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The aim of this study was to evaluate season-of-the-year influences on the moment of calving and insemination, number of days in milk (DIM), and the presence of corpus luteum (CL) in Holstein dairy cows submitted for a timed  artificial insemination (TAI). Testing was done at the onset of hormonal protocols to determine detection rates of conception and estrous return. The protocol was: Day 0 -insertion of progesterone releasing intravaginal device (CIDR®, Zoetis) and an intramuscular injection of 0.02 mg of gonadorelin (GnRH, Fertagyl®, MSD); Day 7 - 12.5 mg of dinoprost trometamina (PGF2?, Lutalyse®, Zoetis) and removal of the device; Day 8 - 1.0 mg of estradiol cypionate (ECP®, Zoetis); Day 10 - TAI. Pregnancy diagnosis was performed 32 days post-AI using ultrasonography.  Cows that had estrous return before pregnancy diagnosis were re-inseminated, conventionally, 12 hours after estrous detection. Trained employees observed cows for signs of estrous twice daily, in the morning and afternoon, for approximately 30 minutes. Cows that did not become pregnant were resynchronized using the same protocol. Data was analyzed by logistic regression in a Minitab program. Significance level of P < 0.05 was adopted. A total of 406 inseminations were performed, which resulted in 101 pregnancies with a conception rate of 24.87%. Estrous return detection rate was 43.27%. Season of year at calving (P = 0.930), DIM (P = 0.508) and presence of CL (P = 0.293) did not affect conception rate. Season of year at calving (P = 0.256), DIM (P = 0.215) and presence of CL (P = 0.076) also did not influence estrous return detection rate. Season of year at the moment of TAI did not interfere with conception rate (P = 0.236); however, there was an affect (P = 0.012) on estrous return detection rate; values found in spring-summer were lower than in autumn-winter (38.24% vs. 53.47%).This suggests the detrimental effect of excessive heat on the occurrence of estrous in Holstein cows.\u

Conception rate and estrous return detection after TAI in Holstein cows