Investigation of high levels of erucic acid in consignments of double-zero oilseed rape varieties

Abstract

Rapid tests for erucic acid content in double-low (also called ‘double-zero' and ‘00’) rapeseed have identified a small proportion of loads with levels well over the 5% legal limit in extracted oil. A new standard of 2% is likely to result in a significant increase in the number of loads that are either rejected or subject to price penalties. This project was commissioned to investigate the possible causes or sources of contamination associated with elevated levels of erucic acid.

A set of 50 samples, collected from commercial oilseed rape crops at harvest 2017, were acquired to provide the core of the study. These were used to compare the analytical accuracy of the new near infrared spectrophotometer scanning (NIRS) with that of the more traditional but slower testing by solvent extraction and gas chromatography (GC). The same samples were also inspected for the presence and influence of oil-bearing and potentially high erucic weed seed. On a subset of 12 samples, tests were conducted at the single seed level to establish whether elevated erucic acid levels resulted from a general drift upwards or from the presence of seeds with elevated levels due to cross-pollination from high erucic rape crops or volunteers. In a separate work package, DNA tests on leaf tissue from volunteer plants, growing in oilseed rape crops for harvest 2018, were conducted to detect the presence or absence of the genetic trait for high erucic acid in the seed. The results from these tests were then related to the erucic acid levels measured in the crop at harvest.

Modern NIRS equipment was found to give a good comparison with GC analysis overall (over an erucic acid range of 0–40%) but gave reduced levels of accuracy in the 0–5% and 0–2% ranges. Consequently, in a small proportion of cases, the rapid test could incorrectly identify loads as having exceeded threshold values. Within the samples inspected, weed seeds were not present at levels that could have resulted in significantly elevated levels of erucic acid. Testing of a small number of oil-bearing weed seeds, identified and removed from the samples, confirmed the high-erucic status of some of these species and this reinforced the importance of controlling them in rape crops. Single seed testing confirmed the stability of the low erucic acid trait. Within the samples investigated, however, clusters of individual seed values, in a range between 10 and 50% erucic acid, clearly indicated the influence of high- or elevated-erucic acid volunteers as the principal causes of elevated levels of erucic acid. DNA extractions on leaf tissue of volunteer plants confirmed the potential value of this as a diagnostic/predictive test of risk levels presented by volunteers in rape crops in individual fields.

Farmers should ensure that any seed – purchased or farm-saved – has been tested and shown not to pose an erucic acid risk. Best cultural practices to minimise volunteers should also be followed, where any background threat exists. Grain samples, from individual loads moved, should also be retained for reference in the event of disputed test results