The aim of this experiment was to evaluate the effect of the duration of progestagen (P4) exposure and sexual maturity of prepubertal and pubertal heifers submitted to timed artificial insemination (TAI). A hundred and fourteen 16- month old Bradford heifers were classified, according treatment and sexual maturity, in 4 groups: PPLPG (prepubertal long exposure to P4, n=21), PPSPG (prepubertal short exposure to P4, n=19), PLPG (pubertal long exposure to P4, n=34) and PSPG (pubertal short exposure to P4, n=40). On day -8, the heifers from PPLPG (prepubertal long exposure to progestagen) and PLPG (pubertal long exposure to progestagen) received an intravaginal sponge impregnated with medroxiprogesterone acetate (MAP). On day 0 the sponge was removed and a new sponge was inserted, together with an injection of 2.5 mg estradiol benzoate (EB). On day 8 the sponge was removed and the heifers received an injection of 0.5 mg EB and 250 ìg sodium cloprostenol. The TAI was performed 54 hours after sponge withdrawal. The heifers from PPSPG and PSPG received the same synchronization treatment, except for the additional MAP from day -8 to day 0. The pregnancy rate was higher in pubertal (60.3%) than in pre-pubertal (35.0%) heifers. However, there was no effect of the duration of treatment in the pregnancy rate between pubertal and prepubertal heifers. The second part of this study aimed to evaluate the economical return of the protocols and compare them with the traditional breeding system at 26 months of age. A better economic response with the use of protocols associating P4 in heifers at 16 months was observed compared to the traditional system at 26 months of age. The prolonged exposure to P4 did not improved pregnancy rate in heifers submitted to TAI.
cattle; economical return; ovulation synchronization; progesterone; puberty
