Effects of conservation tillage systems on soil physical changes and crop yields in a wheat-oilseed rape rotation

Five conservation tillage treatments were applied to an alternating rotation of wheat (Triticum aestivum L.) and oilseed rape (Brassica napus L.) within neighboring clay and clay loam fields in the United Kingdom over three cropping seasons (2013 to 2016) with the objective of determining how the treatments affect soil physical characteristics, weed control, and crop yields. The conservation tillage systems included a relatively disruptive "Farm System" with cultivation to a depth of 200 mm, three intermediate treatments, and a "Low Disruption" treatment with cultivation to a depth of 25 to 100 mm. The disruptive Farm System showed the greatest increase in soil porosity immediately after tillage and resulted in higher wheat and oilseed rape plant counts than the other treatments. Each tillage treatment resulted in mean soil penetration resistances below 2.5 MPa, although a tillage treatment with tine depth to width ratio of 7.5:1 resulted in a higher resistance (2.14 MPa) at 150 to 200 mm than three other treatments (1.36 to 1.67 MPa) in year 1. Despite the above differences, the five tillage systems resulted in statistically similar wheat and oilseed rape yields, except when there was a delay in applying one treatment to the oilseed rape in year 3. In the first two years, the yields of wheat in the clay (9.7 to 10.6 t ha(-1)) and the clay loam field (11.0 to 11.6 t ha(-1)) were similar to yields on other commercial farms. However, in year 3, the wheat yield on the clay soil declined to 8.7 to 9.5 t ha(-1). Some of this is a result of low levels of solar radiation in 2016, but the levels of blackgrass (Alopecurus myosuroides Huds.) increased from 8.2% to 16.0%. There was no statistically significant difference between the capacity of the conservation tillage treatments to control blackgrass on the clay soil, indicating that other measures are needed to control this competitive weed.