Increasing the proportion of grazed grass in the diet in early lactation and its impact on enteric methane emissions and rumen fermentation of pasture-based dairy cows

During the spring period, low grass growth on pasture-based dairy farms often necessitates the supplementation of grass silage to spring calving dairy cows. The objective of this experiment was to quantify the impact of increasing the proportion of grazed grass while concurrently reducing silage supplementation in early lactation on enteric methane (CH₄) emissions in a spring calving pasture-based dairy system. In both 2021 and 2022, a 12-week experiment was conducted, where 80 cows were offered one of two dietary treatments (n = 40): high grass (HG) or low grass (LG). The HG treatment was offered 10.6 kg DM/d per cow of daily herbage allowance (DHA), 1.6 kg DM/d per cow of grass silage and 3.1 kg DM of concentrate in period 1 (P1; weeks 1-6). The LG treatment cows were offered 7.9 kg DM/d per cow of DHA, 4.5 kg DM/d per cow of grass silage and 3.1 kg DM/d per cow of concentrate daily in P1. In period 2 (P2; weeks 6-12), 20 cows from each treatment crossed over to the opposing treatment while 20 cows remained in their original treatment, the HG treatment cows received 15 kg DM/d per cow of DHA, 2.7 kg DM/d per cow of concentrate with no grass silage offered and the LG treatment cows were offered 12.0 kg DM/d per cow of DHA plus 3.0 kg DM/d per cow of grass silage and 2.7 kg DM/d per cow of concentrate. After P2, a 12-week carryover period commenced, where all cows regardless of previous treatment were offered 17 kg DM/d per cow of DHA and 2.2 kg DM/d per cow of concentrate with no grass silage offered. Milk production and gaseous emissions were measured daily and individual animal dry matter intake (DMI) was estimated using the n-alkane technique, and rumen samples were collected at two time points (Week 5, Week 7). No treatment (P > 0.05) differences in CH₄ emissions (g/d), CH₄ intensity (g/kg MS) or CH₄ yield (g/kg DMI) were evident in P1. In P2, enteric CH₄ emissions, CH₄ intensity and CH₄ yield were (P < 0.05) lower for the HG treatment cows in P2. Total rumen fluid volatile fatty acid and butyric acid concentrations were lower (P < 0.05) in the HG treatment cows in P2. The HG treatment cows had a (P < 0.05) lower methane conversion factor in P2. The current experiment findings demonstrate that no effect was observed in P1, but as grass quality improved throughout the experiment and grass silage was removed from the diet in P2, CH₄ emissions were reduced for cows in the HG treatment.