New approaches to weed control in oilseed rape

Abstract

Weed control in winter oilseed rape has always been challenging. Yield loss from poor broad-leaved weed control can range from 3% up to 73% depending on the vigour of the crop and this does not take into account contamination of harvested crop with weed seed which can reduce marketability of the sample. Weed control has been made more difficult because herbicides are being found in drinking water, herbicide availability has declined, legislation changes have introduced restrictions on herbicide use and herbicide resistance in grass weeds is increasing. Two research projects aimed to evaluate new approaches to weed control in winter oilseed rape. The first (3652) aimed to produce a ‘specification’ for an oilseed rape crop which balances the needs of crop performance with those required for weed control. The second (3605) aimed to evaluate new approaches to weed control in winter oilseed rape that use carefully-directed control methods between crop rows to minimise reliance on commonly-used selective residual herbicides.

In situations where weeds are well controlled there is scope to increase row widths up to 48 cm without reducing yield potential. In situations where weed control is poor and coupled with factors which limit the plants potential for compensation (e.g. spring drought), increasing row width to 48 cm and 72 cm was shown to reduce yield potential. There was no evidence that row width affects the optimum seeds/m² sown or the optimum plants/m². However it was shown that it is not possible to establish more than 25 plants per metre of row and yields may decrease above 17 plants per metre of row. This means that it may not be possible to establish more than 50 plants/m² for 48 cm row widths or more than 35 plants/m² for 72 cm row widths.

When winter rape is established on wide rows there is an opportunity to apply targeted applications of a non-selective herbicide such as glyphosate to the inter-row gap, reducing or eliminating the need for residual herbicides. Glyphosate damage can be limited by the use of even-spray nozzles coupled with simple plate type shields that can be produced in the farm workshop or all-round shields from Garfords. Timing the application of glyphosate earlier, at GS 1,3 compared to GS 1,5, limits the potential for damage as the inter-row gap becomes narrower at the later growth stage. A sequence of glyphosate applied at GS 1,5 followed by a second application may be necessary to account for a second flush of weeds.

The targeting of an inter-row non-selective herbicide application can be improved by using Vision Guidance and RTK GPS technologies. Both systems were successful but RTK GPS is simpler to set up and more commonly available. It was found that Vision Guidance coupled to an inter-row cultivator gave weed control equivalent to standard weed control (metazachlor). This result would be particularly useful in organic systems.

Variety type was not important, open pollinated varieties were shown to compensate as well as hybrids against wide row widths and low plant populations. There was no difference in crop damage between hybrid and conventional variety types with inter-row application of glyphosate. There was no evidence that plants in wide rows are attacked more by pigeons or whether the risk of lodging is altered but more work needs to be done to confirm this.