Progesterona injetável previamente ao protocolo de inseminação artificial em tempo fixo em novilhas de corte de raças taurinas ou sintéticas

Abstract

The age at puberty in heifers is considered a key factor for profitability and reproductive performance in breeding systems. The objectives of the study were to evaluate the effect of one or two administrations of injectable progesterone (iP4) on the pregnancy rate after TAI in Angus heifers (14 to 18 months), and on the ovulation rate and serum P4 concentration in Brangus heifers (12 to 14 months). In Experiment 1, 380 Angus heifers were used, with body condition score (BCS; 1-5 scale) ranging from 2 to 3, average live weight (LW) of 243.3±0.88 and reproductive tract score (RTS; 1-5 scale) between 2 and 4 on D -48 (D0: beginning of the TAI protocol). Heifers were allocated into three groups: control (n=152), which did not receive iP4; 1iP4 (n=136), which received one 175 mg iP4 injection (D -24); and 2iP4 (n=92), which received two applications of 175 mg of iP4 (D -48 and D -24). The heifers were submitted to the same TAI protocol: D0: intravaginal device 0.5g P4 and 2 mg estradiol benzoate (EB); D8: 300 IU equine chorionic gonadotropin (eCG); 150 μg cloprostenol (PGF) and 0.5 mg estradiol cypionate (EC); D10: 10,5 μg GnRH analog in heifers that did not show estrus. The average daily gain (ADG) of the heifers between D -48 and D0 was 0.9±0.04 kg (LW on D0= 276±2.5 kg). There was no effect of treatments on pregnancy rate after TAI [Control=50.6% (77/152); 1iP4=44.8% (61/136); 2iP4=46.7% (43/92); P>0.05], nor interaction of treatments with RTS, BCS and LW, evaluated on D -48 (P>0.05). In Experiment 2, 51 Brangus heifers (BCS 2.5-4), were allocated in the same groups previously described, however, without the use of eCG and GnRH. The ADG of heifers between D -48 and D10 was 0.114±0.02 Kg. The heifers had 302.1±4.1 kg of LW on D10. Ovulation rate based on the presence of CL seven days after TAI did not differ among groups [Control=56.2% (9/16); 1 iP4=58.8% (10/17); 2 iP4=61.1% (11/18); P>0.05]. There was no effect of treatments on P4 concentrations on the 7th and 14th day after TAI (Control= 3.7±0.5 and 9.6±0.9; 1iP4= 4.7±0.8 and 11.3±1.8; 2iP4= 5.5±0.7 and 12.2±1.5 ng/mL; P>0.05), being observed an effect of day (P≤0.05). In conclusion, one or two applications of iP4 prior to the TAI protocol did not affect the pregnancy rate 30 days after TAI in taurine heifers, which can be explained by the precocity of the breed and the adequate ADG. In synthetic heifers, the treatments did not affect the ovulation rate and luteal function, which may be due to the low LW and low ADG of the animals. New studies are necessary to identify the conditions that allow a better response to the puberty induction protocol in taurine and synthetic heifers.