In general, herbicide shouldn't be used to prune or trim a trail, although I have used glyphosate a bit to spray poison ivy that is ankle-high on the side of a trail. Poison ivy is not an invasive, so we don't make a special effort to eliminate it, but some people are very sensitive to it, so I think it is a good idea not to have it right next to the trail.
Doug is okay with the use of herbicide at BCSP, so it we wanted to use it to help clear a new trail, it could be considered. Most herbicides have an "exclusion period"; a time where you should prevent access to the area by people who might not have proper clothing and might rub some bare skin up against foliage that is wet with herbicide. I think it is 12 hours for Poast, which is what we sometimes use for stilt grass.
For the first 2 years when we were spraying stilt grass along the trails, I would post a sign indicating what was going on and to warn people to "stay on the trail - herbicide in use". Now most of our spraying is being done only in the area of the trails, mostly 5-100 ft. away from the tread.
Pruning of ground level grass and nettles can be done with a weed wacker and overhead or waist high branches can be trimmed with a pruner. If bikers would occasionally carry a small pruning shear and stop now and then to cut back a few branches that you brush against, most of the problems could be kept at bay. Brushing against branches or grass is a good way to pick up ticks in the spring.
The worst problem I have on my own property is with blackberry bushes, which grow rapidly and like to reach out into the more open area of a trail. I have to prune several times a year, but I am getting a lot of blackberries this year. A similar problem occurs with multi-flora rose, and those can best be treated with herbicide after pruning them back away from the trail edge a bit.
This year, the invasives treatment program at BCSP is focusing mostly on shrubs such as multi-flora rose, Japanese Barberry, and autumn olive and most of the activity has been in the Pine Loop area, part of the North Tower connector, and the saddlebarn horse trails. You will begin to see some brown or bare bushes from treatment that was done in June. So far we have sprayed about 360 gallons of glyphosate, mostly on multi-flora rose. We will have another 6 weeks to work on stilt grass before it goes to seed and the wet weather we have had is producing some very healthy stilt grass.