@SheepGirl, in beef herds the normal replacement rate is a bit lower, more like ~20%. As JHM said, the replacement rate is higher because dairy cows are short-lived or rather, the dairy has a more higher cull rate for various issues from temperament to mastitis. Reproduction and lameness issues also rank high in the list of reasons to cull. Thin cows are not as prolific and fertile as the more "fatter" beef cows, and fertility doesn't seem to be that high on the list of priorities for a dairy herd, even if fertility has low heritability and is mostly affected by plane of nutrition. Priority selection is for more milk, the rest come second. Some dairies are more proactive in selection than others, of course, so I don't want to paint with such a broad brush over all dairies.
@koop it's a different story when talking calving rates and comparing that to conception rates: it's an apples to oranges comparison, really. When concerned with breeding dairy cows (it also applies to beef, but to a lesser extent), the reproductive rates are more concerned with conception rates, pregnancy rates, and heat detection rates. Calving rates come after the fact, and more closely coincide with pregnancy rates. The risk of AI in what JHM was talking about is the conception rate, or whether a cow will settle and become pregnant after being AI'd. This is the measure of a cow's fertility at service. A good dairy herd should have a CR of 50 to 55%, or at least half the herd that has been AI'd should have settled into pregnancy, but it's not uncommon for dairy herds to have rates less than that. JHM, I too have heard and seen records of dairy operations with a CR of around 30 to 40%.
Pregnancy rate is the percentage that is achieved with the number of cows that have been successfully detected to be in heat times the conception rate. In dairy herds it is almost impossible to achieve a PR of 35% or higher. PR is also an indicator of how long it takes, on average, a state/provincial or even national herd to become pregnant. I think here in Alberta the current average PR rate for dairy cattle is only around 18%.
Then there's the issue with calving rate. Cows can become pregnant, but there's the possibility that some cows will reabsorb the embryos or even abort. But, in dairy operations calving rate usually isn't calculated, but I'll have to check my past notes on that and get back to you.
http://cal.vet.upenn.edu/projects/fieldservice/Dairy/REPRO/REPPROG/EVALREPR.htm
http://www.aps.uoguelph.ca/~gking/Ag_2350/measures.htm
