Effects of reproductive tract development and hormonal treatment on estrous synchronization in heifers

Abstract

Evaluation of the reproductive tract development (RTD) is a criterion rarely used when heifers are included in estrous synchronization programs. The objective of the present study was to determine the effect of RTD (mature and immature) and hormonal treatment (EC and EB) on estrous expression rate and pregnancy rate in Bos indicus heifers under tropical conditions. RTD was evaluated with an ultrasound and classified as mature (n=99) or immature (n=101). Heifers received an intravaginal device (DIB^®; day 0), 2 mg of estradiol benzoate (EB) and 250 µg of cloprostenol (PGF_2α). The DIB was removed on day 7 post insertion, and 250 µg of PGF_2α was applied and heifers were divided into two groups: One group of heifers received 0.5 mg of estradiol cypionate (EC group), and on day 8, a second group of heifers received 1 mg of estradiol benzoate (EB group). Estrus was detected by visual observation, and all heifers were inseminated at a fixed-time (FTAI). Fifteen days after insemination, bulls were introduced to the treatment groups. Data were analyzed using general modeling and binary logistic regression procedures. The percentage of estrus was similar for both mature and immature heifers (P>0.05), however, the heifers treated with EB had 17% animals in estrus, than the heifers that received EC (P <0.05). The pregnancy rate after FTAI (74.8%) and total pregnancy (FTAI + natural mating = 91.9%) was greater for the group of heifers with a mature reproductive tract (P<0.05). The hormone treatment had not effect (P>0.05) on the pregnancy rate. In conclusion, the heifers with mature reproductive tracts had the highest pregnancy rate after artificial insemination and total pregnancy. Hormonal treatment did not influence the pregnancy rate.