I have sprayed and pulled the pennyroyal but have to admit defeat. In some paddocks they have literally taken over and they are in huge bunches.
If I were to rotary slash them now, will I be helping spread the seeds ?
Should I leave them alone and continue elimination programme come next spring ?
Mix up some Answer, or what the site sponsors equivalent Methilidone (sp) based product is, and use that, slashing just seems to make them spread sideways more. you'll need to brooadcast clover seed afterwards tho, since answer and Dows stuff are hormonal sprays that kill bycotlydons, leaving monocotlydon grasses behind, and clover is a bicotlydon.
Use a brush spray not a weed spray, pennyroyal in my experience seems to thrive on most sprays 
I was told the best way to get rid of pennyroyal was to test the soil, lime (it prefers acidic soil), fertilise and then resow.
Pennyroyal is very persistant. I have tried slashing my "crop" of pennyroyal (and self-heal, another member of the mint family that looks remarkably like pennyroyal), and while this obviously knocks it back, it does re-emerge no matter when it's slashed.
We've got a paddock that we're intending spraying totally off very soon, but we are wanting to kill everything, grass and pennyroyal. Would it be best to spray the paddock twice, once with Answer and then again with RoundUp (or equivalent), or can you mix the sprays together, or is there a better spray that does everything?