Enteric methane emission of dairy cows in 3 grass-based systems: Unrestricted grazing, zero-grazing, and grass silage

The objective of this study was to evaluate enteric CH₄ emission of 45 lactating dairy cows under 3 different grass-based systems: unrestricted grazing (UG; direct pasture access for 24 h/d apart from milking), zero-grazing (ZG; fresh grass cut and fed indoors), and grass silage (GS; conserved grass fed indoors). The experiment was divided into 2 wk of adaptation followed by 2 wk of measurement and this 4-wk period was repeated 3 times during 2 yr: in April-May (period 1), in June-July (period 2), and in August-September (period 3) of 2020 and 2021. Enteric CH₄ production was measured noninvasively using the GreenFeed system (C-Lock Inc.) in the barn and on pasture. The individual measurements of CH₄ per visit were averaged per cow over the 14-d measurement period and expressed per cow per day (CH₄ production), per kilogram of fat- and protein-corrected milk yield (CH₄ intensity), and per kilogram of DMI (CH₄ yield), with DMI measured using feed bins (for ZG and GS) or estimated based on energy calculations (for UG). In 2020, UG showed the lowest CH₄ yield and intensity in all periods compared with the other grass-based systems (in period 3 not significantly different from ZG), whereas GS showed the highest CH₄ yield and intensity (in period 2 not significantly different from ZG). In 2021, GS again showed the highest CH₄ yield and intensity in period 1 compared with UG and ZG, and differences between grass-based systems were smaller in periods 2 and 3. Independent of the limitations in direct comparison between the grass-based systems in this study due to differences in harvesting moments of fresh grass versus grass silage, the low enteric CH₄ yield of especially UG early in the season (period 1) was remarkable (14.0 g CH₄/kg of DMI in 2020 and 14.9 g CH₄/kg of DMI in 2021). Our results indicate that feeding fresh, high-quality grass early in the season can reduce CH₄ emission per unit of feed intake and per unit of milk produced.