These protocols resulted in an acceptable AI pregnancy rate. When a
controlled internal device that releases progesterone (CIDR) was added to the protocol between GnRH and PGF
[5,6], pregnancy rates to TAI improved. Recently, it has been reported that reducing the interval from GnRH administration and CIDR insertion to PGF administration from 7 to 5 days and extending the interval from PGF to TAI from 60 to 72 hours increased the TAI pregnancy rates in suckled beef cows [8]. Although the 5-day CO-Synch þ CIDR protocol
is effective, it increases the drug and labor cost and may limit adoption due to the necessity of administering
two doses of PGF [9,10] to induce luteolysis of 5 days CL. However, limiting handling number and/or frequency
should not compromise pregnancy success and added pregnancy success may justify the extra investment in
some situations. The comparison of the 5-day CO-Synch þ CIDR to two doses of PGF at CIDR removal with four handlings
versus the 7-day CO-Synch þ CIDR with three handlings in a large-scale beef operation would allow
determination of the program that will yield a better AI pregnancy rate. Based on the necessity of this comparison,
a hypothesis was developed that the 5-day CO-Synch þ CIDR protocol would improve fertility of beef cows
following TAI compared with the 7-day CO-Synch þ CIDR protocol.