OVARIAN RESPONSE AFTER GONADOTROPIN TREATMENT OF HEIFERS IMMUNIZED AGAINST GONADOTROPIN-RELEASING-HORMONE

Angus x Hereford heifers were used to determine whether the ovaries of heifers immunized against GnRH would respond to exogenous gonadotropi ns. Heifers (n = 23; 24 mo; 491 +/- 23 kg) were assigned to a 2 x 2 fa ctorial experiment: immunization against GnRH conjugated to human seru m albumin (GnRH-IMM) or human serum albumin alone (HSA-IMM), and treat ment with gonadotropins (GTH) or saline. The GTH treatment was 2,000 I U PMSG + 1,000 IU hCG, and control heifers were given saline (s.c.). I mmunization against GnRH (GnRH-IMM) caused cessation of ovulation (pro gesterone < 1 ng/mL plasma for 3 wk) and GTH treatment began within 3 to 5 wk after heifers became anovulatory. Heifers immunized against HS A (HSA-IMM) were treated between d 9 and 11 of the estrous cycle. All heifers were given PGF (i.m.) on d 2 (d 0 = start of GTH treatment) an d 2,000 IU of hCG (i.m.) on d 4. Ovaries of heifers were evaluated by ultrasound. Blood was sampled daily from d 0 to d 8 and every second d ay through d 24 to quantify progesterone and estradiol. Concentrations of estradiol increased with time after GTH but the response was reduc ed by GnRH-IMM (IMM x GTH x day, P < .01). Number of follicles greater than or equal to 6 mm (d 0 to 14) was increased after GTH (GTH x day, P < .001) but was decreased (P < .03) by GnRH-IMM. Immunization again st GnRH reduced total follicles greater than or equal to 6 mm on d 4 i n heifers on both saline and GTH treatments. Treatment with gonadotrop ins increased the number of follicles greater than or equal to 16 mm o n d 14 compared with control heifers. The number of CL was increased b y GTH (GTH x day, P < .001) but the response was reduced (P < .03) in GnRH-IMM heifers compared with HSA-IMM heifers. We conclude that treat ment with gonadotropins will induce follicular growth and ovulation in heifers that are anovulatory due to immunization against GnRH.