I have struggled with smooth brome as well as a few other undesirable grasses in my small prairie plantings. I have used a few techniques with success at killing the grass and not killing desirables. One technique was to wear rubber gloves with cotton jersey gloves over them and soak glove in roundup and grab grass to cover it with the herbicide. Another more time consuming technique was using a piece of cardboard to surround and isolate the "bad" grass and spray it. Once the spray dried, i removed the cardboard and moved on to the next victim.

I realize this wouldn't be very efficient in a larger planting but it worked for me. BTW, I would much rather deal with undesirable weeds than grasses.

I've had middling success with grass-specific herbicides, those with fluazifop as the activie ingredient. They seem to be quite effective when applied to my annual beds early in the growing season (March-April here in OK), but much less so when sprayed on warm-season weedy grasses in mid-summer. And tall fescue seems completely immune to these herbicides.