ABSTRACTThe rates of respiration in cut swards ofLolium perenne(S321) were measured while drying under simulated field conditions, the purpose of the experiment being to assess the extent of respiratory losses which can occur in the process of haymaking. Respiration measurements were made on the grass at different moisture contents during the drying process and also at different stages of growth. The results show that both the moisture content and the age of the grass had a significant infiuence on respiratory rates, consequently these factors can be expected to infiuence the DM losses which occur during haymaking. It was estimated that respiratory losses could amount to, at most, hetween 1.28 and 0.26 kg/100 kg DM, depending on the stage of growth at which the grass is cut