That would be your state thinning out chosen species. The recommendation is for trees less than 6 inches in diameter, and not by roots linked up to other trees. We have Buckthorn here which is invasive, wood is hard on chainsaws, and they grow like weeds, so it is sound management using Tordon as it is target specific. It is the quickest, least labor intensive and less noisy method. The State can send college children out to get bit by bugs and it is all part of the life cycle.

Not necessarily invasive species, sometimes just undesirable species. I hunt a 3,500 acre timber company lease in north-central PA. A couple of years before a big sale and cut on a section they come in and remove all the junk species. I've never seen them use the hack and squirt method, they cut all of the bigger junk out, skid it to a landing and feed it into the biggest chipper I've ever seen. My camp is a half-mile from the landing and it shook the place all day long. They blew the chips into semi's. At one point there were more than twenty of themed lined up on my little dirt two-track waiting to be loaded with wood chips.

After the sale and about a year before the good trees are cut they spray the entire area with round up or similar to kill all the underbrush.

pig castrator

Look up safety brush axe. That's what we used to use.
Silvex (2,4,5-T) was the herbicide before it was banned. It worked better than Tordon. With Tordon, you pretty much need to completely girdle the tree for it to work 100%.
With Silvex, you only needed 3 whacks evenly spaced around the trunk.