THE EFFECT OF ESTRADIOL BENZOATE ADMINISTRATION ON ESTROUS RESPONSE AND SYNCHRONIZED PREGNANCY RATE IN DAIRY HEIFERS AFTER TREATMENT WITH EXOGENOUS PROGESTERONE

The objectives of this trial were to determine the effects of 0.5 mg e stradiol benzoate administered intramuscularly 24 h after the removal of progesterone-containing intravaginal devices on the occurrence and timing of estrus, synchronized pregnancy rate and synchronized concept ion rate in dairy heifers. A clinical trial was conducted involving 75 0 dairy heifers in 13 herds. Within each herd heifers were randomly al located to 1 of 2 estrus synchronization treatments. All heifers recei ved a CIDR-B progesterone-containing intravaginal device containing a 10 mg estradiol benzoate capsule for 12 d. Twenty-four hours after CID R-B removal one group received 0.5 mg, im estradiol benzoate while the other group received an intramuscular injection of a placebo. Estrus detection was performed at 48 and 72 h after intravaginal device remov al, and heifers detected in estrus at those times were inseminated. Ad ministration of estradiol benzoate 24 h after removal of CIDR-B device s significantly increased the number of heifers exhibiting estrus with in the observation period (96.1 vs 90.5%; P<0.01). It also altered the onset of estrus so that significantly more heifers were in estrus (86 .6 vs 72.3%; P<0.01) and conceived (47.1 vs 37.5%; P<0.05) by 48 h aft er CIDR-B device removal. The synchronized conception rate was unaffec ted by treatment. The distribution of estrus was such that fixed-time insemination after estrus synchronization with this treatment program may be recommended.