title: Progesterone, follicular, and estrual responses to progesterone-based estrus and ovulation synchronization protocols at five stages of the estrous cycle.
abstract: The objective was to monitor changes in ovarian status in heifers exposed to a progesterone insert with or without concurrent GnRH injection. Estrus was manipulated in 283 heifers (31 breeding clusters) by administering GnRH, progesterone, and PGF(2alpha) at 5 stages of the estrous cycle. Estrus was presynchronized with a progesterone insert (CIDR) for 7 d before PGF(2alpha) was administered 24 h before insert removal. Successive clusters of heifers were assigned to treatments (2 heifers per treatment) on cycle d 2, 5, 10, 15, and 18. Treatments consisted of a progesterone insert (d 0) for 7 d plus: 1) PGF(2alpha) on d 6, 24 h before insert removal (early PGF); 2) GnRH on d 0 + early PGF(2alpha) (GnRH + early PGF); 3) PGF(2alpha) at insert removal (late PGF); and 4) GnRH on d 0 + late PGF (GnRH + late PGF). Controls received GnRH on d 0 and PGF(2alpha) on d 7. Ovaries were scanned by transrectal ultrasonography on d 0, 2, 7, 9, and 11 to assess follicle diameters and ovulation. Blood was collected on d 0, 2, 6, 7, 8, and 9 to quantify serum concentrations of progesterone. Insemination occurred after detected estrus or by timed artificial insemination (TAI) at 64 h after insert removal. Only 25% of 141 GnRH-treated heifers ovulated by d 2; twice as many ovulated when treatment was initiated on d 5 (46.4%) than on other cycle days (20.3%). Diameters of the largest follicle exposed to GnRH on d 0, 2, 7, or 9 did not differ regardless of whether ovulation occurred. Small treatment and stage of cycle differences in diameter of the largest follicle were detected on d 2, 7, and 9. Compared with controls, progesterone concentration was greater in all progesterone-treated heifers on d 2 and 6. Early- vs. late-PGF treatment resulted in less progesterone on d 7 and 8. Pregnancies per AI were less after TAI (44%) than after detected estrus (56%) and were less in controls than in all progesterone treatments. Heifers in which treatments were initiated on d 10 of the cycle had the most consistent (estrus vs. TAI) pregnancies per AI (65.4%) compared with other cycle days. Compared with controls, more progesterone-treated heifers ovulated by 96 h after insert removal. Application of the progesterone insert reduced variance of the interval to estrus after insert removal (or PGF(2alpha) injection in controls) by 1.6-fold compared with controls. These results do not support the use of GnRH in a progesterone-based synchronization protocol.
authors.affiliation: Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-0201, USA. jss@ksu.edu
authors.forename: J S
authors.initials: JS
authors.lastname: Stevenson
mesh_terms: D000818:Animals; D002417:Cattle / Q000502:physiology*; D004971:Estrus / Q000187:drug effects*; D004973:Estrus Synchronization; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D007315:Insemination, Artificial / Q000662:veterinary; D006080:Ovarian Follicle / Q000187:drug effects*; D010060:Ovulation / Q000187:drug effects*; D011247:Pregnancy; D011374:Progesterone / Q000097:blood / Q000494:pharmacology*; D011372:Progestins / Q000494:pharmacology*; D011897:Random Allocation; D013997:Time Factors
chemical_list: D011372:Progestins; D007987:Gonadotropin-Releasing Hormone; D011374:Progesterone