This study investigated the effect of applying a systematic, whole herd P4-based TAI program on reproductive performance of spring-calving, pasture-based lactating dairy cows and examined whether treatment with eCG at P4 device removal at the end of the TAI protocol would affect P/AI. Overall, use of TAI improved reproductive performance, increasing 21-d PR, 42-d PR, and FPR, and reducing the number of days open compared with control cows receiving no hormonal intervention.