Relationship of follicle size and concentrations of estradiol among cows exhibiting or not exhibiting estrus during a fixed-time AI protocol

Authors: PERRY, GEORGE - South Dakota State University; SWANSON, OLIVIA - South Dakota State University; LARIMORE, E - South Dakota State University; PERRY, B - South Dakota State University; DJIRA, G - South Dakota State University; Cushman, Robert - Bob

Submitted to: Domestic Animal Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2014
Publication Date: 6/1/2014
Publication URL: http://handle.nal.usda.gov/10113/58889
Citation: Perry, G.A., Swanson, O.L., Larimore, E.L., Perry, B.L., Djira, G.D., Cushman, R.A. 2014. Relationship of follicle size and concentrations of estradiol among cows exhibiting or not exhibiting estrus during a fixed-time AI protocol. Domestic Animal Endocrinology. 48:15-20.

Interpretive Summary: Cows exhibiting behavioral estrus near the time of fixed-time artificial insemiantion have greater pregnancy success than cows showing no behavioral estrus. The objective of this study was to determine the relationship between follicle size and estradiol concentration in cows that did or did not exhibit behavioral estrus during a fixed-time AI protocol. Ovulation was synchronized in suckled beef cows, and follicle size and ovulation were determined by transrectal ultrasonography. Estrus was detected by visual observation with the aid of estrus-detection patches, and cows that ovulated were classified as exhibited behavioral estrus or did not exhibit behavioral estrus. Among cows that exhibited behavioral estrus, a larger follicle diameter was associated with greater serum estradiol concentrations, but no relationship between follicle size and estradiol concentration was observed among cows not exhibiting behavioral estrus. Cows that exhibited behavioral estrus had a larger follicle and greater peak estradiol concentration than cows that did not exhibit behavioral estrus. Furthermore, cows that exhibit behavioral estrus have a faster rise in estradiol than cows not exhibiting behavioral estrus. In summary, follicle size had a positive relationship with concentrations of estradiol, but only among cows that exhibited behavioral estrus. These greater concentrations of estradiol may contribute to the improved pregnancy rates observed in these cows.

Technical Abstract: Cows exhibiting estrus near the time of fixed-time AI had greater pregnancy success than cows showing no estrus. The objective of this study was to determine the relationship between follicle size and peak estradiol concentration between cows that did or did not exhibit estrus during a fixed-time AI protocol. Ovulation was synchronized in suckled beef cows by applying the CO-Synch protocol [GnRH (100 µg) on d -9, PGF2a (25 mg) on d -2, and a second injection of GnRH 48 h after PGF2a (d 0)] in three replicates (n = 80, 22, and 24). Follicle size (d 0) and ovulation (d 2) was determined by ultrasonography. Blood samples were collected every 3 (replicates 1 and 2) or 4 (replicate 3) h beginning at time of PGF2a injection (0 h). Estrus was detected by visual observation with the aid of estrus-detection patches, and cows that ovulated were classified as exhibited estrus (n = 46) or did not exhibit estrus (n = 63). Among all cows (P < 0.01) and among those that exhibited estrus (P < 0.01), a significant positive relationship was detected between follicle size and peak estradiol concentration, but no linear relationship (P = 0.60) between follicle size and peak estradiol concentration was observed among cows not exhibiting estrus. Cows that exhibited estrus had a larger (P = 0.02) follicle (13.4 ± 0.25 mm) and greater (P < 0.01) peak estradiol concentration (12.4 ± 0.54 pg/mL) than cows that did not exhibit estrus (12.6 ± 0.22 mm and 7.8 ± 0.46 pg/mL). Cows exhibiting standing estrus had greater (P < 0.001) preovulatory concentrations of estradiol beginning 6 h (replicate 1), 0 h (replicate 2), or 12 h (replicate 3) after the injection of PGF2a on d -2 compared with cows not exhibiting standing estrus. Furthermore, cows that exhibited estrus had an increased (P < 0.01) rate in the rise in concentrations of estradiol following the PGF2a to peak estradiol than cows not exhibiting estrus (0.27 ± 0.05 pg/h and 0.10 ± 0.05 pg/h, respectively). In summary, follicle size had a positive relationship with peak concentrations of estradiol, but only among cows that exhibited standing estrus, and estradiol increased earlier in cows that exhibited estrus compared to cows that did not.