Folks, I don't care if you use a hundred units of semen in your cow or not, there is ONE thing that makes those eggs fertile besides semen quality -- and that is very simple -- TIMING.
I breed the donors I am entrusted with ONCE, period -- using two units of semen -- one per side. If you miss the timing, you are screwed. In fact, a large percentage of cows that fail to fertilize come down to one thing -- you went out there thinking that just because she is a donor and is superovulated, you have to start breeding her as soon as she show heat -- that is pure B --- S
Ask any one who has had cows here for the last many many years -- and they will tell you that fertilization is never the issue here. Certain cows will grow thier eggs better than others do, so I usually only have to be concerned for quality issues rather than if they were fertile or not. I quit breeding these donors more than once 20 years ago -- and ever since I quit the early breeding, things have been extremely good.
My point is -- use GOOD semen, breed her on TIME, leave the extra drugs in the bottle -- and she will do just fine! Sorry for the rant here, but I see this so much any more it is agrivating. There is no need to breed more than once, but you must do it at the right time -- GKE mentioned that he had a variance in age, that is only because you had put semen in there early -- and it only got the very first eggs ovulated. Most likely, if you had waited until she had quit riding for several hours, all the eggs would have been fertilized at the same time. They do live several hours after ovulation, and the semen is good for about 20 -- go figure!
Nuf said, I won't bother you any more about it -- but this is what I do here -- and it works VERY well, not changing for nothing. One unit will get them if it is good, but needs to be split between the horns.
Good luck to all -- and I mean that!
Terry