Technical Abstract: Four studies were designed to evaluate minor changes to current estrous or ovulation synchronization protocols in beef cows (n = 1160) and heifers (n = 1404). The objectives of these studies were to decrease the cost of the Select Synch and CO-Synch estrous/ovulation synchronization protocols for use in beef cows (study 1& 2), to evaluate timed insemination with the MGA/PGF (melengestrol acetate / prostaglandin) protocol in beef heifers (study 3), and to evaluate the addition of MGA to the Select Synch protocol for use in beef heifers(study 4). Study 1 evaluated half (50 micrograms) of the recommended dosage of GnRH on synchronization and AI pregnancy rates of cows receiving either the Select Synch or CO-Synch protocol. Blood samples collected from a subset of the cows receiving these treatments will be used to determine the efficacy of using half dose GnRH to induce estrous cycles in anestrous cows. Study 2 evaluated the necessity of a second GnRH injection at the time of breeding to the CO-Synch protocol. Study 3 evaluated use of a single timed insemination with AI approximately 12 hour after the onset of estrus in heifers following synchronization with the MGA/PGF protocol to facilitate use of AI. Study 4 evaluated the addition of MGA in the diet of heifers for 7 days preceding the start of the Select Synch protocol. We are currently analyzing the results of the above studies.