Essential oil blend and monensin for feedlot steers consuming backgrounding and finishing diets: growth performance, methane emissions using the GreenFeed system, and feeding behavior

A study was conducted to assess growth performance, methane (CH₄) emissions, and feeding behavior of feedlot steers consuming backgrounding and finishing diets with an essential oil blend (EO), monensin (Mon), and their combination (EO + Mon). The study was structured as a 2 × 2 factorial, with two feed additive treatments (Control, EO) and two monensin treatments (no Monensin, Monensin). One hundred Angus × steers were evenly distributed across each treatment into four pens, and each dietary phase consisted of four, 28-d periods. Using the GreenFeed system to estimate CH₄ yield (g CH₄/kg dry matter intake [DMI]), the mean CV (coefficient of variation) within the same steer across measurement periods was 14.0% and 15.2% for the backgrounding and finishing phases, respectively. The mean CV of CH₄ production for steer within period and time-of-day block was up to 83.4% and 102.9% for the backgrounding and finishing phases, respectively. There was no difference in overall average daily gain (<i>P</i> ≥ 0.22) or gain:feed ratio (<i>P</i> ≥ 0.26) with EO or Monensin during the backgrounding or finishing phases. There were variable period effects on CH₄ emissions, but overall control had lower (EO × Mon; <i>P</i> ≤ 0.01) CH₄ production, yield, and CH₄ energy as a percentage of gross energy intake (GEI) (Ym, Mcal CH₄ % GEI), during the backgrounding phase. During the finishing phase, CH₄ yield (EO × Mon; <i>P</i> = 0.02) and Ym (EO × Mon; <i>P</i> ≤ 0.01) were greatest in the EO + Mon treatment. Monensin, irrespective of EO, decreased DMI (<i>P</i> < 0.01) and eating rate (<i>P</i> = 0.01), and increased intermeal interval (<i>P</i> < 0.01) for the backgrounding phase. In the finishing phase, Monensin decreased (<i>P</i> < 0.01) DMI, variation of DMI (EO × Mon), meal size, and eating rate. Number of GreenFeed visits was negatively (<i>P</i> < 0.01) correlated with all feeding behavior metrics in the finishing phase. In conclusion, supplementation of EO or Monensin and their combination into backgrounding and finishing diets of feedlot steers had no effect on overall feedlot performance or CH₄ emissions. Additionally, the high variability in CV of CH₄ production and yield suggests a need to improve the accuracy and precision of the GreenFeed system for estimating enteric CH₄ emissions from cattle.