Sorry, gotta tell you that your esteemed father was other than correct.
There are things you can do to limit the behavioral issues involved in heat, and in medicating mares to keep them out of the worst of heat, but copper ain’t a part of it.
I’m in the minority on this one, but my belief has always been that if you explain to your mare that, while she is entitled to her own opinion, she is never entitled to express it, you will never have trouble with a marish mare. Some say it’s wrong, but then my own mare is perfect. Never shows any attitudinal changes that anyone else can see when she’s in heat. I can tell, but then she’s mine and I know her best. Also, I don’t worry about the little ways she shows me that she’s in heat, I do baby her over them, and so no one ever sees us fight about it.
She’s therefore known as a mare with “silent” heats.
One opinion about heat, and a firm answer: ain’t got nothin’ to do with copper…