Beef Steers and Enteric Methane: Reducing Emissions by Managing Forage Diet Fiber Content

**Background:** Enteric methane from ruminants is a major greenhouse gas contributor, particularly in Uruguay where livestock accounts for 70% of total GHG emissions and enteric fermentation contributes 46%. Diet quality, especially neutral detergent fiber (NDF) content, is a key regulator of methanogenesis, but studies on how NDF content affects methane emissions in forage-based diets have yielded inconsistent results. Accurate prediction models for methane emissions from forage diets are needed but validation remains limited.

**Methods:** Thirty-six 18-month-old Angus steers (mean BW 437 ± 7 kg) were blocked by live weight and randomly assigned to two forage diets (n=18 per group) over 97 days (April–July 2021) at INIA La Estanzuela, Uruguay. The moderate-fiber (MF) diet consisted of 100% alfalfa and orchard-grass haylage (<50% NDF), while the high-fiber (HF) diet consisted of 70% alfalfa and orchard-grass haylage plus 30% barley straw (>50% NDF). Animals were housed in three pens with automatic feeders (INTERGADO) that recorded individual daily intake. Enteric methane was measured using the SF₆ tracer technique over five consecutive days, with gas concentrations determined by gas chromatography (Agilent 7890A). Apparent digestibility was determined using the insoluble acid ash technique from fecal samples collected over five days. For model evaluation, six prediction equations (IPCC 2006, Ellis 2007 a,b,c, Moraes 2014 H_AL and DL) were compared against 97 observations from 29 studies across 9 countries. Statistical analysis used mixed models in Infostat 2020, with linear regression and Pearson correlations in R (version 4.1.1).

**Key Results:** The MF diet resulted in higher total DMI (p<0.05) and higher apparent digestibility (DMD) and NDF digestibility (NDFD), both approximately 10 percentage points higher than the HF diet (p<0.05). Animals on the MF diet weighed on average 41 kg more at the end of the period and had twice the average daily gain (ADG) of the HF group (p<0.05). Absolute enteric methane emissions did not differ significantly between diets (p>0.05). However, emission intensities were significantly lower for the MF diet: 21.7 vs. 23.7 g CH₄/kg DMI (p<0.05), 342 vs. 660 g CH₄/kg ADG (p<0.05), and 6.7% vs. 7.5% Ym (methane conversion rate, p<0.05). Emission intensity per kg NDF consumed did not differ (p>0.05). Linear regression showed DMI correlated with absolute methane emissions (r²=0.51, p=0.0043) and NDF intake correlated with absolute methane emissions (r²=0.45, p=0.013). Among prediction models, the IPCC 2006 model performed best with r²=0.70 and RMSE=74.04 g CH₄/day. The Ellis 2007 (a) and (b) models had the lowest correlations (r²=0.28) and highest RMSE values (118 and 131 g CH₄/day, respectively).

**Clinical Implications:** Reducing NDF content in forage diets by at least 10% (52 g/kg DM) reduces methane emission intensity by up to 8% per kg DMI and by nearly half per kg ADG, while simultaneously improving animal growth performance. This dual benefit—lower emissions per unit of product and faster time to slaughter—has both environmental and economic implications for beef production systems. The IPCC 2006 Tier 2 model is suitable for estimating methane emissions from forage-based diets in national inventories, despite its relative simplicity. These findings support dietary fiber management as a practical mitigation strategy for enteric methane in pasture-based beef systems, particularly in regions like Uruguay where high-forage diets predominate.