Below is copy of Vicki the Vets reply to a question back in Septmeber regarding sexing of ponies. I don't know how much it applies to cattle, but it would seem the mechanics would be similar. Maybe she'll chime in and comment on relavancy to cattle.
"Well, I didn't take the time to read all of the answers to the original question, so this may have been answered.
The male determines the sex of the fetus due to the presence of both an X and a Y chromosome. Male spermatozoa swim faster than the female, and die sooner, so if the mare is bred close to the time of ovulation, the highest liklihood would be a male offspring. If bred before ovulation, the highest liklihood would be a female offspring. Some mares will not allow breeding until ovulation, thus more colts, others will allow earlier, thus fillies.
Sexing of spermatozoa is still a bit hit or miss, since you can't guarantee 100% the sex, you can only increase the odds.
I'm really zonked tonight, so PM me if you need further clarification!
V"