Impact of Manipulation of Progesterone Concentrations during Follicular Development on Ovulatory Follicle Growth and Timed AI Pregnancy Rate in Beef Cows
This experiment was conducted to investigate the role of decreased progesterone (P4) concentrations during follicular development on fertility in multiparous beef cows (n=228; Angus x Simmental) from three locations (n=92; n=63; n=73). Ovulation was pre-synchronized with the 5-d CO-Synch+CIDR program with the d of the final GnRH designated as d -6. On d 0, all ovarian follicles were ablated. Cows were stratified by age and days postpartum within location, and assigned to receive either a previously used CIDR and two-25 mg PGF doses 8 h apart (low-P4; L; n=115) or a new CIDR (high-P4; H; n=113) on d 0. On d 5, CIDR were removed, two-25 mg PGF doses administered and estrus detection tail paint applied. Tail paint scoring (TPS; 1 = paint completely removed; 2 = paint partially removed; 3 = paint largely undisturbed and no evidence of mounting), timed-AI (TAI) and administration of 100 µg GnRH were performed on d 8. Blood samples for P4 analysis were collected on d 5. Ultrasonography was conducted on d 0, 5 and 8 to assess ovarian structures, and d 35 for pregnancy diagnosis. Cows without a CL on d 0 were removed from all analyses. Across locations, P4 concentrations on d 5 were greater (P < 0.05) in the H (4.91 ± 0.13 ng/mL) than L (0.99 ± 0.06 ng/mL); a treatment by location interaction (P < 0.05), predominantly due to elevated P4 concentrations in one location, was detected. Follicle diameter on d 5 was greater (P < 0.05) in the L (8.9 ± 0.2 mm) than H (7.4 ± 0.1 mm) treatment, but did not differ between treatments (12.0 ± 0.1 mm) at TAI. Hence, follicle growth from d 5 to 8 was greater (P < 0.05) in the H (1.5 ± 0.1 mm/d) than in the L (1.3 ± 0.1 mm/d) treatment. Distribution of TPS differed (P < 0.05) between treatments with a majority of cows in the H treatment with TPS 3 (57%) and a majority in the L treatment with TPS 1 (54%). Cows with TPS 1 or 2 had a greater (P < 0.05) PR than cows with a TPS 3. However, PR did not differ between the H (67.3%) and L (67.8%) treatments. In conclusion, decreased P4 resulted in an increased percentage of cows in estrus before TAI, but did not impact timed AI pregnancy rate in beef cows.
Progesterone, Fertility, Cows