Effects of treatments to induce puberty and increase pregnancy rate in heifers in native pastures of Pantanal

Early puberty and age at the onset of reproductive life are factors that increase the profitability of cattle production. In extensive pasture systems with low nutritional pastures, such as the Pantanal, the mating age in Nelore heifers is about three years. Ovulation in Bos taurus prepubertal heifers using prostaglandin F2α (PGF2α) has been previously reported. This work aimed to evaluate two hormonal protocols to promote first ovulation in prepubertal heifers and to elevate pregnancy rates in Nelore heifers. 222 Nelore noncyclic heifers (absence of CL after two gynecological exams 11 days apart) with ages averaging 715 days and weighting 275.57 ± 10.43 were submitted to three treatments: Control (n = 73)- heifers received two applications of 2 ml physiological solution on the same days of treatments in T1 group; T1 - PGF (n=74) , heifers received two doses of PGF2α (0.530 mg sodium cloprostenol; Sincrocio; Ouro Fino) 5 days apart, the first coinciding with D0 in T2 group; T2 - CIDR (n = 75)- heifers received an intravaginal device (P4) previously used 3 times (24 days; CIDR, Zoetis, Sao Paulo, SP) on D0, which remained for 8 days and an injection of PGF2α (0,530 mg; Sicrocio; Ouro Fino) on the day of implant removal. After the treatments the diameter of the preovulatory follicle and ovulation rate were evaluated in 4 transrectal ultrasonography examinations 3 days apart. After the evaluation period, heifers were grouped and mated with bulls previously evaluated at a 1:20 bull:cow ratio. Pregnancy diagnosis was performed 40 days after the breeding period by transrectal ultrasonography and pregnancy rates were assessed by logistic regression using PROC LOGISTIC of SAS software. The ovulation rate was not increased in T1 and T2 groups compared to the control (P > 0.05), and pregnancy rates did not differ (P > 0.05) between the control group (36.99%), PGF (35.14% ), and CIDR (46.67 %), despite the 10% difference found in the T1 compared to T2. In conclusion, alternatives like the reutilization of P4 devices or PGF2α to induce ovulation and increase pregnancy rates were not effective in pre pubertal Nelore heifers kept in native pastures in Pantanal.