Immunization of heifers against gonadotropin releasing hormone delays puberty and causes the cessation of estrous cycles

Abstract

Ten prepubertal Angus X Hereford heifers at 12 months of age were used to determine the effects of primary immunization and booster immunizations against GnRH on ovarian cycles. Five heifers were immunized against GnRH conjugated to human albumin using Freund's complete adjuvant. Booster immunizations against GnRH with Freund's incomplete adjuvant were given in Weeks 6 (B1), 25 (B2) and 46 (B3) after the initial treatment. Progesterone was quantified in blood plasma each week for 68 weeks to assess luteal activity (LA). The initiation of pubertal ovarian cycles occurred at 9.6±1.4 and 20.8±2.4 weeks ( P<0.05) after the primary immunization for control and treated heifers, respectively. Control heifers had normal LA throughout the remainder of the study. Immunization against GnRH did not influence the ovulatory surge of luteinizing hormone after puberty, when titers against GnRH were minimal. Luteal activity ceased in three of five heifers at 1.0±0.6 weeks after B2 and was reinitiated 11.3±0.3 weeks later. After B3, LA ceased in four of five heifers within 3.0±0.7 weeks and was reinitiated 8.2±1.9 weeks later. All heifers became pregnant when exposed to a fertile bull between 100 and 160 days after B3. Immunization of prepuberal heifers against GnRH delayed the initiation of normal ovarian cycles. Booster immunizations against GnRH, after immunized heifers initiated ovarian cycles, caused the cessation of cycles for about 2 months.