably DO NOT need to poison every single stem! Even if you miss a few, if you get enough herbicide to the roots, then the entire willow tree clump will die.

(I keep calendar notes of all of my herbicide applications. I have definitely treated things too early or too late in the annual cycle. If you cut and treat some willows this weekend, and then do some more next weekend, keep a note of your treatment areas. If the later treatment has a significantly lower kill rate, then you can determine your optimum time to treat willows in future years. If you are going to be doing a bunch of labor at your property, might as well do it when you get the most bang for your buck!)