The economic importance of accurate spread and establishing the correct bout width from twin disk spreaders has been known for some time. A lot of work is undertaken in understanding spread patterns from twin disk centrifugal spreaders. Spread patterns are either tested by driving the spreader over collection trays or in testing halls, where the spreader is rotated over trays on weigh cells, from which a spread pattern is established by computer algorithm. In both regimes testing is undertaken in ideal conditions. Spreading conditions in the field are often not ideal and little work, apart from that carried out by manufacturers of spreaders for product development, has been undertaken to test the effects of field conditions and variability of spreadpatterns. This paper examines some factors which influence spreading accuracy in field conditions. Spread tests were undertaken on farms to examine the influence of cross winds, slopes, boundary spreading, turning and headland spreading on a range of products and spreaders. Spreadmark (NZ) tests are undertaken to establish bout widths for spreaders. Twin disk spreaders are often adjusted to establish symmetrical patterns which provide the widest pattern overlap that achieves the target accuracy. Repetitive pattern tests were undertaken for all situations and these were compared to standard "ideal" Spreadmark tests and changes in symmetry in differing conditions were measured to observe the impact of varying field conditions. These field conditions have a significant impact on in-field spread accuracy, which current modeling of spread pattern distributions or testing does not accurately reflect.
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