Synchronization treatments previous to natural breeding anticipate and improve the pregnancy rate of postpartum primiparous beef cows.

Timed AI has become a potential tool to bypass postpartum acyclicity, yet only a small percentage of the world bovine herd is inseminated. Most females are still subjected to bull mating; therefore, the frequent occurrence of postpartum anestrus may compromise their reproductive efficiency. Thus, the aim of this study was to develop an approach that allows the early conception of postpartum primiparous beef cows that are exposed to natural breeding (NB). For this purpose, 350 primiparous Nelore cows 35-65 d postpartum were allocated into three groups: Control (no hormonal treatment), TNB (hormonal protocol for timed-NB without equine chorionic gonadotropin; eCG) and TNB + eCG (hormonal protocol for TNB with eCG) groups. The protocol for TNB consisted of the insertion of a 1-g progesterone device and the intramuscular (IM) administration of 2 mg estradiol benzoate on D-9 (nine days before bull exposure), followed by device removal and the administration of 1 mg estradiol cypionate IM on D0. An additional 300 IU of eCG was given only to TNB + eCG cows. All cows were exposed to bull mating from D0 to D105. Pregnancy was checked by ultrasonography every 50 d, gestational age was estimated, and the number of new gestations every 21-d cycle (P21, P42, P63, P84 and P105) was predicted. Control cows had lower pregnancy at P21 (5.7%c , 7/123) than TNB (30.4%b , 35/115) and TNB + eCG (51.8%a , 58/112; P = 0.001) cows. Pregnancy rate increased across P42, P63 and P84 (P = 0.001) but remained higher for TNB + eCG cows. Regarding time to conception, the TNB + eCG cows achieved a greater than 50% pregnancy rate within the first days after bull exposure, while the TNB and Control cows took more than 40 d and 90 d to achieve 50% pregnancy rates, respectively. At the end of the breeding season (BS), TNB + eCG cows had a 21% higher pregnancy rate than the Control cows and a 16% higher rate than the TNB cows. The probability of conceiving increased 1.5-fold for cows treated with TNB (P = 0.0079) and 2.2-fold for cows additionally treated with eCG (P < 0.0001). The average interval between the onset of the BS and conception was reduced (P < 0.0001) for the TNB + eCG cows (26.5 ± 3.8c ) compared to the TNB (35.7 ± 4.1b ) and Control (64.7 ± 3.9a days) cows. Thus, the use of TNB, especially when associated with eCG, efficiently improved the early conception of postpartum primiparous beef cows after exposure to NB. The increased number of cows conceiving early in the BS is crucial to improve reproductive efficiency by reducing the interval between parturitions and improving the number of pregnant cows at the end of BS, thus resulting in greater farm income.