Methane emissions from ruminants fed white clover and perennial ryegrass forages : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palmerston North, New Zealand

Ruminant enteric methane (CH4) emissions account for ~35% of New Zealand’s total greenhouse gas (GHG) emissions and a commitment has been made for their reduction. Previous research suggested lower CH4 yields (g/kg dry matter intake; DMI) from sheep fed white clover (Trifolium repens) compared to perennial ryegrass (Lolium perenne; ryegrass), and the initial focus was to account for that difference. However, measurements undertaken here showed little difference between diets in CH4 yield. The objective of this thesis was amended to better understand causes of variation in CH4 emissions from ruminants fed white clover and ryegrass forages. A database analysis showed greater variation in CH4 yield from sheep fed ryegrass forages with measured intakes using the SF6 technique, compared to respiration chambers (23.4 ± 5.70 vs. 23.1 ± 2.90 g/kg DMI). The composition of ryegrass fed to sheep predicted