the aim of this study was to compare the efficiency of five synchronization protocols, economically, in holstein cows during heat stress. this experiment was conducted at a commercial dairy farm in summer 2015 (temperature humidity index 73–81). five hundred holstein dairy cows with ˃25 kg/d milk yield were randomly assigned to five experimental groups: 1) ovsynch; 2) presynch-ovsynch; 3) doubl...