The Institutional Animal Care and Use Committee approved all of the procedures used in this experiment at the University of Nebraska. Postpubertal female cattle of composite breeding (n = 21; 1/4 Hereford, 1/4 Angus, 1/4 Red Poll, 1/4 Pinzgauer; 439.9 ± 12.2 kg of body weight) were used in this study. Estrus was synchronized to a common day with two injections of prostaglandin F2a (PGF2a; Lu-talyse; Pharmacia & Upjohn, Kalamazoo, MI) given 11 days apart.
To synchronize stage of dominant ovarian follicle development among animals so that dominant follicles would be at the same stage of development at the time of luteol-ysis, all follicles larger than 5 mm were aspirated by trans-vaginal procedures 4 days before the second treatment of PGF2a. Aspiration of follicles was performed using a 5.0-MHz convex array probe with a device attached to guide a needle into ovarian follicles that were aspirated while visualized on a monitor (Aloka 500V Corometrics, Wallingford, CT).
Animals were randomly assigned to one of the following treatments: 1) control (n = 5; 5% mannitol); 2) LHRH antagonist; ^-Ac-D-Nal!,4Cl-D-Phe2,D-Pal3,D-Cit6,D-Ala10-LHRH) starting 2 days before initiation of the preovulatory surge of LH (Antagonist [Ant] -2; n = 6); 3) LHRH antagonist at initiation of the preovulatory LH surge (Ant 0; n = 5); and 4) LHRH antagonist starting 2 days after the preovulatory LH surge (Ant 2; n = 5). LHRH antagonist was administered s.c. every 24 h at 10 ^g/kg BW until Day 7 of the estrous cycle to all females in treated groups.