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

Abstract

In a 172-d finishing trial (Exp. 1), 210 recently weaned crossbred heifers were allotted to six growth promotant treatment groups, involving implanting initially with Synovex-C (C) or H (H) followed by reimplanting with Finaplix-H (F) or H and F. Melengestrol acetate (MGA) was provided in the diet to 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 with follicles but also had lower (P = .01) gain/DMI. In a 182-d finishing study (Exp. 2), 270 recently weaned crossbred heifers were allotted to the following six implant (d 0)/ reimplant (d 70) groups using no implant (N), Ralgro (R) or H: N/R, R/H, R/R, N/R, H/H and R/R for Treatments 1 through 6, respectively. On d 70, all heifers were implanted with F. Heifers were fed MGA from d 70 to 182 (Treatments 1, 2, and 3) or for the entire trial (Treatments 4, 5, and 6). Implanting on d 0 increased (P < .05) overall ADG. Differences (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. In 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 with 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 was not beneficial for young heifers, particularly if they are provided an initial estrogenic implant followed by a second implant. In older (yearling) heifers, increased gains and gain/DMI were obtained by feeding MGA and implanting initially or 56 d later with F.