I still believe in the old rule of see them in heat in the morning, breed in the afternoon, see in the afternoon, breed in the morning... and with the sexed semen, I have heard 2 thoughts.. but sooner rather than later is what I have heard since the female carrying sperm live longer and will meet the egg, further up the reproductive tract when the male carrying sperm will die off before they get there...
But it also has a lot to do with the ph of the repro tract too... and the general thoughts are the more alkaline the female tract, the better the capacitation (maturing) of the sperm, the better motility and so they will swim farther, live longer...so the female carrying sperm should "outlast" the male carrying sperm to reach the egg. The later the breeding, the closer the egg is so the sperm do not have to live as long or travel as far....so the male carrying sperm win the race.
But then again, some years it seems we have 2/3 bull calves to 1/3 females and other years more females... so diet may play a part in the body's reproductive tract ph also...
I have always bred early for the hope of a female calf... JUST MY WAY....No scientific studies in that...