Continuous administration of low-dose GnRH in mares I. Control of persistent anovulation during the ovulatory season.
Three experiments were conducted during the operational breeding season to confirm that continuous, subcutaneous infusion of low-dose GnRH would not disrupt established estrous cycles (Experiment 1), and test the hypotheses that a similar treatment would stimulate secretion of LH and induce development of ovulatory follicles in persistently anovulatory mares (Experiments 2 and 3). Treatment with GnRH (5 microg/h) increased (P<0.001) serum P4 during the luteal phase (7.7+/-0.5 versus 6.4+/-0.5 ng/mL), tended to increase serum LH (2.6+/-0.27 versus 1.9+/-0.25 ng/mL), and did not modify interovulatory intervals. In Experiment 2, GnRH treatment (2.5-5 microg/h) of persistently anovulatory mares increased (P<0.001) serum LH compared to controls (0.5+/-0.08 versus 0.1+/-0.03 ng/mL), with all GnRH-treated and no Control mares ovulating. Mares exhibiting Delayed Recrudescence (n=29) or Lactational Anovulation (n=18), were assigned randomly in Experiment 3 to receive either (1) GnRH/GnRH (n=23); 2.5 microg GnRH/h for 14 d (Period I) and 5 microg/h during the subsequent 28 d (Periods II and III); or (2) Control/GnRH (n=24); no treatment during Period I (control period) and GnRH treatments as in 1 during Periods II and III. Percentage of mares ovulating and pregnant during Period I was greater (P<0.05) for GnRH-treated than Control mares. Thereafter, cumulative ovulation frequency (85%), pregnancy (72%) and cycles/conception (1.3+/-0.2) were similar between groups; however, interval to conception was reduced (P<0.01) by 10.3 d in GnRH/GnRH compared to Control/GnRH.
Williams
G L
GL
Animal Reproduction Laboratory, Texas A&M University Agricultural Research Station, Beeville, TX 78102, USA. glwilliams@tamu.edu
Amstalden
M
M
Blodgett
G P
GP
Ward
J E
JE
Unnerstall
D A
DA
Quirk
K S
KS
D000818:Animals; D000858:Anovulation / Q000188:drug therapy / Q000662:veterinary*; D004971:Estrus / Q000187:drug effects*; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000008:administration & dosage / Q000627:therapeutic use*; D006734:Horse Diseases / Q000188:drug therapy*; D006736:Horses; D007986:Luteinizing Hormone / Q000378:metabolism; D006080:Ovarian Follicle / Q000187:drug effects / Q000254:growth & development; D012621:Seasons
D007987:Gonadotropin-Releasing Hormone; D007986:Luteinizing Hormone