I was thinking those tags were called Detect-Hers, but that's a different marking system. So, OK, I checked Hoard's, and they have an ad running for Kamar Heat Mount Detectors.
www.kamarinc.com/We don't run herd bulls where I work, so I can't vouch for their effectiveness, although it certain seems like feasible technology.
Now, as far as keeping your cow bred back in time to avoid winter milking, hmm, if she's due in mid-May, the normal 90-day voluntary waiting period would mean rebreeding in mid-August, with a subsequent due date around May 24. Assuming she settles the first time around, you'd lose 12 days with each lactation. If it takes two heats to settle her, that's roughly 33 days, so yeah, she would be trending toward fall calving all right!
A couple of options: you could try breeding her back earlier. My boss likes to start at around 50 days, although personally I think that's pushing the girls a little hard. But my favorite cow in the herd, Henrietta, had her last two calves 11 months apart, and just settled to her first breeding postpartum. (I think she was about 75 days out.) Of course, with a due date in May, you'd start breeding your girl in late July, the hottest part of the year in most places, and (as you point out) that has a negative effect on the conception rate.
If you're doing A.I., you could try "setting her up" (Ovsynch or similar program of hormone shots to time the A.I.), to increase your chances of success in the first round, but it sounds like you're using a bull. (And actually, your chances are probably better with a bull!)
Dude, you ain't got a lot of options! I'd say your best bet would be to put her in with the bull about 50 days postpartum, and try to optimize conditions as much as possible -- for instance, making sure they have a cool barn with fans going to get out of the heat.
Another option, which you're probably not gonna like (lol) would be to sit out the dance, so to speak. If you want a March calf, you'd want to breed her between Dec. 8 and Jan. 7. So if she calves in mid-May, you'd let her go seven or eight months before trying to re-breed. Her milk is probably going to fall off pretty quickly after she settles, and she might end up having an extended dry period. (Be careful not to let her get too fat, either before or after breeding.) But the advantage here is that you'd be breeding during cooler weather, with a more optimal conception rate.
That's about all I've got.
