Methane production in dairy cows

Greenhouse gas emissions from the agricultural sector represent about 14.5% of total emissions related to human activity. Approximately 40% of agricultural sector emissions derive from enteric methane (CH₄) production by ruminants, due to their microbial digestion of feed. Level of CH₄ production varies according to feed type, feed intake and even among individual animals raised under similar conditions, but the underlying mechanism is not well known. This thesis investigated the effects of feed, feed additives, and rumen microbiota on CH₄ production within dairy cows and in a gas in vitro system. Effect of individual cow was stronger than effect of diet for both CH₄ production and methanogenic population when two different levels of forage proportions were fed. Dividing Methanobrevibacter species into two groups better explained the variation in CH₄ production. The effect of individual was evaluated in cows fed the same diet during mid-lactation. High, low and medium emitters were identified and selected for further studies on rumen microbiota. These revealed that CH₄ production was associated with archaeal and bacterial community structure. Differences were observed in volatile fatty acid proportions between communities, but not in fibre digestion or milk production. Tests on feed additives, cashew nut shell extract (CNSE) and glycerol in a gas in vitro system for their ability to reduce CH₄ production showed that CNSE reduced CH₄ production by 18% and had a strong impact on microbiota, while glycerol increased CH₄ production by 12% and had less effect on microbiota compared with the control. Comparison of microbial composition in inoculum from the in vitro control and in inoculum from the donor cow before incubation revealed that the bacterial community was relatively similar, while relative abundance of some species changed for archaeal population. This effect of transfer into another system should be considered when evaluating in vitro data. Evaluation of the in vitro system by comparing predicted and observed CH₄ production on 49 test diets showed an overall good relationship, with small root mean square error for prediction (12.3% and 9.5% of observed mean for fixed and mixed models, respectively). However, the in vitro system had limitations in prediction of concentrate proportion