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) using 10 mg human serum albumin (HSA) (control); (2) primary and booster (day 41) immunizations using 3.3 mg prostaglandin F2 alpha PGF-HSA conjugate; (3) primary and booster (day 83) using 3.3 mg PGF-HSA; (4) primary and booster (day 210) using 3.3 mg PGF-HSA; (5-7) as in treatments 2-4 except 10 mg PGF-HSA were used. Plasma progesterone concentrations were used to determine the onset of puberty and the presence of a corpus luteum (CL); plasma PGF antibody titres were determined at 28-day intervals. There was no effect (P > 0.05) of conjugate dose or booster interval on mean antibody titres and there was no interaction between them. However, heifers in the 83- and 210-day booster treatments had higher (P < 0.05) peak antibody titres than heifers in the 42-day booster 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 persistent CL. There was a positive correlation between mean titre and onset of puberty but not with duration of persistent CL.