ABSTRACT 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 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.
Kar-Klootwijk et al. (Sun,) studied this question.