175 Effect of estrus manifestation on follicular diameter, endometrial thickness, and circulating estradiol concentration in Bos indicus cows submitted to timed artificial insemination

This study aimed to evaluate the effects of oestrus manifestation on follicular growth, endometrial thickness, ovulation, and pregnancy outcomes in Bos indicus cows submitted to timed AI (TAI) protocol. A total of 50 suckled cows from a commercial farm in Brazil were used. Cows were treated with a P₄/E₂/eCG-based protocol (10 days) for TAI (D0 = insertion of P₄ device). At P₄ device removal (D8), cows were painted with chalk on their tailheads, and removal of chalk on Day 10 (D10) was used as an indication of oestrus (PE = presence of oestrus and AE = absence of oestrus). The ovaries and endometrium were examined by ultrasonography (US; DP10 Vet, Mindray^®) every 12 h, from D8 until ovulation (96 h). Blood was collected from a subgroup of cows (n = 18) during every US examination, and E₂ concentrations were determined by ELISA. The following variables were evaluated according to the presence of oestrus: largest follicle diameter (LFD), endometrial thickness diameter (ETD), circulating E₂ concentration (CE2), ovulation rate (OVR), and pregnancy per AI (P/AI). Statistical analyses were performed using the software of SAS^© (9.4), and differences were considered significant when P ≤ 0.05 and a tendency when the P-value was between 0.05 and 0.10. Continuous variables with repeated-measures (LFD, ETD, and CE2) were evaluated using logistic regression with the Glimmix procedure. There was an interaction (P = 0.03) between the presence of oestrus and LFD on the time (D8: PE = 12.7 ± 0.8 mm vs AE = 10.0 ± 1.0 mm; P = 0.03; D8.5: PE = 13.9 ± 0.7 mm vs AE = 11.8 ± 0.9 mm; P = 0.05; D9: PE = 14.6 ± 0.8 mm vs AE = 12.1 ± 1.0 mm; P = 0.04; D9.5: PE = 15.1 ± 0.6 mm vs AE = 12.8 ± 0.8 mm; P = 0.02; D10: PE = 15.5 ± 0.6 mm vs AE = 13.1 ± 0.7 mm; P = 0.02; D10.5: PE = 16.1 ± 0.9 mm vs AE = 12.0 ± 1.0 mm, P = 0.01). The LFD was greater between D8 and D10.5. However, no LFD effect was observed after D10.5 (P > 0.05). No interaction endometrial thickness diameter × presence of oestrus on time was observed (P = 0.45). The endometrial thickness diameter was greater in cows that exhibited oestrus (P = 0.04) according to the time (D8: PE = 6.7 ± 0.4 mm vs AE = 6.7 ± 0.3 mm; D8.5: PE = 8.5 ± 0.3 mm vs AE = 7.9 ± 0.3 mm; D9: PE = 9.5 ± 0.3 mm vs AE = 8.6 ± 0.3 mm; D9.5: PE = 9.6 ± 0.2 mm vs AE = 9.5 ± 0.3 mm; D10: PE = 10.0 ± 0.1 mm vs AE = 9.7 ± 0.2 mm; D10.5: PE = 11.0 ± 0.3 mm vs AE = 10.6 ± 0.2 mm; P < 0.001). No interaction oestradiol concentration × presence of oestrus on time was observed (P = 0.88). The circulating E₂ concentration was greater (P = 0.02) in cows with presence of oestrus and increased (P = 0.0005) according to the time (D8: PE = 3.6 ± 1.0 pg mL⁻¹ vs AE = 3.4 ± 1.0 pg mL⁻¹; D8.5: PE = 7.2 ± 1.6 pg mL⁻¹ vs AE = 5.0 ± 0.4 pg mL⁻¹; D9: PE = 9.2 ± 2.1 pg mL⁻¹ vs AE = 5.1 ± 1.6 pg mL⁻¹; D9.5: PE = 10.0 ± 2.5 pg mL⁻¹ vs AE = 5.3 ± 1.8 pg mL⁻¹; D10: PE = 9.84 ± 2.8 pg mL⁻¹ vs AE = 6.0 ± 2.0 pg mL⁻¹; D10.5: PE = 9.4 ± 2.0 pg mL⁻¹ vs AE = 5.7 ± 3.1 pg mL⁻¹). An oestrus effect on ovulation rate was observed (PE = 89.0% (25/28) vs AE = 73.0% (16/22); P = 0.03). Also, the P/AI was higher in cows that exhibited oestrus (PE = 43.0% (12/28) vs AE = 18.0% (4/22); P = 0.04). In conclusion, cows that exhibited oestrus after the TAI protocol presented larger endometrial thickness diameter and higher P/AI.