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 co njugated 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 lat er 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 trea tment; 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 beh aviour was observed twice daily. In addition, the follicular status of ovaries of cyclic (n = 4), prepubertal (n = 6) or prolonged luteal ph ase ( > 200 days, induced by immunization against PGF) heifers (n = 5) were monitored daily for 23 consecutive days by ultrasound examinatio n. Antibody titres were higher (P < 0.05) immediately following booste r 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 wa s delayed (P < 0.05), relative to control heifers, by 185 and 122 days (pooled standard error of the difference, 89) in heifers immunized wi th NUFA-1 mg and DEAE-dextran-10 mg, respectively. After first ovulati on at puberty, persistent corpora lutea (CL) formed in two of six heif ers 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 immuni zed with DEAE-dextran-10 mg. The duration of persistence was 103 +/- 1 9, 301 +/- 33, 124 +/- 23 and 100 days, respectively, The mean (+/- SE M) number of dominant follicles was not different (P > 0.05) for the t hree 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 max imum size of dominant follicles (11.2 +/- 0.4, 9.6 +/- 0.3 and 12.4 +/ - 1.8 mm, respectively). In summary, immunization of prepubertal heife rs against PGF delayed the onset of puberty and reduced subsequent oes trous 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 hei fers which have long-term persistent CL.