An effective weed control strategy in oilseed rape has benefits that reach beyond the target crop, writes Colin Lloyd, research development manager at Masstock.

Unlock the potential of oilseed rape to get on top of a blackgrass problem and the yield benefits will come rolling in throughout the rotation. But squander that opportunity and you’ll be left with a higher burden of tough-to-tackle weeds than you started with.

At Stow Longa, Masstock’s high-population blackgrass trials site, the yield advantage when using a subsoiler and seeder over more conventional min-till techniques has been at least 1t/ha. While some of this is down to deeper rooting, the weed population in the poorer plots was painfully obvious.

Then there’s the harvestability and admixture issues from cleavers. Add in the effect of cranesbill on canopy thinning and the crop variability volunteers can bring. With such a high-value crop, it’s worth putting a bit more thought into your weed control strategy than putting on a bit of pre-emergence herbicide before switching your focus to your wheats.

Which weeds should you focus on?

For many growers, blackgrass is the most important weed.

There is not just the question of keeping it controlled in the crop to preserve rapeseed yield. This is also the opportunity in the rotation to hit populations hard with different chemistry and establishment techniques to reduce the overall seed burden.

This is also true for ryegrass and bromes.

Broad-leaved weeds will compete for space and nutrients, but more importantly a high population will affect canopy development. The rape crop will produce spindly plants with a poor seed yield.

Cranesbill is one of the biggest broad-leaved culprits. At cotyledon stage the kidney-shaped leaf looks like an oilseed rape seedling, but in reverse, and is much hairier. Cleavers’ characteristic notched cotyledon distinguishes them from other broad-leaved weeds.

Charlock, or runch, is not such a big problem, but can take advantage of open crops in the spring. Its cotyledons look identical to rape’s, but the first true leaves have a short blunt tip and surface hairs.

On lighter land, look out for shepherd’s purse, with its short stalk and oval cotyledons, and poppies, that have small, narrow and pointed cotyledons.

What role does canopy play in weed control?

Variety choice is important. Rape crops with a prostrate growth that cover the ground quickly will help smother weeds and give them less chance to compete.

Hammer, DK Cabernet and Excel work well – you have to manage the height, but their competitiveness is useful. But it’s questionable whether semi-dwarf varieties should be grown where there is a high blackgrass burden.

You’re looking for a competitive crop going into the winter, with typical target populations of at 20-30 plants/sq m for hybrids and 40-50/sq m for in-breds. Lower populations should be sufficient on wider spaced rows established with a subsoiler where the soil has not been moved between the rows.

A thick canopy will give excellent weed control but could result in an inefficient canopy and lodging, and yield will suffer. Provided you have good rooting and a good plant structure you will get a competitive canopy that will also deliver the right green area index at flowering.

The biggest issue tends to be variability – weeds will take over in parts of the field where establishment is poor.

Late October to mid-November is the best time for applying late residual herbicides, as products can break down quickly in soil temperatures above 14C.

Oilseed rape presents an excellent opportunity to control blackgrass and brome, with the simple aim being to reduce the population that will compete with your following first wheat.

In most years, you have to discount any chance of achieving a chit that can be sprayed off with glyphosate before drilling. So it’s important to disturb the soil as little as possible. This keeps the blackgrass roots near the surface, which aids chemical control.

In Masstock trials, using a subsoiler and seeder or direct drilling has, therefore, achieved a better result than min-till.

The chemistry used in rape crops works on blackgrass in a completely different way from the herbicides used in cereals. Carbetamide and propizamide are residual herbicides that work through the root, while post-emergence cereal herbicides rely on foliar or shoot uptake. There are no known resistance issues to the chemistry either, offering the opportunity to hit resistant populations hard.

Soil temperature is an issue, however, and products can break down quickly if it is above 14C, making late October to mid-November the best timing.

Establishment technique also plays a hand in product selection. Propyzamide works best when blackgrass is germinating from the top few centimetres of soil. Where oilseed rape has been established following deeper cultivations then carbetamide, which tends to move more easily in the soil, is useful for deeper rooting blackgrass.

How do you master broad-leaved weeds?

The real secret of the job here is to get good pre-emergence control. Make sure you use chemistry that matches the spectrum of weeds you anticipate. Mixtures might be needed for good cleavers control, for example. The pre-em will also help to sensitise the blackgrass.

Where you have poor seed coverage, it may be an idea to delay the pre-em until rape cotyledons have emerged, but this will reduce control of any weeds that have also emerged.

Unlike in cereal crops, opportunities to clean up weeds that escape control in the spring are limited. Products are available, however, and a high population of charlock in a small, struggling rape crop can make a treatment worthwhile.

The best time for volunteer control coincides with wheat drilling, so is often left too late.

These need to be removed, or your rape crop will have lumps missing where the combine went through, leading to a variable crop that will bring you headaches later on. Again there’s rarely a chance to take these out pre-drilling, so an application of a foliar-applied "fop" or "dim" herbicide works best.

The difficulty is that the right timing will probably coincide with wheat drilling so volunteer control is all too often left too late, which will often have an impact on yield.

What stewardship issues are there?

There are relatively few herbicides used in oilseed rape, but they are being detected more frequently in water at levels exceeding the EU Drinking Water Directive limit.

More soluble herbicides can wash easily into drains and water courses. Those that dissolve less easily bind to soil particles that can run off as a result of flooding or heavy rainfall. To limit your herbicide being washed away:

• Establish a 6m buffer zone next to water courses
• Try in-field erosion control measures, such as tramlines following contours and grass strips
• Wait for dry, cracked soils to moisten and close up before applying herbicides
• Do not spray if drains are running or heavy rain is predicted
• Spray headlands last, to avoid picking up herbicide on sprayer wheels.

Three golden rules

1 Minimise soil disturbance to keep blackgrass near the surface
2 Aim for an even crop with a canopy that will help smother weeds
3 Keep on top of volunteers