Challenges and opportunities to capture dietary effects in on-farm greenhouse gas emissions models of ruminant systems

This paper reviews existing on-farm GHG accounting models for dairy cattle systems and their ability to capture the effect of dietary strategies in abatement. The focus is on methane (CH4) emissions from enteric manure (animal excreta) sources nitrous oxide (N2O) animal excreta. We identified three generic modelling approaches, based degree which diet-related characteristics: ‘none’ (Type 1) ‘some’ by combining key diet parameters with emission factors (EF) 2) ‘many’ using process-based 3). Most selected have adopted a Type 2 approach, but few hybrid / 3 approaches been developed recently that combine empirical (through use CH4 and/or N2O factors; EF) (mostly through rumen whole tract fermentation digestion). Empirical comprising inputs (i.e., dry matter intake organic digestibility) can predict reasonable accuracy. However, impact mitigation often needs be assessed more integrated way, 1 frequently lack biological foundation do this. Only represent underlying mechanisms such as ruminal total-tract digestive processes excreta composition effects manner. Overall, better model simulate function, greater opportunity include characteristics addition commonly used variables, thus strategies. value capturing additional feed prediction carefully balanced against gains accuracy, need input activity data, variability encountered on-farm.