Winter Oilseed Rape Update – August 2020

The monitored commercial crop data for the adopted crops this week is shown in the table below.

This data is generated in commercial crops in your area so is indicative of the disease pressure and growth stages on farms at the moment. It should give you a guide to the generic disease pressure in your area.

Pests

During the oilseed rape harvest keep an eye on the sides of the trailers. The presence of any blue-black beetles in harvested oilseed rape seed or seen on the trailers is a warning that there could be a problem in the next winter oilseed rape crop from cabbage stem flea beetle. The beetles won’t harm the seed in the store and will be keen to get out of the store and find an oilseed rape crop to feed on.

Weeds

There are now more options for varying approaches to weed control in oilseed rape. For fields that have cruciferous weeds or volunteer oilseed rape, Clearfield varieties are a good choice. Volunteer oilseed rape and cruciferous weeds can reduce the yield and may also give rise to deductions due to raised erucic acid levels. Clearfield herbicides used with these varieties are applied early post-emergence which can be another benefit of this system.

If there isn’t a specific cruciferous weed problem, the decision depends on how the crop is established and on soil conditions. Method of establishment has a big effect on what is the best approach to weed control. Pre-emergence herbicides work best where the soil is warm and moist, seed well covered by 1.5 cm soil (2cm if clomazone is being used) and there is a low risk of crop failure. They generally give most cost-effective control of the widest spectrum of weeds – it is the best option for ploughed ground in good conditions with predicted rapid crop establishment. Pre-emergence treatment is particularly beneficial on shepherd’s purse, crane’s-bill, cleavers and poppy – these are more difficult to control if using metazachlor co-formulations post-emergence. The aim is to apply to moist soil immediately after drilling, particularly if using straight metazachlor or metazachlor + clomazone. Clomazone is a pre-emergence only herbicide, it cannot be applied post-emergence.

For crops drilled into stubble and seed not well covered, or if it is likely for any reason that the crop may fail to establish it is generally best to avoid a pre-emergence residual. If the seed is not well covered by soil, treatment should be delayed until the crop has two true leaves.

In the event of crop failure, a field treated with a metazachlor product would need to be ploughed to 15 cm and then sown with wheat in the autumn, or left until the spring before re-sowing. If clomazone had been used, the field would need to be ploughed to 25 cm and only winter cereals or beans could be sown in the autumn, after 6 weeks had elapsed, or left until the spring before re-sowing.

For crops sown direct into stubble, a pre-sowing spray with glyphosate will ensure a clean start, then a follow-up will control any weeds that germinate. If the seed is well covered by soil, then the follow-up can be a residual. If the seed is not well covered or there is doubt over the crop’s viability, it is better to wait until the crop has emerged. It can be treated early post-emergence with a metazachlor-based product once the crop has two fully expanded colyledons, or slightly later with a halauxifen-mthyl + picloram product, which controls a broad range of weeds early post-emergence including shepherd’s purse, poppy and cleavers.

Metazachlor gives a good start to annual meadow-grass, chickweed, mayweed, shepherd’s-purse and groundsel control. Dose of metazachlor depends on timing, soil type (the lighter the soil the lower the dose) and weed species expected – higher doses are needed for shepherd’s purse and poppy.

Metazachlor + clomazone is an alternative option where shepherd’s purse and/or cleavers are the main weeds. It can also control hedge mustard which is a problem in some parts of the borders and north of England, and can lead to raised erucic acid levels. This has proved a useful alternative to improve control of cleavers, shepherd’s-purse and red deadnettle amongst a wide range of other broad-leaved weeds plus annual meadow-grass. A metazachlor / clomazone tank mix may only be used pre-emergence of the crop. There can be a transient bleaching of the crop, but it recovers rapidly. Note the use of clomazone has been found to sensitise grass weeds such as volunteer cereals, brome and black-grass to following graminicides.

Metazachlor + dimethenamid-p contains 200g/l metazachlor with the addition of 200g/l dimethenamid-p recommended at 2.5 l/ha. The dimethenamid-p has root and shoot uptake compared to just root uptake from metazachlor. This mix gives improved pre-emergence residual control of geranium spp. and poppy. It also has useful activity on shepherd’s-purse pre-emergence to 1 true leaf.

Metazachlor + quinmerac. Quinmerac is very soluble but highly bound to the soil and taken up by leaves and roots and compliments the root uptake of the metazachlor. Compared to straight metazachlor, it gives a little more activity on fumitory, cleaver control pre-emergence to cotyledon, control of poppy, red-dead-nettle, speedwells, shepherd’s purse and mayweeds.

Metazachlor + dimethenamid-p + quinmerac. The three active ingredients combine to give root, shoot and leaf uptake with improved and more consistent pre-emergence and post-emergence control of problem weeds – better control of shepherd’s purse and poppy than straight metazachlor applied pre-emergence.

