Grass carp will eat primrose if they are stocked at high densities, like 10 per acre. I've seen large older grass carp consume primrose, but as you've read here on the forum, they don't eat as much (as a percentage of their body weight) when they grow old and large. You can try glyphosate, but soon realize that it is a pain in the butt to deal with spraying weeds. Primrose will return relatively quickly, wind can work against you, and spray equipment may not be sufficient to cover significant portions of infestation at a time.
I don't see the need, necessarily, to remove old grass carp unless you have soft water. In Lexington area you should have relatively hard water and thus may not have to worry about carp causing muddy water. Grass carp will live 10-15 years.
Another option is to use Sonar Q at 90 ppb concentration, which is a pelleted form of sonar that is quite environmentally friendly and will work for up to an entire season. It is easy to apply, but can be expensive. Contact me if you need more specific recommendations for Sonar Q.
