title: Distribution and repeatability of anterior pituitary responses to GnRH and relationship of response classification to the postpartum anovulatory interval of beef cows.
abstract: Our objectives were to investigate the phenotypic variation in anterior pituitary responsiveness to GnRH (100 microg, i.v.) of beef cows between d 5 and 8 postpartum, estimate repeatability, and determine the relationship between response classification and duration of the postpartum anovulatory interval (PPI). Brahman x Hereford (F1) cows (n = 137) and primiparous heifers (n = 58) were evaluated. Response classifications (Class) included peak LH (Low, Intermediate, or High; Class I) and time to peak LH (Early, 10 to 30 min or Late, 60 to 120 min; Class II). The independent effects of Class I and II on PPI were determined in 145 of 195 cows through twice-weekly serum samples analyzed for progesterone. For Class I, pituitary responses to GnRH approximated a normal distribution and, by definition, differed (P < .001) in magnitudes of peak LH and area under the curve (AUC). For Class II, 111 and 84 cows exhibited early and late peaks, respectively; mean AUC was greater (P < .05) in cows exhibiting late compared with early peaks. Pretreatment LH (P < .01) and estradiol-17beta (P < .004) influenced responses in one or both response classes. Pluriparous cows had shorter (P < .035) PPI than primiparous cows. Class I did not influence the duration of the PPI; however, in Class II, cows with late peaks exhibited an average PPI that was 8 d shorter (P < .025) than in those with an early peak. To estimate repeatability of pituitary responses, 18 classified cows were subsequently rechallenged with GnRH at d 170 of gestation and at the next postpartum period. Although means for each of these challenges differed (P < .05) throughout in both Classes I and II, the small sample size used to make the estimate failed to yield significant (P > .10) interclass correlations. Nevertheless, overall results provide evidence that variability in individual pituitary responses to GnRH could be targeted as a selection marker to improve reproduction.
authors.affiliation: Animal Reproduction Laboratory, Texas A&M University, Agricultural Research Station, Beeville 78102-9410, USA.
authors.forename: P
authors.initials: P
authors.lastname: Fajersson
authors.forename: R L
authors.initials: RL
authors.lastname: Stanko
authors.forename: G L
authors.initials: GL
authors.lastname: Williams
mesh_terms: D000818:Animals; D000858:Anovulation; D001800:Blood Specimen Collection; D002417:Cattle / Q000502:physiology*; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D016018:Least-Squares Analysis; D010903:Pituitary Gland, Anterior / Q000187:drug effects*; D049590:Postpartum Period
chemical_list: D007987:Gonadotropin-Releasing Hormone