23 Effect of extended days on feed on growth performance, efficiency, and carcass characteristics of steers and heifers of different proportions of angus and Limousin genetics.

Abstract Recently, cattle feeders have extended days on feed (DOF) resulting in hot carcass weights (HCW) exceeding the limits of existing serial slaughter datasets. Our objective was to determine the impact that extended DOF exerts on growth performance, feed efficiency, and carcass characteristics of steers and heifers with varying proportions of Angus and Limousin genetics. In 2022, commercial Angus cows from two MT ranches were artificially inseminated to 9 sires from 3 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, SD and pre-conditioned until study initiation in January 2024. Calves were allotted to 1 of 3 pens (~72 hd/pen) and fed a common finishing diet ad libitum. Insentec nodes (12/pen) captured individual feed disappearance. 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 heifers to accommodate feeding melengestrol acetate. 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. Respective groups were weighed and shipped to a commercial abattoir for slaughter on d 200, 235, and 270, respectively. Data were analyzed with fixed effects of sire breed, sex, and DOF and all resulting interactions. Source herd was a random effect; individual animal was the experimental unit. 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 G:F (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 G:F (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, less RF and marbling (P < 0.05) than heifers. Different genotypes responded similarly to extended DOF. Increasing DOF resulted in fatter, heavier carcasses with increased marbling in steers, but did not increase marbling in heifers.