196 Dominant follicle removal in Gyr × Holstein crossbreds accelerates development of ovarian follicles

Dominant follicle removal (DFR) has been shown to promote follicular growth in Bos taurus females. This study was conducted to test whether DFR has a similar effect on follicular development in Gyr × Holstein females, a popular dairy cross-breed in certain parts of the world. The ovaries of (28) 1/2 Gyr: 1/2 Holstein (1/2 Gyr; 34.7 ± 9 months of age) and (29) 3/4 Gyr: 1/4 Holstein (3/4 Gyr; 22 ± 1.4 months of age) females were subjected to ultrasound scanning (USS), and the numbers of corpora lutea (CL), large follicles (LF; diameter 6–10 mm), medium follicles (MF; diameter 3–6 mm), and small follicles (SF; diameter < 3 mm) were recorded for each donor. Each animal was subjected to USS, first at the time of DFR (Day zero; D0), and second two (D2) or three (D3) days after DFR. Additionally, each animal was subjected to USS during an adjacent oestrus cycle without subjecting to DFR (as internal negative controls). The effect of DFR on follicle numbers was determined using a mixed effects model (PROC MIXED procedure of SAS). At the first USS (D0), the number of observed total follicles (TF) was higher (P < 0.05) in 1/2 Gyr (29 ± 2) than in 3/4 Gyr (22 ± 1.9) females. This might be due to the different age groups of the donors. As a percentage of TF, SF were the most common type of follicle observed in both groups (89% in 1/2 Gyr and 85% in 3/4 Gyr), followed by LF (8.7% in 1/2 Gyr and 8.6% in 3/4 Gyr), and MF (2.5% in 1/2 Gyr and 6.7% in 3/4 Gyr). On D2 and D3 USS, the number of TF had not changed in response to DFR (P > 0.1); however, the percentage of MF had increased (P < 0.01; from 2.5% to 44% in 1/2 Gyr; from 6.7% to 43% in 3/4 Gyr) and the percentage of SF had decreased (P < 0.01; from 89% to 51% in 1/2 Gyr; from 85% to 54% in 3/4 Gyr), suggesting that DFR accelerates development of SF into MF. Further, higher percentages of MF (P < 0.01) were observed on USS at 48 h after DFR (63% ± 3.3; 65% in 1/2 Gyr and 60% in 3/4 Gyr) than at 72 h after DFR (50% ± 3.2; 53% in 1/2 Gyr and 47% in 3/4 Gyr) suggesting that the optimal time point for ovum pickup (OPU) in Gyr × Holstein cross-breds is closer to 48 h after DFR than to 72 h after DFR. These findings suggest that DFR promotes growth of SF into MF, possibly by removing the suppression exerted on SF by the dominant follicle. Further studies using age-matched groups of GH crossbreds are necessary to determine effects of DFR on the quantity and quality of the oocyte harvest from OPU, and subsequent embryo development.