Effects of <i>Chenopodium album</i> L. Substitution Levels and Harvest Time on In Vitro Rumen Fermentation and Methane Production in Early-Fattening Hanwoo Steers

This study investigated the feasibility of incorporating <i>Chenopodium album</i> L. (CAL) into ruminant feed ingredients through evaluating the effects of harvest time and substitution levels on in vitro rumen fermentation. In the first phase, a sole-substrate experiment was conducted using CAL harvested from June to August, analyzing its chemical composition and total saponins content. The impact of harvest time on fermentation parameters was assessed with CAL as the sole substrate. The second phase involved a mixed-substrate experiment using an early-fattening Hanwoo diet (30% rice straw and 70% concentrate), where increasing proportions of CAL (control: 0%, T1: 5%, T2: 10%, T3: 15%, and T4: 20%) replaced rice straw. Seasonal variations in CAL composition influenced the fermentation characteristics. CAL harvested in July exhibited higher fermentability, with total volatile fatty acids (TVFAs) reaching 103.87 mM at 72 h. In contrast, CAL harvested in August showed lower fermentability and digestibility. However, August-harvested CAL was selected for the subsequent experiment, as it provided a more practical balance of sufficient biomass yield and a higher saponins concentration, aligned with the study’s methane mitigation objectives, while also exhibiting a fiber composition comparable to that of rice straw. We hypothesized that the saponins in CAL contribute to methane reductions. Supplementation with 15% of CAL significantly reduced methane production per gram of inoculated and digested dry matter (<i>p</i> < 0.05), likely due to differences in crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), and saponins content. However, despite having the lowest fiber content, T4 (20% CAL) exhibited the lowest in vitro dry matter digestibility (IVDMD), suggesting that factors such as saponins, CAL’s chemical composition, or microbial shifts may have hindered digestibility. Ammonia–nitrogen production increased from 0 to 3 h, but it continuously decreased between 3 and 9 h due to microbial growth and nitrogen assimilation, as microbes incorporate ammonia into their biomass (<i>p</i> < 0.05). Fermentation characteristics further revealed that the acetate-to-propionate (A/P) ratio decreased with increasing CAL levels, with T4 showing the lowest ratio (1.55 at 72 h), confirming a shift toward propionate-based fermentation. Notably, T2 (10% CAL) showed an optimized fermentation efficiency, producing the highest TVFA concentration at 24 h (98.28 mM). These findings highlight the potential for using CAL as a functional feed ingredient, with moderate substitution levels (10–15%) enhancing fermentation efficiency while reducing methane production.