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Journal of Animal Science, Vol 74, Issue 5 1092-1097, Copyright © 1996 by American Society of Animal Science

JOURNAL ARTICLE

Ovarian response after gonadotropin treatment of heifers immunized against gonadotropin-releasing hormone

D. K. Bishop, R. P. Wettemann, J. V. Yelich and L. J. Spicer
Animal Science Department, Oklahoma Agricultural Experiment Station, Stillwater 74078-0425, USA.

Angus x Hereford heifers were used to determine whether the ovaries of heifers immunized against GnRH would respond to exogenous gonadotropins. Heifers (n = 23; 24 mo; 491 +/- 23 kg) were assigned to a 2 x 2 factorial experiment: immunization against GnRH conjugated to human serum albumin (GnRH-IMM) or human serum albumin alone (HSA-IMM), and treatment with gonadotropins (GTH) or saline. The GTH treatment was 2,000 IU PMSG + 1,000 iu hCG, and control heifers were given saline (s.c.). Immunization against GnRH (GnRH-IMM) caused cessation of ovulation (progesterone < 1 ng/mL plasma for 3 wk) and GTH treatment began within 3 to 5 wk after heifers became anovulatory. Heifers immunized against HSA (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) and 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 day through d 24 to quantify progesterone and estradiol. Concentrations of estradiol increased with time after GTH but the response was reduced by GnRH-IMM (IMM x GTH x day, P < .01). Number of follicles > or = 6 mm (d 0 to 14) was increased after GTH (GTH x day, P < .001) but was decreased (P < .03) by GnRH-IMM. Immunization against GnRH reduced total follicles > or = 6 mm on d 4 in heifers on both saline and GTH treatments. Treatment with gonadotropins increased the number of follicles > or = 16 mm on d 14 compared with control heifers. The number of CL was increased by 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 treatment with gonadotropins will induce follicular growth and ovulation in heifers that are anovulatory due to immunization against GnRH.


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