173 Association between follicular characteristics and ovulatory response to the last GnRH of the Double Ovsynch and pregnancy per AI in first service Holstein multiparous cows

Our aim was to determine the association of follicular characteristics with ovulatory response and pregnancy per AI (P/AI) in multiparous Holstein cows (n = 1289) submitted to a Double Ovsynch protocol for first service. Ovaries were scanned by ultrasound at the first (G1) and last (G2) GnRH treatments of the breeding-Ovsynch and 2 days later to determine follicle size and ovulatory response. Only cows that ovulated to G1 (70.8%) and with serum P4 > 1.00 ng mL⁻¹ at PGF_2α of the breeding-Ovsynch were considered in the analysis for ovulatory response to G2 and P/AI (n = 855). Associations were analyzed by logistic regression in SAS. At the time of G2, 0.6% of the cows had no follicles ≥ 9 mm, 54.3% had one follicle, 34.9% had two follicles, 7.6% had three follicles, and 2.7% had more than three follicles. Serum P4 at PGF_2α was negatively associated (P < 0.01) with the number of follicles ≥ 9 mm at G2; however, milk production was not associated (P = 0.26) with the number of follicles ≥ 9 mm at the time of G2. Ovulation to G2 was 93.7% and was not associated (P = 0.40) with the number of follicles ≥ 9 mm at G2. In cows with more than one follicle ≥ 9 mm at G2, the number of follicles ≥ 9 mm was associated (P < 0.01) with double ovulation to G2. Cows with three or more follicles had greater double ovulation (48.0%) than cows with two follicles (24.9%). The number of follicles ≥ 9 mm at G2 was not related (P = 0.70) to P/AI at Day 32 post-AI (overall: 50.4%). In cows with one follicle ≥ 9 mm at G2, follicle diameter was positively associated with ovulation to G2 (P = 0.01) and P/AI (P = 0.03). In cows with two follicles ≥ 9 mm at G2, 48.4% of cows ovulated the largest follicle, 26.7% ovulated the smallest follicle, and 24.9% had double ovulations. There was no association between the diameter of the largest (P = 0.15) or smallest (P = 0.18) follicles and ovulation to G2; however, the largest follicle diameter was negatively associated with double ovulation to G2 (P < 0.01). The same correlation was not observed with the smallest follicle diameter (P = 0.22). Interestingly, the largest follicle diameter tended to be positively associated with P/AI on Day 32 (P = 0.08), and the smallest follicle diameter was positively associated with P/AI on Day 32 (P = 0.03). Largest/smallest follicle ratio had the strongest negative correlation with double ovulation to G2 (P < 0.01) in cows with two follicles ≥ 9 mm at G2; however, it was not correlated with ovulatory response to G2 (P = 0.65) or with P/AI on Day 32 (P = 0.46). In cows with three follicles ≥ 9 mm at G2, 25.4% ovulated the largest follicle, 22.3% ovulated the middle follicle, and 6.8% ovulated the smallest. Moreover, 15.3% double ovulated the largest and the middle follicle, 23.7% double ovulated the middle and the smallest follicle, and 6.8% double ovulated the largest and the smallest follicle. The diameter of the largest, middle, or smallest follicle did not correlate with ovulation or double ovulation to G2 and P/AI on Day 32 in cows with three follicles ≥ 9 mm. In summary, follicle diameter at G2 was associated with ovulation to G2 only in cows with one follicle ≥ 9 mm, and the number of follicles ≥ 9 mm at G2 was correlated with double ovulation in multiparous Holstein cows.