Abstract:
Two experiments were performed in a commercial farm located in Lara, Venezuela. The objective was to evaluate the pregnancy rate in postpartum dairy cows with different genetic composition of Holstein (H) and Carora (C) breeds (1/2, 3/4, and 7/8) during two seasons of the year using estrus synchronization programs. The first experiment evaluated the pregnancy rate of three different breeding groups in two seasons of the year. All cows (n = 305) were treated with the Heatsynch program (Select Sires, Inc., Miami, FL, USA): Cycling cows between Days 36 and 42 after parturition were injected with two doses of PGF2� 14 days apart. Seven days later, after the last injection of PGF2�, the cows were injected with a GnRH analog (Buserelin). Seven days later cows were injected with PGF2� and after 24 h cows were injected with 1 mg of estradiol cypionate (ECP). Cows were inseminated when heat was detected, and after 48 h all cows that did not present heat were inseminated. During the period from January to May, 152 cows were treated. The average temperature (T) was 25.22�C and relative humidity (RH) was 70.16%. During the period from June to October, 153 cows were treated. The T was 25.26�C and RH was 72.58%. The pregnancy rates for the groups were 1/2H _ C, n = 89, 28.08%; 3/4H _ C, n = 143, 25.17%; and 7/8H _ C, n = 73, 26.02%; they were not significantly different (P < 0.14). However, the pregnancy rate was higher in January-May (30.92%) than in June-October (22.87%; P < 0.05). The second experiment evaluated pregnancy rates in the different breeding groups (H _ C), using two protocols, Heatsynch vs. Ovsynch (Select Sires, Inc.). The Ovsynch program is similar to the Heatsynch program; however, the last injection of ECP in the Heatsynch program is changed to the GnRH analog after 48 h from the last injection of PGF2�. Cows were inseminated 24 h after the GnRH injection. Pregnancy rate was lower for cows treated with the Heatsynch program (21.15%) than for those treated with the Ovsynch program (45.28%; P < 0.01). Furthermore, cows in the 1/2 and 3/4 breeding groups had higher pregnancy rates than those in the 7/8 breeding group (1/2 = 42.05%, 3/4 = 38.33%, and 7/8 = 16.72%; P < 0.01). Data were analyzed using logistic regression analysis. In conclusion, the introduction of the Carora breed in the dairy herd increased the adaptability of the Holstein cows to tropical conditions by increasing their reproductive efficiency. Furthermore, pregnancy rates were increased using synchronization programs that did not require heat detection. This demonstrated the possibility of increasing reproductive performance in the tropics by incorporating estrus synchronization systems that do not require heat detection. These studies also demonstrated that the incorporation of dairy breeds adapted to tropical conditions, such as the Carora breed, aids in improving reproductive performance under high RH conditions.