Training: Tom Redmayne of Paragon Advanced Breeding What is a cow-stopper embryo? is the question answered by Paragon Vet, Tom Redmayne. It is not unusual for some cows to repeatedly fail to hold in calf and continue to return to oestrus every 21 days. Using a cow-stopper (repeat breeder) embryo could be an effective way to solve this problem. Cow-stopper embryos are transferred in to cows in addition to serving or artificially inseminating them. The addition of the cow-stopper embryo can help the establishment and recognition of pregnancy. An embryo (originally frozen at 7 days old) is thawed and implanted 7 days post oestrus/service. Matching the stage of cow-stopper embryo to the stage the cow is in her reproductive cycle is critical to improving conception rates.