Animal factors affecting enteric methane production in late lactation pasture based dairy cows in Ireland : a thesis presented in partial fulfilment of the requirements for the degree of Master of Animal Science at Massey University, Palmerston North, New Zealand

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Ireland currently has a national commitment to reduce 51% of total greenhouse gas emissions by 2030. In order to achieve these reductions, factors that affect enteric methane production in a pasture-based system need to be further investigated. The objectives of this study were to 1) investigate the repeatability on enteric methane emissions in grazing dairy cows, 2) assess the relationship between enteric methane and other animal traits at grass and 3) investigate the potential of a new trait called residual enteric methane emission (RME) to select for lower enteric methane emitting cows without impacting productivity. Enteric methane emissions were measured on forty-five late lactation grazing dairy cows using the GreenFeed monitoring system at Teagasc, Moorepark, County Cork, Ireland. The average enteric methane produced was 351.8 g per day with a daily coefficient of variation of 13%. The cows were averaging 16.6 kg dry matter intake (DMI) while producing 1.62 kg milk solids (MS; fat plus protein) per day. The repeatability of the enteric methane measurements was 0.67 indicating that the enteric methane measurement is reliable. Through the partial correlations conducted, it showed that milk, MS, fat and protein yields, milk urea, live weight and DMI all have positive correlations with daily CH4 production. Therefore, it is expected that an increase in any of these traits would lead to an increase in enteric methane production. While body condition score (BCS) had a significant negative correlation with enteric methane production. This could be due to higher energy demands at a lower BCS for an animal to increase their body condition score leading to an increase in DMI. Residual enteric methane emissions were estimated through two methods: multiple regression and Irish national inventory calculations. For each method, animals were split into three groups with high, medium and low ranking of RME with 15 animals per group. The rank correlation between the two methods was 0.79 (P<0.001) showing that the two methods are able to rank animals to a similar level as each other. The low ranked animals produced between 16.2% and 6.9% less enteric methane per day than both the high and medium ranked animals. Despite this reduction in enteric methane production, there was no effect on the milk production, composition, live weight, BCS or DMI. Therefore, these low ranked animals produced less enteric methane per kg of milk solids, live weight and DMI indicating that the RME has potential be used in future strategies to reduce methane through for example breeding for lower enteric methane producing animals while not affecting the production and income of farmers.
Figure 2.2 is re-used under a Creative Commons Attribution 4.0 International (CC BY 4.0) license. Figure 2.5 (=Hristov et al., 2015 Fig 1) was removed for copyright reasons.