In the CO-Synch protocol, PGF is administered 7 days after GnRH followed by a second GnRH injection and TAI 66
to 72 hours later. The Ovsynch protocol is similar, except that the second GnRH injection occurs at 48 hours after PGF
and the TAI occurs 12 to 24 hours later. 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.