title: The effects of GnRH administration postinsemination on serum concentrations of progesterone and pregnancy rates in dairy cattle exposed to mild summer heat stress.
abstract: The objective of this study was to evaluate whether administration of GnRH postinsemination would improve reproductive performance in heat-stressed dairy cattle. Estrous cycles of Holstein cows were synchronized using the OvSynch protocol and cows were artificially inseminated. Cows were then administered the following treatments: control (no GnRH; n=37), GnRH (100 microg) on Day 5 (GnRH-D5; n=34), or GnRH (100 microg) on Day 11 (GnRH-D11; n=34) postinsemination. Cows were provided access to both fans and sprinklers, and environmental data was collected hourly. Rectal temperatures and blood samples were obtained from cows on Days -9, -2, 0 (AI) and on alternate days from Day 5 to Day 19 postinsemination. Blood serum was collected for the analysis of progesterone (P(4)) by RIA. In a subset of cows (n=6/treatment) ultrasonography was performed on alternate days from Day 5 to Day 19 postinsemination to assess numbers of corpora lutea (CL) and CL cross-sectional areas. Pregnancy status of cows was confirmed at Day 30 postinsemination. Environmental data indicated that cows experienced mild heat stress during the trials (mean daily THI=73-77). Serum P(4) was greater (P<0.05) after Day 9 for GnRH-D5 cows and after Day 15 for GnRH-D11 cows through Day 19 postinsemination. The number of CLs present for GnRH-D5 cows was greater (P<0.05) on Day 17 than in either the control or GnRH-D11 treatment groups. On Day 17 postinsemination, both the GnRH-D5 and GnRH-D11 cows were observed to have greater (P<0.05) total CL tissue area than control cows. The interval from insemination to when serum P(4) returned to <1 ng/ml (i.e. luteolysis and return to estrus) did not differ (P>0.10) among treatment groups. Control cows (19%) tended to exhibit lower pregnancy rates (P<0.08) compared to the GnRH-D5 and GnRH-D11 treatment groups combined (35%). In summary, the treatment of heat-stressed dairy cows with GnRH postinsemination (Day 5 or 11) results in the appearance of more CL tissue, increased serum concentrations of P(4) and a tendency toward greater pregnancy rates.
authors.affiliation: Department of Animal and Dairy Sciences, Mississippi State University, Box 9815, Wise Center, Mississippi State, MS 39762, USA. swillard@ads.msstate.edu
authors.forename: S
authors.initials: S
authors.lastname: Willard
authors.forename: S
authors.initials: S
authors.lastname: Gandy
authors.forename: S
authors.initials: S
authors.lastname: Bowers
authors.forename: K
authors.initials: K
authors.lastname: Graves
authors.forename: A
authors.initials: A
authors.lastname: Elias
authors.forename: C
authors.initials: C
authors.lastname: Whisnant
mesh_terms: D000818:Animals; D001831:Body Temperature; D002417:Cattle / Q000502:physiology*; D003338:Corpus Luteum / Q000033:anatomy & histology / Q000502:physiology; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000008:administration & dosage*; D006358:Hot Temperature; D006813:Humidity; D007315:Insemination, Artificial / Q000662:veterinary*; D011247:Pregnancy; D011374:Progesterone / Q000097:blood*; D012621:Seasons; D013696:Temperature
chemical_list: D007987:Gonadotropin-Releasing Hormone; D011374:Progesterone