But Mesosulfuron-methyl is sold as Atlantis here.  All of the Sulphonyl Ureas are notorious for rapid development  of resistance  in  our annual ryegrass.  The better it works, the quicker it happens. Some advisers suggest as little as   4 hits, and a significant resistant population exists.

I can't say i have actually used Atlantis, but ive used other closely related ones,and I DO have several populations that are all but unaffected now by that family of  products.  

Interestingly, since you have such excellent results, that the Atlantis label only lists suppression for  ryegrass. 

Thanks for the suggestion.  I learned some  from following it up.