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

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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.
G. A. Perry , O. L. Swanson , E. L. Larimore , B. L. Perry , G. D. Djira , R. A. Cushman
USDA Scientist Submission
Domestic animal endocrinology 2014 v.48
Journal Articles, USDA Authors, Peer-Reviewed
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