Submission Number: GAR-4-10-48

Abstract Number: 302

CONTROL OF LACTATIONAL AND IDIOPATHIC ANOVULATION IN MARES WITH LOW-DOSE, NATIVE GNRH: ANTERIOR PITUITARY AND OVARIAN RESPONSES TO CONTINUOUS, SUBCUTANEOUS INFUSION.

GL Williams* ¹, M Amstalden* ¹, MR Garcia* ¹, SW Williams ¹, J Ward ², GP Blodgett ³ and KS Quirk ⁴

Animal Reproduction Laboratory, Texas A&M University Agricultural Research Station, Beeville, TX ¹
McDermott Ranch, Waller, TX ²
6666 Ranch, Guthrie, TX ³
Millennium Farms, Belton, TX ⁴
Valor Farm, Pilot Point, TX ⁵
JEH Stallion Station, Frisco, TX ⁶

Abstract:
Objectives were to 1) determine the efficacy of continuous s.c. infusion of GnRH for inducing follicular development and ovulation in mares exhibiting idiopathic or lactational anovulation, and 2) characterize anterior pituitary responses associated with this treatment. Forty-seven Thoroughbred and Quarter Horse mares at 6 locations were used. Idiopathic anovulation was defined as the condition of maiden or barren mares (n = 29) failing to develop a 35 mm follicle or to ovulate by April 1, or by March 1 if supplemental lighting had been supplied since mid- December. Lactational anovulation was defined as the condition of foaling mares (n = 18) failing to develop a 35 mm follicle or to ovulate within 21 days of foaling, or within 21 days of a non-fertile foal heat. Treatment groups were 1) GnRH/GnRH; s.c. Alzet osmotic pump delivering 2.5 g GnRH/h for the first 14 d (period 1), followed by a pump delivering 5 g/h during periods 2 and 3 if a 35 mm follicle or ovulation failed to occur during the preceding period, and 2) Control/GnRH; treatment identical to Group 1, except that no treatment was administered during period 1(control period). The percentage of GnRH/GnRH treated mares developing a 35 mm follicle, being bred and ovulating during period 1 was 50 and 37.5% in Idiopathic and Lactational mares, respectively, which was greater (p < 0.05) than in Control/GnRH mares (14 and 10% for Idiopathic and Lactational, respectively). Mares in the Control/GnRH group exhibited an abrupt increase (p < 0.05) in follicular development, breeding activity, and ovulation after GnRH pump insertion at the start of period 2. By the end of period 3, 80 to 85% of mares in both groups had ovulated and been bred, with pregnancy rates of 68.4 and 78.9% for Idiopathic and Lactational mares, respectively (p > 0.10). Average interval to conception was reduced (p < 0.01) by 10.3 d in GnRH/GnRH compared to Control/GnRH groups, all mares in Group 1 conceiving earlier than those in Group 2. Continuous GnRH infusion (2.5 g/h) resulted in a modest increase in mean LH (0.3 0.05 ng/ml) compared to that of untreated mares (0.1 0.01 ng/ml). This increase accompanied development of a preovulatory follicle and spontaneous ovulation. Low-dose, s.c. infusion of native GnRH induced ovulation in a high proportion of anovulatory mares during the operational breeding season.

Keywords: GnRH anovulation mares

This abstract is being presented at: 8:00 AM in session:
Follicular Development II