Ovarian evaluation of Girolando (Holstein × Gir) heifers submitted to a GnRH–PGF2α–GnRH protocol in the dry or rainy seasons in the tropical savannah

The Girolando breed is used in pasture-based dairy production systems in Brazil to associate the high production of Bos taurus to the rusticity and thermal adaptation of Bos indicus. This study was designed to evaluate the physiological response to a gonadotropin-releasing hormone (GnRH)–prostaglandin F2α (PGF2α)–GnRH protocol to synchronize the ovulation in 40 Girolando heifers of a pasture-based dairy production system and its relationships with the temperature and humidity index (THI) during the dry (DS) and rainy season (RS) in the tropical savannah—Brazil's cerrado biome. Responses were characterized by follicular and corpus luteum number and diameter, ovulation (D9), and pregnancy rates after first AI. Total follicle number (8.1 ± 0.3 × 8.8 ± 0.3), D9 ovulatory follicle diameter (11.9 ± 0.4 × 10.1 ± 0.4 mm), corpus luteum diameter (8.6 ± 1.3 × 3.9 ± 1.5 mm), corpus luteum score (3.7 ± 0.8 × 1.8 ± 1.0), corpus luteum diameter after AI (9.6 ± 1.6 × 3.9 ± 1.5 mm), and corpus luteum score after AI (3.2 ± 0.4 × 0.9 ± 0.6) in DS and RS differed (P < 0.01). D9 ovulation rate was 40 % (DS) and 20 % (RS), without differences (P > 0.05). Pregnancy rate was 45 % (DS) and 11 % (RS), with differences (P < 0.01). THI differed between DS and RS (P < 0.01). THI may interfere in the follicular and luteal dynamics and in the response of Girolando heifers to the GnRH–PGF2α protocol in the tropical savannah, thus reducing the chances of pregnancy at the first artificial insemination.