PS5-24. Effects of Peripartum Supplementation of Chromium, Rumen Protected Choline, or Both on Feed Efficiency Postpartum and Post Supplementation in Holstein Dairy Cows.

Abstract Feed efficiency (FE) is critical for improving nutrient utilization and reducing production costs in the dairy industry. Nutritional additives, such as chromium (Cr) and rumen-protected choline (RPC), are known to improve metabolic efficiency and energy partitioning. The objective of this study was to evaluate the effects of peripartum supplementation of Cr, RPC, or both (RPC+Cr) on FE metrics in dairy cows. Pregnant multiparous Holstein cows (n = 90) were randomly assigned to control (CTL), RPC (15g pre- and 30g postpartum, CholiGEM®, Kemin Industries), Cr (0.5mg/kg of diet DM, KemTRACE® Chromium, Kemin), or RPC+Cr for 22 ± 4d pre- and 23 ± 3d post-calving. Cows had ad libitum access to treatment (TRT) diets via Insentec Feeders (Hokofarm Group; 4 feeders/TRT). Following the postpartum supplementation period (SP), cows received a common lactation diet until 100 DIM (postSP). Milk yield (MY) was recorded twice daily, and milk composition was analyzed weekly. Feed efficiency metrics were based on milk/dry mater intake (DMI), energy-corrected milk (ECM/DMI), 4% fat-corrected milk (FCM/DMI), and milk energy output per net energy for lactation intake (MilkE/NEL). Data from SP and PostSP were analyzed separately using mixed models (SAS 9.4), with Cr, RPC, week (wk), and their interactions as fixed effects and random effect of cow nested within TRT and wk of enrollment. Statistical significance was declared at P ≤ 0.05, with tendencies defined as 0.05 < P ≤ 0.10. Means were separated by SLICE when wk interactions were P ≤ 0.10. Mean MY and DMI for CTL, Cr, RPC, and RPC+Cr during the SP were 47.3, 49.4, 50.6, 50.2 kg/d for MY and 24.3, 24.2, 25.6, 25.0 kg/d for DMI, respectively; corresponding values during the postSP were 53.6, 54.6, 56.6, 55.3 kg/d for MY and 29.5, 29.1, 30.0, 30.0 kg/d for DMI. During SP, an RPC × wk interaction was observed for Milk/DMI but means could not be separated. There was a Cr × wk interaction, where in wk 3, Milk/DMI tended to be greater (P = 0.10) for Cr-supplemented cows compared to cows that did not receive Cr (2.06 vs 1.96). Chromium × wk interactions were also observed (P = 0.02) for ECM/DMI and FCM/DMI during SP but means could not be separated. A tendency for an RPC × wk interaction was observed for energy-based efficiency (P = 0.06), where in wk 3, cows that received RPC tended to have greater MilkE/NEL than cows that did not receive RPC (0.80 vs 0.74). No differences were observed among TRT for any FE metrics during the postSP period (P ≥ 0.15). Overall, Cr and RPC supplementation led to wk-dependent responses, with improved FE particularly in wk 3, with no residual effects after supplementation.