SUMMARY The effects of mechanical conditioning, which crimps mown grass, and of either spreading or gathering into windrows are estimated from a field trial with repeated measurements. Except for treatmentdependent scaling terms, drying curves are found to have a common shape that is described by a cubic spline. The rate of drying is approximately proportional to potential evaporation from a freewater surface, as predicted by Penman's equation, which is a function of basic weather variables. A series of field and laboratory trials has been conducted at the Scottish Institute of Agricultural Engineering to ascertain the benefits of various mower conditioning systems (Klinner, 1975) in Scottish climatic conditions. Effects of either spreading the mown grass to dry or gathering it into ridges called windrows, were also investigated. The field trials had, as a secondary objective, the identification of a relationship between drying rate and weather variation for use in operational research studies of forage conservation systems. Methods of analysis and results are illustrated using a silage wilting trial which was conducted over 4 days in June 1985. There were three replicates of a 4 x 2 factorial treatment structure laid out as a randomized block design. The first factor had four levels denoting whether the grass was conditioned by one of three mower conditioners which crimp the grass and break its waxy surface, or left unconditioned. The two levels of the second factor corresponded to the grass being spread or gathered into windrows. Five samples of grass were taken at random from the swath, that is the strip of mown grass, in each plot both at the beginning and end of the experiment. These were separately weighed, dried in an oven at 102?C for 24 hours, and reweighed. Moisture contents (dry basis), the ratios of water to dry matter in the grass, could then be calculated. In order to estimate moisture contents during the trial, repeated measurements were taken by weighing a length of each swath at regular intervals of time, as well as at the times when the samples were collected. To ensure that the same lengths were monitored throughout, they were contained within nets supported on lightweight aluminium frames. Also recorded were basic weather variables: air temperature, vapour pressure deficit, net radiation above the swaths, wind speed 2 m above the ground, and rainfall. Further details of the trial are given by Spencer et al. (Scottish Institute of Agricultural Engineering Dep. Note, No. SIN/472, 1986). Analysis was in two stages, reflecting the twofold objectives. Initially, the effects of treatment on drying rates were examined, then the relation to weather variability was