Technical Abstract: Moving the time of insemination and second GnRH injection of the CO-Synch protocol which includes the first two injections of the Select Synch protocol and timed insemination with a second GnRH injection at 48 hours after PGF to 64 hours after PGF resulted in higher pregnancy rates in some herds and lower pregnancy rates in other herds (unpublished data). We interpret this to mean that in some herds, we have lost the opportunity to induce a fertile ovulation with a second GnRH injection by moving the GnRH injection later than 48 hours after PGF injection. When estrous detection efficiency is high during the time period from -30 to 72 hours after PGF (0 hours), breeding by estrus until 72 hours coupled with timed AI at 72 hours should result in high pregnancy rates. However, when estrous detection efficiency is low or in herds that have not had a high estrous response (> 50%) by 72 hours after PGF, it might be more economical to continue to observe cows and AI following observation of estrus for an additional 2 days rather than mass inseminating cows. We conclude that incorporating a timed AI at 72 hours after PGF for cows synchronized with the Select Synch protocol may be feasible if the 72-hour estrous response is high, but caution against timed AI at 72 hours in herds when the 72-hour estrous response is low. Also, addition of a second injection of GnRH at 72 hours is not necessary.