In this study, the 5-day CO-Synch þ CIDR protocol included two doses of PGF. Although one exogenous dose of
PGF can induce rapid luteolysis in the mid-luteal phase, the early CL (up to Day 5 post-estrus) is somewhat refractory to
the luteolytic action of PGF. In this study, PGF administration occurred 5 days following the first GnRH injection. If
the first GnRH injection induced ovulation and/or luteinization, then the CL would have been 5 days old at PGF
administration, which may be the reason for decreased luteolysis to a single dose of PGF. Although variable
responses in luteolysis have been noted when a single dose of PGF was given on Day 5 of the estrous cycle, the results
from the previous studies indicated that two doses of PGF will overcome the refractoriness of the early CL, induce
luteolysis, and result in a greater AI pregnancy [9]. In this study, the interval between first and second PGF was 6
hours. This interval was chosen based on the result from previous study in which we have shown that cows
receiving a second PGF dose between 4 and 5 hours and between 6 and 7 hours resulted in 63.4% and 60.8% TAI
pregnancy rates, respectively, which was greater than any other interval examined.