Declining rapeseed yields in Finland: how, why and what next?

Average seed yields per hectare of Brassica oilseed crops in Finland, mainly summer turnip rape (Brassica rapa L. var. oleifera subvar. annua), which covers 0.93-0.99 of the total oil crop cultivation area depending on year, have fallen dramatically during the last 15 years. This downward trend is contrary to those in other temperate regions, where rapeseed yields have increased or levelled off after reaching a relatively high level. The 5-year moving averages for Finland show that seed yield started to diminish gradually after reaching its highest level of over 1700 kg/ha in the early 1990s. By 2005 it had fallen to 1270 kg/ha. The present study evaluated the possible reasons for the recorded collapse in Finnish turnip rape yields. All the statistical analyses were based on large, previously produced, datasets from multi-location Agrifood Research Finland (MTT) Official Variety Tests, Finnish Food Safety Authority (EVIRA) Seed Testing datasets and the Information Centre of the Ministry of Agriculture and Forestry in Finland (TIKE) national production datasets. Results from MTT trials indicated that the latest turnip rape cultivars were more sensitive to elevated temperatures at late seed set and during seed fill - and such temperatures often occurred during the years of greatest yield reduction. When taking into account how commonly sown these cultivars were at national level during the last 10 years, increased sensitivity contributed to up to two thirds of the recorded yield reduction. Even though the growing area of turnip rape has slightly exceeded 100 000 ha, after long being 60000-70000 ha, by extending cultivation to more northern areas of Finland, such changes do not explain the yield collapse according to data from TIKE. Furthermore, lower national yields do not stem from larger, but rather are associated with narrower within year variation in seed yield. Additional empirical work is needed to understand the causes of increased temperature sensitivity in modern cultivars (e.g. possible linkage to drought, diseases and/or drastically increased seed energy content). Furthermore, a national survey is essential for a thorough and up-to-date picture of the prevalence of pests and diseases in turnip rape and their contribution to reduced yields.