The enzyme polyphenol oxidase (PPO) requires both plant tissue damage and the presence of oxygen to become activated. It has been shown that with a large window of opportunity for oxygen activation, such as during ensilage of red clover, increases in both dietary-nitrogen utilisation and the concentration of polyunsaturated fatty acids in ruminant products can be achieved. However, there is little evidence as to whether such responses can be achieved during the grazing of fresh red clover, where the window of opportunity for oxygen activation is limited to the mastication period (Lee et al. 2006). This experiment investigated whether mastication of fresh red clover would result in the activation of PPO and its potential effects on protein and lipid profiles in the resulting bolus.