PROSTAGLANDIN-F2-ALPHA IMMUNIZATION OF PREPUBERTAL BEEF HEIFERS - EFFECTS OF CONJUGATE DOSE AND TIMING OF IMMUNIZATION RELATIVE TO PUBERTY ON THE ONSET OF PUBERTY AND SUBSEQUENT OVARIAN-FUNCTION

Fifty-six prepubertal Hereford cross Friesian heifers were assigned to seven treatment groups: (1) primary (day 0) and booster (day 41) usin g 10 mg human serum albumin (HSA) (control); (2) primary and booster ( day 41) immunizations using 3.3 mg prostaglandin F-2 alpha PGF-HSA con jugate; (3) primary and booster (day 83) using 3.3 mg PGF-HSA; (4) pri mary and booster (day 210) using 3.3 mg PGF-HSA; (5-7) as in treatment s 2-4 except 10 mg PGF-HSA were used. Plasma progesterone concentratio ns were used to determine the onset of puberty and the presence of a c orpus luteum (CL); plasma PGF antibody titres were determined at 28-da y intervals. There was no effect (P > 0.05) of conjugate dose or boost er interval on mean antibody titres and there was no interaction betwe en them. However, heifers in the 83- and 210-day booster treatments ha d higher (P < 0.05) peak antibody titres than heifers in the 42-day bo oster treatments. Puberty was delayed in 40% (16/40) of PGF-immunized heifers and 40% (16/40) of heifers formed persistent CL after puberty. Overall, eight of the heifers with delayed puberty also formed a pers istent CL. There was a positive correlation between mean titre and ons et of puberty but not with duration of persistent CL.