Effects of husbandry factors and harvest method and timing on oil content and chlorophyll retention in rapeseed

Abstract

A three-year study was undertaken to increase understanding of the agronomic factors that influence the oil and residual chlorophyll content of winter oilseed rape seed. Increases in seed yield or oil content can contribute to improved returns for growers, through the oil premium. Recent varieties on the HGCA Recommended List have differed by up to 4%, but the extent to which growers can alter oil content through crop husbandry has been unclear.  High concentrations of chlorophyll in the seed at harvest causes problems during refining and processing of the oil and consequently, crushers may be unwilling to accept seed lots with a high percentage of green seed.

To date, the scope for reducing seed chlorophyll retention under UK conditions through agronomic practice has not been established. Field trials were established on a range of soil types in the north, south-west and east of England over three successive seasons, to examine the effects on seed yield, % oil, and chlorophyll content of nitrogen and sulphur fertiliser regime, crop canopy manipulation through plant density and fungicide strategy, and pre-harvest treatment method and timing.

Seed oil contents differed between locations, seasons and varieties. Oil yields for Royal were very similar between 2002/03 and 2004/05, but lower in 2003/04 (when autumn establishment was slow). Increasing total spring nitrogen dose from 150 to 240 kg/ha increased seed yields, but reduced oil contents by an average of 1.1%. Oil yields, output values and margins peaked with 190 kg/ha nitrogen. Delaying part of the nitrogen dose until later, or applying a larger proportion early, had little impact.

There was evidence of seed yield penalties from not applying sulphur in most trials, but these were only significant on the lighter soil in the South-West, where reductions in oil content also occurred.

Spring fungicide strategy had no effect on oil content, other than in one trial affected by light leaf spot where the autumn spray was omitted, which showed increases in both seed yield and oil content with spring treatments. In another trial where phoma was present, applying an autumn fungicide increased seed yield whilst maintaining oil content, but there was no advantage to additional spring fungicides.

Halving the plant density tended to reduce seed yield but had no consistent effect on oil content. Swathing the crop often resulted in lower harvested seed yields than desiccation with glyphosate, although higher seed losses were in some cases partly responsible and earlier harvesting might have reduced this. Treatment timing had no consistent effect on oil content, but there was a tendency for swathing earlier to result in slightly lower values, whereas desiccating earlier had little effect.

Variety choice is the main method by which growers can improve the % oil content of their rape seed. Nitrogen fertiliser dose was the only factor that consistently affected oil content in this study, and the right balance must be struck between seed yield and oil content to optimise margins. Applying sulphur fertiliser is important to maximise seed yields and oil content in deficient situations. Fungicide applications in autumn or spring that give increases in seed yield should also benefit oil yield and output value. Swathing too early has the potential to reduce oil content as well as seed yield, whereas timing of desiccation with glyphosate is less likely to be critical.

Seed chlorophyll concentrations differed significantly between sites, but over the three experimental years were, in general, within the range acceptable to the crushers. Harvest method had no significant effect on the concentration. Similarly, the timing of swathing or desiccation also had no consistent effect.

In one year (2003/04), early swathing increased the concentration at some sites, but decreased it at others compared to later swathing. Fertiliser regime had the most consistent effect on seed chlorophyll. Concentrations were increased by high N doses and by a failure to apply sulphur.