A physiological analysis of oilseed rape yields: Past and future

Oilseed rape yields on farms have not increased in several countries, including the UK, since the mid 1980s. This may be because the yield potential for the environment in these countries has been reached, or due to a lack of genetic improvement, or due to changes in the environment of the growing crop caused by crop management practices. The present paper investigates which of these factors may be causing the yield of farm crops in the UK to remain at 3 t/ha. The yield potential for the UK that would be possible by combining the best characteristics that have been observed with the best crop management is estimated to be significantly greater than average farm yields at 6·5 t/ha (90 g/kg moisture content). In order to achieve 6·5 t/ha, a crop would have a flower cover of less than 0·4 to facilitate the production of 130000 seeds/m2. Seed filling must last 46 days and have a solar radiation use efficiency of 0·75 g of seed/MJ. A tenth of the yield must come from stem reserves and the seed weight must be 5·0 mg. All of these characteristics have been achieved; therefore the challenge lies in combining these traits within the same crop. The ultimate yield potential for water retentive soils in the UK is estimated at 9·2 t/ha. This would require new characteristics to be bred into the crop and represents a long-term target. In the UK, new oilseed rape varieties are introduced each year and the yield of these varieties under optimum growing conditions is estimated to have increased by 62 kg/ha/year between 1978 and 2005. Lack of genetic improvement is therefore unlikely to explain the halt in farm yields. There have been trends for farm crops to be grown in shorter rotations, established using minimal cultivations rather than ploughing and to receive less nitrogen fertilizer. Sulphur applications have increased, but probably have not kept pace with the reduction in deposition from the atmosphere. Fungicide applications to farm crops are less than applied to the variety testing system and are unlikely to offer complete disease control. It is concluded that a combination of these crop management factors has caused the halt in yield improvement on UK farms.