Estrus was synchronized for 656 suckled beef cows with the 7-d Co-Synch +con-trolled internal drung release (cidr) protocol (100 ug gnrh+cidr [1.38 g progesterone] on d 0,25 mg PGF at cidr removal on d 7,and 100 ug GnRH on d 10,66 h after cidr removal Estrus detection aids (estrotect) were applied at PGF and CIDR removal on d 7 ,and estrous expression was recorded at Gnrh on d 7,and estrous expression was recorded at GnRH on d 10