Semi-dwarf varieties could trump blackgrass issues

Semi-dwarf oilseed rape varieties can have a valuable role in tackling blackgrass but how do you get the most out of them. Jackie Crame takes a look

Semi-dwarf oilseed rape is the way to win in the battle against blackgrass in the rotation, according to promising trials this year.

The work being carried out by Dick Neale and colleagues at Hutchinsons, on four- to six-hectare plots within crops at Brampton, Cambridgeshire, was set up specifically to focus on controlling blackgrass.

“We are showing that it is wrong to believe that semi-dwarfs are no good at weed control, are more susceptible to pigeon damage and have a poor root system,’ says Mr Neale, “Growers should not be fearful of choosing a semi-dwarf variety to compete with weeds, especially blackgrass.” The varieties tested in the trial were PR DO6, PR DO7 and Troy.

The Hutchinsons’ blueprint for blackgrass control begins with cultivations which are crucially important. They recommend going straight in behind the combine and rolling, two to three times in different directions to get the shed blackgrass seed into the soil moisture and induce early germination.

Drilling date must be between 15 and 25 of August to make maximum, good ground cover. The competitiveness of the crop is highly reliant on strong, early growth.

The morning of drilling, the stubble should be sprayed off with glyphosate mixed with Sprite Aqua as an adjuvant.

“Its best to spray because blackgrass will be there even if you can’t see it. Lots of seed will have been shed before harvest and will have been germinating in the bottom of the wheat,” warns Mr Neale. “There only needs to be a three-hour turnaround between spraying and drilling.”

He recommends a seed rate of 80 seeds/sq m, which for semi dwarfs equates to about 2.5-3kg/ha, depending on the thousand grain weight. The aim is to achieve 70 plants/sq m. Semi-dwarfs can be drilled at higher rates without the risk of lodging as in conventional varieties.

The focus has to be on moving the minimum amount of soil so discs should definitely not be used. The aim is to drop the seed in rows behind the legs of the drill with minimum soil disturbance, then to roll.

Fertiliser at the drilling stage, fed into the air system at the base of the main seed unit, targeted to feed the newly emerging crop and not the weeds is also essential.

To achieve this Hutchinsons used microgranular, high phosphate, Primary P, to encourage “hard” crop growth not the “soft’, green, disease prone growth stimulated by high nitrogen.

“At drilling, 10 kg/ha, equivalent to 4kg/ha of P and 2kg/ha of N was placed immediately next to the seed without causing any damage as it has low acidity. The phosphate is readily available and not locked up in the soil so the benefits last all season.”

The programme included a pre-emergence herbicide mix of metazachlor with or without Centium (clomazone), rate depending on the target broad-leaved weeds.

When the crop is at the three to four-leaf stage and the blackgrass about two leaves the recommendation is to apply carbetamide at 1.5-2 kg/ha.

“Do not be tempted to apply a higher dose, because of the risk of damage to the small rape plants, particularly if it rains,” warns, Mr Neale.

He advises using the lower dose in a wet year and the 2kg rate in a drier, warm season. This will not kill the blackgrass, but it will damage the roots, keeping them short and within the top few centimetres of soil.

If this is followed up four weeks later in early November with a full dose of cortizamide (2.1 litres/ha), the weakened weed seedlings will show a really good level of kill.

“Following this regime rigidly results in over 98% of blackgrass being wiped out,” estimates Mr Neale, “but it does not work if all the rules are not followed.”

The biggest thing that growers tend to get wrong is the cultivation element. The objective should be to achieve a plant with a shallower, multi-rooted system, not a plant with one long taproot. Cultivations should not disrupt the worm burrows, the crop needs these to achieve the ideal eight to nine root forks. Stirring up the soil also decreases the activity of the chemicals.

Semi-dwarf differences

Semi-dwarfs only differ from conventional varieties in stem extension in spring.

They can smother blackgrass by throwing out sideways an interlocking rosette of big leaves, close to the ground in the autumn. Under this canopy virtually all the light is excluded. Under a conventional crop the taller plants allow lots of mottled light through so weeds continue to grow slowly. This does not present a problem to the crop but the vital rotational advantage is lost, concludes Mr Neale.

These findings are confirmed by ADAS trial work which shows that the overall biomass of semi-dwarfs is virtually the same as conventional varieties. “Although PR45 DO3 compared with Excalibar has a stem approximately 40% shorter, it requires virtually the same amount of nitrogen to build the same unit area of canopy,” says Pete Berry of ADAS. “It produces lots of branches close to the ground.”

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