Cleavers

If you have cleavers a clomazone + metazachlor tank-mix can be used as a more flexible alternative. As cleavers can germinate from depth the pre-emergence herbicides may not give 100% control and a post-emergence follow-up may be needed. Halauxifen-methyl + picloram is a useful autumn post-emergence option for cleavers.

Volunteer oilseed rape and charlock

Volunteer oilseed rape, particularly spring rape, and charlock can interfere with crop quality. Spring rape has survived some recent winters quite happily and flowered before and during the winter rape to re-contaminate the ground as well as contaminating the crop. If there is time, and you expect a problem, prepare the land for sowing rape after the winter barley harvest as soon as possible and hope to encourage germination of the volunteer rape and charlock, then use glyphosate to burn out the weeds, sowing the rape crop directly afterwards. This is only advisable if there is plenty of soil moisture.

Bifenox, which is applied post-emergence, may have some effect on charlock, reducing its winter hardiness.

If there is a serious cruciferous weed problem the most effective solution is to sow a Clearfield variety. A herbicide such as imazamox + metazachlor or imazamox + quinmerac or imazamox + metazachlor + quinmerac can then be used – the weed spectrum includes charlock, runch, shepherd’s purse, chickweed, fumitory, and OSR volunteers of non-Clearfield varieties.  Note that these herbicides are for exclusive use on Clearfield branded varieties of winter rape, they will kill non-Clearfield varieties. They are applied post-emergence from two expanded cotyledons up to eight true leaves (GS18). Imazamox is an ALS inhibitor so it is important to remember which fields have grown Clearfield rape as the volunteers in following cereal crops will not be controlled by other ALS inhibitors (e.g. tribenuron etc).

Volunteer cereals and other grass weeds

Metazachlor controls annual meadow-grass. It has some effect on volunteer cereals and some other grass weeds but needs a follow-up treatment for problem grass weeds. The easiest option is a ‘fop’ or ‘dim’ graminicide that has foliar action. Control of volunteer cereals, wild oats etc. can be very effective, although sometimes weeds are missed if they are shielded by the oilseed rape canopy, so it is generally best to apply a lower dose graminicide early to ensure good coverage and optimize oilseed rape establishment. This may be followed if necessary by a second dose, or in fields that have problem grass weeds, by propyzamide once the soil temperature has fallen below 10°C. Propyzamide is root acting and controls a range of grasses and broad-leaved weeds, including grass weeds resistant to fop and dim or ALS herbicides. But note that weeds need to be rooting near the soil surface so propyzamide works particularly well in undisturbed soil such as that between sub-cast or band sown rows. In deeply cultivated soils, weeds that have germinated from depth may be missed. For broader weed spectrum, propyzamide + aminopyralid can be used.

 Protection of water

With all residual herbicides in oilseed rape, it is important to remember that all the common rape herbicides are regularly being detected in water courses above EU drinking water limits. To help protect metazachlor, there are label restrictions – no more than 1000g/ha active ingredient applied to the same field over a three year rolling period and no more than 750g active per crop per season.

The latest VI Water Protection Advice Sheet advocates a cut-off for application of metazachlor or quinmerac to drained soils – they should preferably be applied by 30^th September, and should not be used after 15^th October. Additionally the general VI best practice advice to protect water should be followed:

• Fill sprayer in a bunded area and clear up any spills immediately
• Ensure there is a 6m grass buffer strip next to water courses
• Wash sprayer down in the field or in a bunded area
• Do not apply when soils are cracked, dry or saturated, or if drains are flowing.
• Do not apply if heavy rainfall is expected within 48 hours of application as this can lead to significantly higher losses to water.

In high risk areas metazachlor and quinmerac should not be applied after 30^th September, and at least six of the following should be met in order to reduce the risk:

1. Soils are moist and NOT dry, cracked or saturated.
2. Field drains are NOT flowing and are unlikely to flow within 7 days of application.
3. Field slope is less than 5% (a 5% gradient is 1 metre fall in 20 metres).
4. The field is NOT bordered by a watercourse.
5. Metazachlor is applied at less than 750g ai/ha or quinmerac at less than 250g ai/ha, especially in combination with other herbicides. Check required dose with your BASIS-registered adviser.
6. The field has a 5m no-spray zone or 6m grass buffer strip adjacent to water.
7. The field has NOT been deep sub-soiled (below plough layer) in the preceding 6 months
8. The crop has been established early with minimum tillage of only the top 4-6 cms or by direct drilling.
9. There is NO risk of heavy rainfall within 48 hours of application.

 For more information on weed control in winter oilseed rape click here