Characterisation and modelling of white mustard (Sinapis alba L.) emergence under several sowing conditions

White mustard is sown as a catch crop in Northern Europe to limit nitrate leaching during winter. Its crop establishment is difficult to control as growers attempt to drill at minimum cost, and the weather may be hot, dry, or with rainstorms. This study was carried out to characterise the sensitivity of emergence of white mustard to sowing conditions. Sowing was performed in three locations with varied soils and climates, either after mouldboard ploughing or stubble disking, or directly under the straw without tillage. We compared thereafter these observations to the predictions obtained with a crop emergence model, SIMPLE, designed, to assist tillage and sowing decisions and parameterised for white mustard. Final emergence in the field experiments was high in all cases, 88-100%, the reasons identified for non-emergence being disease, clods and soil crusting. In contrast, half emergence time varied widely from 3 days to 2 weeks, depending on sowing depths, soil temperature and water content. The comparison of the germination rates observed in these field experiments to SIMPLE predictions indicated that observed/predicted germinations, using a simplified hydro-thermal time model, were in good agreement under the wide set of wet to dry conditions that occurred. Final emergence was also well predicted at all sites. Predicted progress of emergence with time was slightly earlier than observed in most cases, the difference being 6-42 degrees Cd (0.5-3 days). Temperatures over the optimum (27 degrees C), dryness and mechanical resistance in the seedbed during shoot elongation could be the reasons for those differences. Emergence observations in field conditions and modelling indicated that coarse seedbeds and large quantities of straw were not as important as soil water content, temperature, and sowing depth in explaining variations in emergence. Although SIMPLE can still be improved, it gives reasonably accurate predictions that could be used in addition to field experiments to evaluate the effects of a range of sowing dates on emergence over a wide range of soil and climatic conditions. (c) 2004 Elsevier B.V. All rights reserved.