@Angusjohn sorry this slipped by me. My personal experience tells me a CIDR heat is not as reliable as a natural heat. But, I have had my vet explain it to me.
If you have ever had setting chickens, you will understand that when a hen lays so many eggs to set/hatch, they can only be so many days old. After a certain number of days, the egg gets "stale" and will not develop.
If a cow is just on the verge of cycling, her egg is ready to release, then we tell it to "hold on" until another 9-10 days. Her egg gets old and not very viable for fertilization. Whether this is FACT or not, not sure, but it explains to me why "I" have experienced lower conception with CIDR. Don't get me wrong. I use them. They definitely will get a cow cycling. But, many times, I will let her come into heat, then Lutalyse her and bring her back in 7-10 days later. I rarely am using inexpensive semen - some as much as $100-$200/unit - most at least $50. I try to make sure I'm confident on the heat we are breeding. Sometimes it's a catch 22 case, because I'm under a time limit with a 60 day breeding season.