85. Interaction Between Breed and Optaflexx Supplementation and Implant Strategy on Intake and Carcass.

Abstract Beef cattle breeds differ by fat deposition, muscle accretion, growth rate, and nutrient partitioning, and therefore may respond differently to growth-promoting technologies. The objective was to determine whether breed-type influenced response to Optaflexx supplementation and implant strategy. Treatments were randomly applied (stratified by breed of sire) for 1,390 steers from the Germplasm Evaluation (GPE) herd at the U.S. Meat Animal Research Center (Clay Center, NE). The GPE herd is comprised of 18 breeds highly utilized in U.S. beef production. Steer breed composition was partitioned and classified by biological-type, including British, Continental, and Brahman. All steers received an initial Revalor-IS implant (80 mg trenbolone acetate and 16 mg estradiol) and were assigned to a moderate implant strategy using Revalor-S (120 mg trenbolone acetate and 24 mg estradiol) or an aggressive strategy using Revalor-200 (200 mg trenbolone acetate and 20 mg estradiol). Within the moderate strategy, a subset received Optaflexx (Elanco Animal Health, Indianapolis, IN, n = 361) during the last 28-34 days of finishing, whereas the remainder did not (n = 341). Similarly, within the aggressive strategy, a subset received Optaflexx (n = 330), and the remainder was unsupplemented (n = 358). Models included fixed effects of feed additive, implant strategy, growing season, expected heterosis, age at slaughter, and breed-type. Three-way interactions among breed-type, Optaflexx supplementation, and implant strategy, as well as two-way interactions among breed-type x Optaflexx and breed-type x implant strategy were evaluated using linear regression models in R. Carcass characteristics and average dry matter intake (DMI) during the Optaflexx supplementation period were evaluated. British-influenced steers fed Optaflexx had a lower USDA yield grade (-0.19; P < 0.01), reduced 12th-rib fat thickness (-0.10 cm; P = 0.01), and greater slice shear force (+0.03 kg; P = 0.01) compared to unsupplemented steers. Ribeye area and dressing percentage tended to be greater (+1.50 cm2 and +0.30 percentage units; P = 0.09 and P = 0.06, respectively) in British-influenced steers fed Optaflexx than those unsupplemented. Brahman-influenced steers fed Optaflexx had greater fat deposition than steers not fed Optaflexx (+0.44 cm; P = 0.01). Continental-influenced steers supplemented with Optaflexx had dressing percentage 0.50 percentage units less than unsupplemented steers (P = 0.03). No breed type x supplementation interactions were observed for DMI during the Optaflexx period (P ≥ 0.29). Breed-type did not interact with implant strategy (P ≥ 0.12) for carcass characteristics or DMI during the Optaflexx period. Overall, although some carcass traits exhibited breed-specific responses to Optaflexx supplementation, most traits were unaffected, suggesting similar management strategies may be appropriate across breed types.