Technical Abstract: Three experiments were conducted to evaluate whole soybeans (SB) and either dried distillers grains (DDG) or wet corn gluten feed (WCGF) as protein and energy sources in beef heifer development diets. Crossbred heifers were fed diets containing 1.25 kg (DM) SB or 2.0 kg WCGF beginning at 10 mo of age (Exp. 1; n=104), 1.25 kg SB or 1.25 kg DDG beginning at 6 mo of age (Exp. 2; n=100), and 1.25 kg SB beginning at 7 or 10 mo of age (Exp. 3; n=100). Heifers were fed melengestrol acetate for 14 d followed by an injection of prostaglandin F2' (PGF2') 19 d later to synchronize estrus. Heifers were artificially inseminated for 5 d after PGF2', and subsequently exposed to bulls for ~60 d beginning 10 d after the last day of AI. Pregnancy to AI was determined by ultrasound 45 d after the last day of AI. Heifers fed SB in Exp. 1 had a lower (P < 0.05) synchronization rate (81 vs. 96%) and longer interval (P = 0.05) from PGF2' to estrus (3.2 vs. 2.9 d) compared to heifers fed WCGF. In Exp. 2, synchronization rate (84 vs 88%) and timing of estrus after PGF2' (2.9 vs. 2.9 d) did not differ due to diet. In Exp. 3, age heifers were started on SB diets did not alter synchronization rate (79%) or timing of estrus after PGF2' (3.2 d) There were no differences due to diet (P > 0.10) for AI conception (76.5, 68.5, and 60%), AI pregnancy (67, 59, and 46%), or final pregnancy rates (92, 90 and 90%) in Exp. 1, 2, or 3, respectively. An HPLC analysis of extracted SB detected presence of three phytoestrogens. The interaction of phytoestrogens and age of heifers at the time of feeding may have resulted in the lower synchronization rate and delayed estrus response in Exp. 1 when compared to Exp. 2. In summary, SB, DDG and WCGF all appear to be feasible protein and energy supplements in heifer development diets at the inclusion rates used in these studies.