I have seen a few cows re breed at 18 days after calving. Two years ago, I had a cow that had a backwards calf that ended up being a difficult birth. It took me a couple hours to get it out and I was really wishing I had taken her to the vet for a c- section. Miracle #1 was that the calf was still alive when I got it out. I was afraid I may have damaged the cow as she tore her uterus a little in my attempts to get the calf up into the birth canal. Miracle #2 occured when she was in standing heat 18 days following this difficult birth. I bred her to one of our herd sires and she stuck to this service and had a beautiful heifer calf last year( we are thinking of showing her this fall). In 2011, she again calved and gained 20 days over her calf last year, so she obviously bred back very quickly after calving. I am not sure if there is any research on this topic, but I have seen some cow lines that seem to be very fertile and rebreed faster than some others.