Feeding strategies and their impact on milk production, milk composition and enteric methane emissions throughout the lactation in pasture-based dairy systems

Enteric methane (CH4) emissions account for 62.5% of Ireland’s agricultural greenhouse gas (GHG) emissions, highlighting the need for effective mitigation strategies. The objective of this thesis was to evaluate how feed quality influences enteric CH4 emissions, milk production, and milk composition in spring-calving pasture-based dairy systems. This was examined through four separate research questions, 1) evaluating the digestibility of grass clover and grass only silage and their impact on dry matter intake (DMI), CH4 emissions and post-partum milk production during the pre-partum period (Chapter 3), 2) investigating the effect of silage supplementation over grazed grass on CH4 emissions and rumen fermentation in early lactation when grass availability is limited (Chapter 4), 3) assessing the impact of lactic acid bacteria silage inoculants on CH4 emissions and milk performance in late lactation (Chapter 5), and 4) comparing ranking methods for identifying low CH4 emitting cows without impacting performance (Chapter 6). It was hypothesised that greater forage digestibility would reduce CH4 yield and the Ym factor of pasture-based dairy cows, furthermore residual methane emissions (RME) could be used as a ranking tool that is independent of productivity traits. Pre-partum supplementation with grass–clover silage increased DMI (P<0.001) and reduced CH4 yield (P<0.05), with measured CH4 emissions 10% lower than Tier 2 inventory estimates. During early lactation, higher levels of silage supplementation increased CH4 emissions (+23 g/cow/day; P<0.05) without affecting DMI, reflecting the lower digestibility of silage compared to grazed grass. In late lactation, inoculation of silage with lactic acid bacteria did not enhance digestibility, milk performance, or CH4 mitigation. Ranking cows by RME proved effective: low-RME cows emitted 11.1% less CH4 (P<0.001) with no adverse effects on performance in early lactation. Overall, these findings demonstrate that improved forage digestibility can reduce CH4 yield, whereas lactic acid bacteria inoculants provide limited mitigation potential in late lactation. Residual methane emissions represent a promising phenotypic tool for identifying low-emitting animals. Furthermore, the emission factors generated in this thesis deliver country-specific data to refine national inventory models and enhance the accuracy of greenhouse gas reporting in Irish dairy systems.