This is an opinion piece by the Head of the Department of Livestock Sciences at the Swiss Research Institute of Organic Agriculture (FiBL). In it he combines his knowledge of normal rumen functioning with reviews of several studies of low protein feed diets for dairy cattle. He argues that new perspectives on optimal rumen functioning need to be found that allow for the development of low-input feed evaluation systems for dairy cattle. Future research could identify diets that are low in protein if they are high in roughage quality and secondary plant compounds. Their use could improve animal health, increase their use of protein and fibre, and decrease excess urea and methane emissions.