One of the most important factors of animal production intensification is the supply of a proper quantity and quality of roughages, especially hay and silage. The paper is a survey of research results concerning the sources of losses during field drying of green forage for hay or silage. The analyses were based on research results from scientific centres all over the world as well as the author's own research. Not only the loss factors were analysed, but also mathematical models which generalize the influence of many factors on the losses. Firstly, an analysis was made of the influence of height and frequency of cutting on the quantity and quality of harvested green forage. The choice of a proper cutting height allows both to maintain the plantation in a good state and to obtain high yields during vegetation. Next, the factors were discussed that affect respiration losses during field drying of green forage for hay. Atmospheric conditions were considered, as well as the kind of mechanical processes applied in order to enhance forage drying rate. Also, mathematical models were presented taking into consideration the influence of temperature and moisture of the harvested material on the intensity of respiration of the cut plants and on dry matter losses. Further considerations involved the influence of rainfall on the level of nutrient losses, changes in the chemical composition and digestibility of hay, as well as dry mass losses. Two mathematical models were presented, describing the process of rewetting of swaths. The first one concerns changes of general water content in the dried fodder. The second model takes into consideration changes of particular water categories during fodder rewetting from rain. The results of computer simulations of water content changes in green forage were compared to experimental results. The essential part of the paper is devoted to discussion of the influence of the application of different technologies, kinds of mechanical treatments, and way of harvesting on dry mass and nutrient losses. Special attention was paid to the influence of cutting units (knife cutting, cutting rotors with horizontal or vertical axis of rotation of working elements) on stubble losses and crushing of the cut plants. Also, analysis was performed of the influence of mechanical treatment (conditioning, maceration, tedding, spreading) on quantitative and qualitative losses of the obtained fodder. Mathematical models were presented of losses caused by cutting and initial mechanical treatment, depending on the kind and growth stage the cut plants and on the type of cutting and conditioning units. These analyses also take into consideration the influence of the kind of cutting material, harvest term, as well as the type of applied machines and their design-operation parameters on quality and quantity losses.