Desk study to evaluate contributory causes of the current yield plateau in wheat and oilseed rape

Abstract

To address the lack of progression in UK average farm yields for wheat and oilseed rape, national yield trends were analysed in relation to cropped area, genetic improvement, weather patterns, economic influences, crop nutrition and protection, plus other aspects of agronomy. Farm-specific data were evaluated to investigate the effects of changes or differences in agronomic practice, and research evidence examined to quantify their likely yield impact. Opportunities for overcoming yield constraints were considered, in the context of legislative, environmental and technical barriers, along with potential impacts on grain quality or end use.

From 1980 to 1996, wheat yields improved rapidly, by an average of 0.10 t/ha per year, aided by a fall in the proportion of second wheats. Since then, yields have stagnated despite the potential of new varieties increasing by 0.05 t/ha per year. A number of weather variables have influenced annual yield variation. Increased crop protection measures have minimised yield loss from weeds, pests and diseases, while a move to earlier sowing has contributed positively to yield. The transition to reduced tillage may have had a negative yield effect in the short term, with a longer-term impact possible from deep soil compaction. Crop nutrition has also been a factor in yield limitation, as a result of sub-optimal applications of nitrogen (N) fertiliser in at least some situations, and the area of crop receiving sulphur (S) fertiliser initially rising more slowly than the area at risk of deficiency.

From 1984 to 1994, oilseed rape yields declined but, after varying wildly, they have improved since 2004. Yield potential has increased at nearly 0.05 t/ha per year through genetic improvement but prior to 2004 poor uptake of higher-yielding varieties meant that over half of this was not being deployed. Increased cropping frequency has undermined yield improvement over the whole period. From 1984 to 1994, the net impact of agronomy was negative, with decreasing N fertiliser doses and increasing S deficiency. An increase in spring oilseed rape and unfavourable weather patterns also reduced yield improvement. From 1994 to 2004, yields benefitted from rising S fertiliser use but a shift to shallow cultivation was detrimental. From 2004 to 2011, better uptake of new varieties, strengthening crop protection and favourable weather combined to give a rising yield trend.

No single factor has had a dominant influence on yield trends. Changes to agronomy have had a number of mainly small effects, with growers aiming to maximise profit not yield. To restore rising yields in the face of warmer conditions, economic or environmental pressures and evolving weeds, pests or disease threats, a more holistic approach to agronomy is needed. Recommendations include improving selection and management information for varieties, sowing wheat earlier on light land to mitigate drought and a focus on improving N use efficiency. Benchmarking of yields, resources to ‘health check’ cropping systems and increased utilisation of survey data are vital to guide and measure change. Further studies should include the yield effects of changing weather, the incidence and severity of deep soil compaction and pollination and seed set in oilseed rape.