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Information from the literature about forage conservation losses in the field has been reviewed, contrasted and analysed in the categories respiratory loss, leaching loss and mechanical loss. An attempt has been made to summarise the information in terms of relationships with various climatic, crop and machinery factors, suitable for incorporation in an operational research model of forage conservation. An equation developed to relate respiratory loss to the temperature, moisture content and water soluble carbohydrate content of forage material gives a good representation of results of a range of laboratory studies, and is in reasonable agreement with less accurate field studies. A relationship between leaching loss for a given quantity of rain and moisture content is proposed, but on the basis of very limited information. It was found helpful to regard mechanical loss as the sum of two processes, namely true shatter loss and pick-up loss for windrowing, baling and forage havesting operations. An equation relating shatter loss to moisture content and severity of mechanical treatment is proposed. It is suggested that a roughly constant level of pick-up loss per unit area of stubble cleared by the operation might be assumed, because this is the only conclusion justified on the basis of the rather varied information available.