Growth-promoting systems for heifer calves and yearlings finished in the feedlot

In a 172-d finishing trial (Exp. 1), 210 recently weaned crossbred heifers were allotted to six growth promotant treatment groups, involving implantin g initially with Synovex-C (C) or H (H) followed by reimplanting with Finap lix-H (F) or H and F. Melengestrol acetate (MGA) was provided in the diet t o four of the treatment groups. Heifers fed MGA and administered only F as the terminal implant had the greatest (P = .01) number of mature ovaries wi th follicles but also had lower (P = .01) gain/DMI. In a 182-d finishing st udy (Exp. 2), 270 recently weaned crossbred heifers were allotted to the fo llowing six implant (d 0)/reimplant (d 70) groups using no implant (N), Ral gro (R) or H: N/R, R/H, R/R, N/R, H/H and R/R for Treatments 1 through 6, r espectively. On d 70, all heifers were implanted with F. Heifers were fed M GA from d 70 to 182 (Treatments 1, 2, and 3) or for the entire trial (Treat ments 4, 5, and 6). Implanting on d 0 increased (P <.05) overall ADG. Diffe rences (P > .05) in performance were not found between MGA treatment groups . Using an H implant/reimplant regimen decreased (P = .01) ovarian and(or) follicular development when compared with an R implant/reimplant regimen. I n a 126-d finishing trial(Exp. 3), 360 crossbred yearling heifers were used to evaluate F and estrogen (Implus-H) implants when used in combination wi th an MGA feeding program. Heifers receiving only F in combination with MGA had greater (P <.05) ADG, whereas all heifers fed MGA had greater (P < .05 ) gain/DMI than heifers not fed MGA. These data suggest that feeding MGA wa s not beneficial for young heifers, particularly if they are provided an in itial estrogenic implant followed by a second implant. In older (yearling) heifers, increased gains and gain/DMI were obtained by feeding MGA and impl anting initially or 56 d later with F.