PSXI-15 Effect of daily low doses of hCG in the luteal and follicular dynamic of Bos indicus Nelore beef heifers.

Abstract The goal was to evaluate the effect of daily low doses of hCG starting on D13 of the estrous cycle in the luteal and follicular dynamic of bos indicus Nelore beef heifers (n=31). Heifers were induced to ovulate using estradiol base synchronization protocol (estradiol benzoate [2mg] and a 0.5g intravaginal P4 insert [D-10]; insert was removed, followed by eCG [200IU], estradiol cypionate [0.5 mg], and cloprostenol sodium [0.5 mg] treatments [D-2]. On day zero (D0), all heifers received buserelin (0.08mg) but were not inseminated. Ovarian ultrasound was performed on D5 to confirm ovulation by checking the presence of at least one corpus luteum (CL) with a dominant follicle (≥ 8mm in diameter). On D13, heifers were randomized to either receive daily low doses of hCG (100 IU) until D23 (hCG; n=15) or 1mL of saline (Control; n=16). From D13 to D25, blood samples were collected, and ovarian ultrasounds were performed to track follicular dynamics and CL blood perfusion using the color Doppler. Functional luteolysis was defined as the first day with P4 < 1ng/mL, and structural luteolysis was considered the first day CL had less than 25% of the total area with blood perfusion. The largest follicle on D13 was assumed to be the dominant follicle of the second follicular wave and was similar (P=0.53) between hCG (8.4 ±0.4) and Control (8.8 ±0.5). There was an effect of circulating progesterone for TRT (P=0.03) and Day (< 0.01) but no interaction of Trt*Day (P = 0.18). On D15, specifically, there was a greater CL volume (cm3; P=0.03) for hCG (9.3 ±0.8) compared with Control (6.9 ±0.7), while there was a tendency for greater circulating P4 for hCG (3.0 ±0.3) compared with Control (2.3 ±0.2), and similar (P=0.61) CL blood perfusion (hCG 69% ±5% vs. Control 65% ±6%). The dominant follicle on D13 underwent atresia followed by a new follicular wave emergence in all heifers in the Control group, while in 53.3% (8/15) of hCG-treated heifers, the dominant follicle of the second follicular wave kept growing until ovulation (two wave patterns [hCG_2wave]). Day of functional and structural luteolysis was similar (P=0.98 and P=0.41) among groups (Control [17.6 ±0.24 and 18.5 ±0.5], hCG_3waves [17.6 ±0.36 and 18 ±0.5], and hCG _2waves [17.6 ±0.3 and 19.3 ±0.6]. The size of the largest follicle at structural luteolysis and at ovulation was greater (P < 0.01) for hCG_2waves [17.4 ±1.2 and 19.1 ±1.4], intermediary for hCG_3waves (13.4 ±1.1 and 13.8 ±1.4), and smaller(Control [9.1 ±0.5 and 11.5 ±0.4]. Day of ovulation was similar among groups (Control 20.2 ±0.6; hCG_2waves 18.7 ±1.0; and hCG_3waves 20.7 ±0.7). Daily low doses of hCG change the follicular dynamic of bos indicus beef cattle but do not affect time to luteolysis.