If you are considering AI on your replacement heifers, then some sort of synchrony programme may be advisable. There are a number of different options, depending on what resources are available, for heat detection, drafting, AI etc.
Single PG involves mating to detected heat for 5 days, and then injecting all remaining heifers with Prostaglandin (PG). Most should respond within 5 days of injecting, and should be served to detected heat. (i.e. 10-11 days heat detection and spot mating before putting the bulls out).
Double PG involves 2 injections 11 days apart, with the 2nd injection 2 days before Planned Start of Mating (PSM). This offers fairly tight synchrony, and heifers should be mated to detected heat for 3 to 4 days before putting the bulls out.
CoSynch offers tight synchrony to allow fixed time insemination. Heifers are treated on 3 occasions:
Day -9: Insert CIDR & Inject GnRH
Day -2: Remove CIDR & Inject PG
Day 0: Inject GnRH & Inseminate
This is subtly different from the CIDRSynch programme for non-cycling cows, where cows are inseminated approx 16-20 hours after the final GnRH injection. The reason for this is that heifers ovulate faster than cows in response to PG, and the good news is that it involves one fewer yarding for these animals.
These programmes should only be considered on heifers that are well grown and showing good cycling activity. In the run-up to mating, attention should be paid to trace element status, parasite control and BVD status. 40% of predicted adult body weight should be the target for PSM
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