My youngstock usually graze it and leave field fairly even, so don't leave the knapweed but I do graze & cut when leafy .Our main mob are in a knapweed heavy field atm. I left them in the thickest section for slightly longer than normal as it still looked quite shaggy, when I moved them, they'd taken all the grass , clovers, trefoils etc down to the ground and not touched a leaf of the knapweed. Nice to have the odd patch, but I'd say not good fodder. At least the seeds won't be spread to the next field in the dung. Maybe it is just too dry now, after Treg's comment...
I've now cut a field with it in that was much more stalky so probably at the stage it's less palatable for stock.
Bit like chicory it's gazing / cutting it at the right stages.