Grace Olinger1, Zach Smith1, Forest Francis2, Becca Grimes Francis1, Riley Leeson1, Michael Gonda1, Robert L. Weaber3, and Warren Rusche1 Department of Animal Science, South Dakota State University, Brookings, SD 57007, 2Department of Animal Science and Veterinary Technology, Texas A&M University, Kingsville, TX 78363, 3Eastern Kansas Research and Extension Centers, Kansas State University, Manhattan, KS 66506 1 Cattle feeders have extended days on feed (DOF) in recent years resulting in hot carcass weights (HCW) exceeding the upper bounds of existing serial slaughter datasets. The goal of this study was to determine the impact extended DOF has on growth performance, feed efficiency and carcass characteristics of steers and heifers with differing degrees of Angus and Limousin genetics. In 2022, commercial Angus cows from two Montana ranches were artificially inseminated to nine sires from three breeds (Angus, AN; Lim-Flex, LF; and Limousin, LM) with parentage confirmed by DNA. In 2023, 216 calves (72 AN, 73 LF and 71 LM; 105 steers and 111 heifers) were shipped to Brookings, South Dakota, and pre-conditioned until study initiation January 2024. Calves were allotted to one of three pens (~72 hd/pen) and fed a common finishing diet ad libitum. Individual feed intake was measured via Insentec nodes (12/pen). Cattle were implanted on d 28 (100 mg trenbolone acetate and 14 mg estradiol benzoate; SYNOVEX Choice, Zoetis, Parsippany, NJ) and reimplanted on d 104 (200 mg trenbolone acetate and 28 mg estradiol benzoate; SYNOVEX One Feedlot, Zoetis). On d 78, Insentec nodes were designated for steers or heifers to accommodate melengestrol acetate administration to heifers. On day 141, cattle were assigned to one of three slaughter groups with equivalent body weights and balanced across sex, sire breed and sire within breed. Outcome groups were targeted for 200, 235 and 270 DOF at which point they were weighed and shipped to a commercial abattoir for slaughter the following morning. Data were analyzed using PROC GLIMMIX with fixed effects of sire breed, sex and DOF with source herd as a random effect. Individual animal was the experimental unit. Percent USDA Choice and Prime exceeded 90% for all slaughter groups. There was a breed × sex tendency for dressing percentage with greater values for LM and LF heifers (P < 0.09). Extending DOF increased final weight (FBW) and HCW (P < 0.01), ribeye area (REA: P < 0.01), and rib fat (RF: P = 0.01). Increased DOF increased carcasses with HCW > 1100 lbs (P < 0.01) and increased marbling in steers (P = 0.02) but not heifers (P = 0.93). Additional DOF did not affect Cumulative ADG (P = 0.24) but did worsen F:G (P < 0.01). Sire breed did not affect FBW, HCW or ADG (P ≥ 0.76), but increasing Limousin influence reduced DMI (P < 0.01), improved F:G (P < 0.01) and reduced percentage YG 4 & 5 (P = 0.04). Greater AN influence increased marbling, RF, and YG (P < 0.05). Steers were heavier and more efficient (P < 0.01) with greater REA and less RF and marbling (P < 0.05) compared to heifers. Cattle with different genotypes responded similarly to extended DOF and increasing DOF resulted in fatter and heavier carcasses with increased marbling in steers. STRENGTH IN GENETICS, EXCELLENCE IN PERFORMANCE! A Huge Thank You To Our 2025 Bull Sale Buyers! We greatly appreciate the trust and confidence our buyers placed in our genetics. Your support drives us to continue offering top-tier livestock that deliver superior performance and value. It’s a pleasure to be a part of your operation’s success. We look forward to seeing the results from these exceptional bulls. BL Money Talks 426M HP • HB • 52% Lim-Flex MIDL Genisus 159G x BL Kit and Kaboodle 267K CE: 14 BW: 0.1 WW: 94 YW: 163 MK: 29 RE: 0.61 MB: 0.38 $TPI: 114 Thank you, South Fork Limousin of Fort Pierre, SD. BL Legion 3123L HP • HB • 51% Lim-Flex AHCC Growth Fund 266J x BL Howdy Do 022H CE: 9 BW: 0.5 WW: 76 YW: 125 MK: 29 RE: 0.69 MB: 0.30 $TPI: 112 Thank you, Rafter DP Farm & Ranch of Geary, OK. Wade & Vicki Beckman 208.313.0235 • Sedar & Danna Beckman 208.521.4628 • Roberts, ID • APRIL 2025 37