The interval to estrus was 14 h longer for cows treated with PGF2á between d 10 and 13 of the cycle compared with cows treated on d 6 to 9 or d 14 to 18 of the cycle. Similarly, Tanabe and Hann (1984) and Watts and Fuquay (1985) found a longer interval to estrus when dairy heifers were treated with PGF2á on d 8 to 11 of the cycle than when treated on d 5 to 7 of the cycle.