EFFECTS OF ACTIVE IMMUNIZATION OF PREPUBERTAL HEIFERS AGAINST PROSTAGLANDIN-F2-ALPHA ON THE ONSET OF PUBERTY AND SUBSEQUENT OVARIAN-FUNCTION

The objective of these two experiments was to determine the effect of immunization of prepubertal heifers against prostaglandin F-2 alpha conjugated to human serum albumin (PGF-HSA) on the onset of puberty and subsequent ovarian function. Heifers that were 2-3 months of age (n = 34; mean +/- standard error of the mean (SEM) bodyweight 75 +/- 1.4 kg) were given a primary immunization (Day 0) and a booster 183 days later using one of two adjuvants; non-ulcerative Freund's adjuvant (NUFA) and diethylaminoethyl-dextran (DEAE-dextran) and one of two doses of PGF-HSA as immunogen (1 and 10 mg), with six to seven heifers per treatment; control heifers (n= 7) were untreated, The experiment lasted up to 562 days. Plasma antibody titres every 2-3 weeks and progesterone concentrations two to three times weekly were determined. Oestrous behaviour was observed twice daily. In addition, the follicular status of ovaries of cyclic (n = 4), prepubertal (n = 6) or prolonged luteal phase ( > 200 days, induced by immunization against PGF) heifers (n = 5) were monitored daily for 23 consecutive days by ultrasound examination. Antibody titres were higher (P < 0.05) immediately following booster immunization in heifers immunized with DEAE-dextran than with NUFA. High titres were maintained for longer (P < 0.05) in heifers immunized with NUFA than with DEAE-dextran. The mean number of oestrous periods was decreased (P < 0.05) in PGF-immunized heifers in comparison with control heifers following booster immunization (3.1, 0.5, 0.9, 1.5, 1.1 in control heifers and in heifers immunized with NUFA-1 mg, NUFA-10 mg, DEAE-dextran-1 mg and DEAE-dextran-10 mg, respectively. Puberty was delayed (P < 0.05), relative to control heifers, by 185 and 122 days (pooled standard error of the difference, 89) in heifers immunized with NUFA-1 mg and DEAE-dextran-10 mg, respectively. After first ovulation at puberty, persistent corpora lutea (CL) formed in two of six heifers immunized with NUFA-1 mg, five of seven immunized with NUFA-10 mg, two of seven immunized with DEAE-dextran-1 mg and one of seven immunized with DEAE-dextran-10 mg. The duration of persistence was 103 +/- 19, 301 +/- 33, 124 +/- 23 and 100 days, respectively, The mean (+/- SEM) number of dominant follicles was not different (P > 0.05) for the three groups of heifers that were examined by ultrasound (3.7 +/- 0.2, 3.8 +/- 0.3 and 3.2 +/- 0.3 for cyclic, prepubertal and persistent CL, respectively); neither was their rate of growth (1.8 + 0.2, 1.5 +/- 0.1 and 1.5 +/- 0.2 mm day(-1), respectively), rate of atresia (1.3 +/- 0.1, 1.6 +/- 0.1 and 1.8 +/- 0.2 mm day(-1), respectively) or the maximum size of dominant follicles (11.2 +/- 0.4, 9.6 +/- 0.3 and 12.4 +/- 1.8 mm, respectively). In summary, immunization of prepubertal heifers against PGF delayed the onset of puberty and reduced subsequent oestrous behaviour due to the formation of persistent CL following first ovulation and there was a similar growth pattern of dominant follicles before first ovulation in prepubertal heifers, cyclic heifers and heifers which have long-term persistent CL.