You could have a sterile marble inserted in the uterus. That usually stops them from cycling for about 3 months. I believe it is the larger one - about quarter sized? You have to do it just as they go out of heat.
raspberry leaves- it just takes the squealing, squirting, flirting down a notch.
I am kind of shocked at some responses. I have only had one mare boarded here that would squeal/squirt etc at a fence post. ALL the rest have needed to be actually presented to the stallion, or an occasional strong gelding, nose to nose, for at least a few seconds to tell if they are in heat or not. Not something that ever happens unless we directly are teasing them. None have ever shown any signs just being in the same arena being worked, and that even goes for mares that have been live covered in the past.
Out of 3 mares, two got nothing. The other often got nothing the first couple of years but sometimes needed depo. We switched to Regumate March thru October. Because it was more consistent, took any guessing out of it and did the job.
I tried several of the others, including rasberry leaves and they did nothing for that particular mare.
And that particular mare had an awful time with transitional estrus in March and April and 10 times worse in late Sept and October. No squirting or squatting, just moody as hell and not fun to be around. Mid winter she was a loving lap dog. More like an enraged pit bull in those months.
Believe it or not, being around stallions at the shows sometimes? Nothing. But she'd get herself into a snit in a heartbeat.
When opportunity knocks it's wearing overalls and looks like work.