The objective was to compare the reproductive performances of Brown Swiss cows and heifers synchronized for estrus and ovulation using prostaglandin F 2 alpha (PGF2a) alone or with gonadotropin-releasing hormone (GnRH). Primiparous and multiparous cows (45 to 75 days postpartum; n = 26) and nulliparous heifers (16 to 20 months; n = 26) were randomly assigned to 2 treatment programs. GnRH injection was followed in 7 days by PGF2a injection and then by another GnRH injection 9 days later. Artificial insemination (AI) was performed 16 h after the second GnRH injection. Injections of PGF2a were given 11 days apart AI was only carried out after estrus was detected following the first and second PGF2a injections or, timed AI in the absence of detected estrus was carried out 72 h after the second PGF2a injection. The conception rate in first insemination, pregnancy rate and the interval from PGF2a to estrus were not different between the 2 groups. However, the estrus detection rate was greater for the group II cows (61.5%) than for the group I cows (15.4%) (P < 0.05). Pregnancy rates tended to be higher in group I cows. The GnRH-PGF2a -GnRH method allowed effective management of AI in cows and heifers without the need for estrus detection.

The objective was to compare the reproductive performances of Brown Swiss cows and heifers synchronized for estrus and ovulation using prostaglandin F 2 alpha (PGF2a) alone or with gonadotropin-releasing hormone (GnRH). Primiparous and multiparous cows (45 to 75 days postpartum; n = 26) and nulliparous heifers (16 to 20 months; n = 26) were randomly assigned to 2 treatment programs. GnRH injection was followed in 7 days by PGF2a injection and then by another GnRH injection 9 days later. Artificial insemination (AI) was performed 16 h after the second GnRH injection. Injections of PGF2a were given 11 days apart AI was only carried out after estrus was detected following the first and second PGF2a injections or, timed AI in the absence of detected estrus was carried out 72 h after the second PGF2a injection. The conception rate in first insemination, pregnancy rate and the interval from PGF2a to estrus were not different between the 2 groups. However, the estrus detection rate was greater for the group II cows (61.5%) than for the group I cows (15.4%) (P < 0.05). Pregnancy rates tended to be higher in group I cows. The GnRH-PGF2a -GnRH method allowed effective management of AI in cows and heifers without the need for estrus detection.