A large part of the rumen research we use here in NZ is drawn from work done overseas, routinely on cows fed lots of grain, while there is perishingly little hard data on the high production, grass based rumens that are the direction NZ dairy appears to be heading in. Despite this, or maybe because of this, there are a considerable number of loud and strong opinions around rumen function here in NZ, and particularly about the high producing cow. Although it seems there are a lot of issues around rumen function spoken about, in reality there is just one central theme that is most commonly discussed, and this is just recycled in different forms. This paper will look at what we know, and don’t know, about rumen function in grass based dairy cows under NZ conditions, and outline the results of the three years of rumen research on the Lincoln University Dairy Farm (LUDF), and what this tells us about the role of the rumen in lameness here in the South Island.