Do they have to be removed or just killed? Killing is easy.Yes, check with your NRCS/FSA office. In our county (MN) FSA is requiring land owners to remove trees from CRP. A neighbor lady had to relinquish her contract because the cost of tree removal was too high yet she was told she had to eliminate them in order to continue recieving payments. We have a real problem with cottonwood trees invading. Thankfully got rid of ours this summer with bush hog.
I don't know the details, but in this case the trees are approaching 20 feet tall and 4-5 inches in diameter, so I think they wanted them felled at a minimum, not just sprayed. I noticed another neighbor went in with a chain saw and felled his, they are still laying where they fell, probably they will burn them this winter. In both cases tbe cottonwoods were invading riparian buffers, they grow fast in those moist soils.