198 Emir. J. Food Agric ● Vol 32 ● Issue 3 ● 2020 *Corresponding author: Jorge Alonso Peralta-Torres, División Académica de Ciencias Agropecuarias, Universidad Juárez Autónoma de Tabasco. Carretera Villahermosa-Teapa Km. 25, R/A La Huasteca 2ª Sección, CP.86280, Villahermosa, Tabasco, México. Tel: (993) 3581585, 1429151, Fax: (993) 1429150. E-mail: jorge.peralta@ujat.mx Received: 12 June 2019; Accepted: 24 December 2019 INTRODUCTION In the bovine production systems under tropical conditions, the most common criteria for replacement of heifers are body condition score, body weight, and age of heifer (Barcellos et al., 2014; Lardner et al., 2014). Evaluation of the reproductive tract is rarely applied, probably because it requires transrectal palpation of the internal genital organs to check the development of the uterus and ovaries. However, it has been shown to be a useful tool for decision making and tends to improve the breeding success of the herd by excluding heifers that probably cannot become pregnant during their first breeding season (Holm et al., 2009; Gutierrez et al., 2014; Dickinson et al., 2019). In this sense, the evaluation of reproductive tract development (RTD) has been used to determine the age at puberty (Monteiro et al., 2013; Holm et al., 2015; Bruinjé et al., 2019), fertility in postpartum cows (Baez et al., 2016; Holm et al., 2016) and the response to estrus synchronization (Kasimanickam et al., 2016; Kasimanickam et al., 2020). Young et al. (2011) indicated that the size of the reproductive tract at the time of artificial insemination could be related to the fertility of dairy cattle. Moreover, a high percentage of Bos indicus and cross beef heifers in fixed-time artificial insemination (FTAI) programs have been reported to have immature reproductive tracts (Claro et al., 2010; Kasimanickam et al., 2017). This was associated with a poor pregnancy rate because of the asynchrony between estrus and ovulation (Bó and Baruselli, 2014). One option to improve the synchrony between 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 2 α). The DIB was removed on day 7 post insertion, and 250 µg of PGF 2 α 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. Keywords: Bos indicus; Estradiol benzoate; Estradiol cypionate; Pregnancy rate; Tropics. ABSTRACT Emirates Journal of Food and Agriculture. 2020. 32(3): 198-203 doi: 10.9755/ejfa.2020.v32.i3.2061 http://www.ejfa.me/ Effects of reproductive tract development and hormonal treatment on estrous synchronization in heifers Jorge Alonso Peralta-Torres 1 *, Jesús Ricardo Aké-López 2 , Carlos Luna-Palomera 1 , José Candelario Segura-Correa 2 , Oswaldo Margarito Torres-Chablé 1 , Nadia Florencia Ojeda-Robertos 1 1 División Académica de Ciencias Agropecuarias, Universidad Juárez Autónoma de Tabasco. Carretera Villahermosa-Teapa Km. 25, R/A La Huasteca 2ª Sección, CP.86280, Villahermosa, Tabasco, México. Tel. (993) 3581585, 1429151, Fax: (993) 1429150, 2 Facultad de Medicina Veterinaria y Zootecnia. Universidad Autónoma de Yucatán. Carretera Mérida-Xmatkuil Km. 15.5, Apdo. 4-116 Itzimná, CP 97100, Mérida, Yucatán, México. Tel. (999) 942 3200; ext. 23 Fax (999) 942 3205. RESEARCH ARTICLE