Uneven crop residue distribution influences soil chemical composition and crop yield under long-term no-tillage

Conservation agriculture has three main components: crop residue retention; minimal soil disturbance; and diverse rotations. Whilst retaining crop residues on the soil surface is important to prevent erosion and conserve water, high residue amounts can also have negative effects. Residue spread behind harvesters is not always uniform, therefore, it is important to understand the impact of uneven residue spread on crop performance. These problems are likely to increase as more farmers windrow and burn residues for weed control, harvesters become larger and controlled traffic systems are implemented. The effect of residue distribution on soil properties and crop establishment and yield was studied under no-tillage and controlled traffic in cereal (wheat-wheat-barley) and diverse (wheat-legume-canola) rotations, over the final six years of a larger 12-year (2007-2018) rotation trial, in the Wheatbelt of Western Australia. Within each crop and rotation, residue management consisted of either spreading or windrow-burning and measurements were taken from three locations relative to the harvester wheel tracks: centre (between harvester wheel tracks), mid (2.25 m perpendicular) and outer (4.5 m perpendicular), representing the similar to 9 m working width of the machinery. In most years, the distribution of crop residue behind the harvester was uneven, with up to twice as much residue directly behind the harvester compared with mid and outer locations. Differences in residue amount had positive, negative or no effect on crop establishment and yield, depending on the crop and residue type and climatic conditions. Increasing residue amount had a positive effect on establishment and yield when conditions were dry around seeding and early crop growth stages, possibly associated with increased water availability with increased residue load. Conversely, there were negative effects when frost occurred at anthesis or under high rainfall conditions, the latter due to physical impairment, increased disease or lack of available nitrogen. Negative effects only occurred with cereal residue and could be minimised by growing more break crops, even under high residue loads. In the longer term, higher residue amounts resulted in increased soil nutrients behind the harvester, which increased yields in some years. Therefore, research to improve uniformity of residue spread behind harvesters is crucial. Otherwise, variation in crop performance will increase as harvesters get bigger and, in the long term, periodicities in soil chemistry under controlled traffic systems will occur.