Estrus in Relation to Time of Administration of Prostaglandin F2 to Heifers

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Abstract
Holstein heifers (95) were given 2 injections of prostaglandin F2.alpha., the 1st on day 0 and the 2nd either at 0600 or 1800 h on day 12. The interval from the 2nd treatment to 1st observation of estrus did not differ between heifers treated during the morning (69.8 .+-. 4.4 h) or evening (72.7 .+-. 5.2 h). Estrus was detected sooner after treatment of heifers during July, 1975 (49 .+-. 2 h) than after treatment during June, 1976 (85 .+-. 8 h). Estrus began at 64 .+-. 5, 73 .+-. 7 and 76 .+-. 7 h after the 2nd injection during Aug. 1975, Oct. 1976 and Dec. 1975. Heifers treated during the morning in Oct. were detected in estrus sooner (57 .+-. 3 vs. 95 .+-. 13 h) than those treated during the evening. An interaction of season by time may affect interval from prostglandin F2.alpha. treatment to onset of estrus.