The palatability of weeds (vegetation) to grass carp not only involves taste but also texture. If your weeds taste better than fish food then the GC will probably eat more of the weeds but if the fish food tastes better than the current weed growth then the GC may be dining a lot on fish food.
From my limited observations, I think the GC will eat primarily weeds until the present weed growth diminishes or is heavily grazed. Then, in exploring new food sources the GC find out about fish pellets, maybe even from observing other fish eat the floating food. Once this happens some or all of the GC may prefer fish food over the current, prevaling weed growth.
One should always allow for some natural variation. I repeat, I think individual fish have some food preferences. I think, not all GC absolutely favor the exact same foods with the same "enthusiaum".
