Researchers at the U.S. Meat Animal Research Center compared two estrus synchronization systems for timed artificial insemination. The crossbred beef cows used in the trial ranged from 3 to 10 years of age and 26 to 91 days postpartum. Researchers synchronized 661 cows in the fall and 579 in the spring using one of two treatments:

* Co-Synch: Injection of gonadotrophin-releasing hormone (GnRH) followed by injection of prostaglandin (PGF2α) seven days later.

* Co-Synch plus CIDR: Same as above plus a CIDR (progesterone-releasing device) inserted between the two injections listed above.

At 60 hours after PGF2α administration, the team treated all cows with GnRH and artificially inseminated them, regardless of estrus, and placed them with bulls 10 days after AI. They collected blood samples at two weeks before initial GnRH injection, at initial GnRH injection and at PGF2α injection, and analyzed progesterone levels. They monitored estrous behavior with heat detectors and determined pregnancy using ultrasound 72 to 77 days after AI.

At first GnRH injection, progesterone did not differ but was higher in Co-Synch + CIDR at PGF2α injection. At 60 hours after PGF2α, Co-Synch + CIDR cows had 67 percent in estrus versus 58 percent for Co-Synch, and also 55 percent were pregnant to AI versus 44 percent. When progesterone levels at PGF2_ injection were low, Co-Synch + CIDR had higher than a 65 percent pregnancy rate versus 31 percent. There was no difference between treatments when progesterone was high. Regardless of treatment, all cows expressing estrus within 60 hours before AI had higher pregnancy rates. The authors concluded that the addition of CIDR increased the progesterone level, percent detected in estrus and pregnancy rate, primarily in cows with low or decreasing levels of endogenous progesterone.