Weeds

Can anyone identify this plant? I have a TON of them growing in my yard. When they first come up, they almost look like rhubarb. Then they start growing at an astronomical rate - they end up about 8-10 feet tall. The stem/stalk is hollow when you cut it, very fibrous and leaks a milky fluid.

I have not found any chemicals that kill it - I have sprayed it with at least 3 different things and the leaves I spray (on the small, young plants) curl up & die but the plant keeps growing. If you cut it down, about 15 more grow up around it. I have tried digging up the smaller plants, but they have a very large, fibrous rhizome/tuberous root that is about 20 x the size of the plant (a small, 4" plant could have one that is larger than an ear of corn), so digging them up is not feasible.

I have been searching the internet, using weed-identification programs, etc. and haven't found anything that quite matches it. I think it may be pokeweed, pigweed, knotweed or some type of sorrel, but these are just speculation. I really just want to get rid of them because they are everywhere! I call it the malignant rhubarb.