Here is a link to types of vegetation that grass carp prefer to eat, coontail and and eurasian milfoil are low on on a grass carps prefernce list. Typically if a paticualr type of weed is low on the prefernce list a higher stocking rate of grass carp is recomended.

http://aquanic.org/publicat/usda_rac/efs/srac/3600fs.pdf

As a general rule of thumb, LMB can typically eat something as long as 40% of their own body length. A 22" LMB should be able to eat another fish up to 9" long.