Rainfall, rotations and residue level affect no-tillage wheat yield and gross margin in a Mediterranean-type environment

Wheat yield was obtained over nine years (2007–15) of a long term experiment in a Mediterranean-type climate, to understand the effects of rotation and residue retention on rainfed wheat establishment, yield and gross margin under a no-tillage system. The three treatments were based on increasing levels of diversity in the rotation, from ‘monoculture wheat’, ‘cereal rotation’ and ‘diverse rotation’. These treatments, except monoculture wheat, were based on three phase (year) rotations with every phase presented every year. Any winter/spring cereal may be grown in the ‘cereal rotation’ treatment, while the diverse rotation was based on a wheat–legume–brassica sequence. For the period 2007–2010, residue was spread across the plot behind the harvester. The plots were split after 2010 with residue spread on half of each plot, and the other half having the residue windrowed and burnt prior to seeding, which reduced residue levels by 40–66%. This reduction in residue level had a positive effect on wheat yield in years with high levels of cereal residue and had negative, or no effect, when residue levels were relatively low (<∼3000 kg ha⁻¹). By contrast, the effect of windrow burning of canola residue on following wheat yield was negligible, even at high residue levels. Therefore the effect of crop residue on wheat yield depended on the type and amount of material. Monoculture wheat and cereal rotation had the highest cumulative 9-year average gross margins, despite the diverse rotation showing higher grain protein concentration in most years and improved wheat yield over time. Lower gross margins in the diverse rotation were associated with poor legume performance in many years and low canola yields in dry seasons. Improving the reliability of these break crops in this growing environment is the key to increasing their uptake by farmers. Cover crops in the rotation negatively impacted gross margins, without any observed yield benefits in the following years, therefore should not be recommended to replace the one cash crop per year in this low rainfall Mediterranean-type environment.