130 Late-Breaking: Sustainability assessment of sub-species and management on growth performance and gas flux of steers finished in summer conditions.

Abstract Objectives of this study were to evaluate the influence of cattle sub-species and management on the comparative growth performance, carcass characteristics, and gas flux in yearling Bos indicus (BI; Brahman) and Bos taurus (BT; Angus) steers managed with (TRT) and without (CON) the use of growth-promoting technology (GPT). One hundred BI (initial body weight (IBW) = 342 ± 31 kg) and 100 BT (IBW = 341 ± 21 kg) steers were fed for 180d in 2 consecutive phases. In Phase 1, d 0-83, cattle of each sub-species were blocked by weight and randomly assigned to management treatment (20 pens, 5 pens/treatment). In Phase 2, d 84-180, cattle were moved and randomly assigned to a Climate Smart Research Pen (CSRP; 1 pen/treatment) each equipped with 1 automated head chamber system (AHCS; C-Lock, Rapid City, SD, USA) and 5 SmartFeed bunk systems (C-Lock, Rapid City, SD, USA) for measuring individual gas flux (methane (CH4), carbon dioxide (CO2), oxygen (O2), and hydrogen (H2)) and feed intake, respectively. Data were analyzed with R (R Core Team, 2021, v. 4.4.1) software to assess the fixed effects of cattle sub-species, management treatment, and their interaction. In phase 1, IBW did not differ (P > 0.75) by sub-species or management. In phase 1, dry matter intake (DMI), average daily gain (ADG), and feed efficiency (G:F) were greater (P < 0.01) for BT, resulting in greater final body weight (FBW) for BT compared to BI. Within sub-species, DMI did not differ (P > 0.35) between treatments in Phase 1, but ADG, G:F, and FBW were greater (P < 0.01) for TRT compared to CON. In Phase 2, DMI increased with GPT, with additional growth performance observations in Phase 2 being consistent with Phase 1. However, there was a sub-species × treatment interaction (P ≤ 0.04) where greater increases in ADG and FBW were observed between TRT and CON for BT relative to BI. Furthermore, BI had a greater proportion of less desirable, Standard and Select quality grades relative to BT. Daily CH4 production (g CH4/d), CH4 yield (g CH4/kg DMI), and yield of CH4 (% of gross energy intake) were less (P < 0.01) for BI than BT. The use of GPT decreased emissions intensity per unit ADG and carcass gain; furthermore, a sub-species × treatment interaction existed (P ≤ 0.05), where a greater decrease in EI was observed between CON and TRT for BT when compared to BI. Ultimately, BT had greater growth performance and carcass quality, but BI produced less CH4, highlighting the complex tradeoffs resulting from sustainability assessments in beef production systems.