Post-emergence grass-weed control and cover crop termination advice for northern growers

Crop Progress

Cereal drilling for most of the north of England is complete, except for after some root crops. The forecast as we approach the end of October is a bit more wet for the next 10 days so drilling might become a little stickier, but I would expect growers to be able to find a window to drill still.Earlier drilled crops have emerged well, but in some places so have the grass-weeds.Oilseed rape is a real mixed bag – some crops are racing away, while there are others that are not nearly as good. Agronomists tell me that around the same area of oilseed rape was planted but with higher levels of cabbage stem flea beetle damage, there will be more lost this season.

Tom’s agronomy tips for November

1. Apply pre-emergence to late drilled wheat

While there’s not a lot of wheat left to be drilled, there will be a few crops after roots or even where delayed drilling is being used as a tactic against difficult grass-weeds.In the latter situation, hopefully every opportunity to remove these weeds before drilling will have been taken, which, together the delayed drilling, lessen the threat.Even so, a pre-emergence residual herbicide is likely to be needed – probably based on Liberator (flufenacet + diflufenican). Add in other partners, such as pendimethalin, prosulfocarb or Proclus (aclonifen) as risk dictates.Most root crop land is likely to be lighter soil types and perhaps lower risk from grass-weeds. A pre-emergence Liberator should deal with most weeds coming through, although again add a partner if you have a known threat from a weed it struggles on.

2. What about earlier drilled wheats?

Residual herbicides may well be running out of steam in earlier drilled crops, with the longevity of many reduced with quicker breakdown in the warmer soils. Couple that with earlier use and generally drier conditions, and it’s no wonder some of the residual treatments might have struggled.There are three options for dealing with difficult grass-weeds now coming up in emerged wheat crops – although no easy decisions.The first is where the crop is getting swamped, consider spraying off with glyphosate. While this is difficult to stomach having spent on the crop already and at current commodity prices, an earlier decision will save spending any more money on a crop that is going to struggle, as well as lowering seed return. There is still time to start again.Where weeds are still no bigger than one true leaf, there is the possibility of going with a residual top up spray. This is more likely on those later October drilled crops. Options include Liberator – although remember it needs to be applied six weeks after a previous application of Liberator – and the metribuzin-containing products Alternator Met and Octavian Met (flufenacet + diflufenican + metribuzin).Metribuzin will add a little bit of contact to the residual activity, which will be useful on these very small grass-weeds. Both products can be used at 0.5 L/ha, up to the end of November. Again other residuals that can be used post-emergence can be added as required.If the weeds are bigger than one true leaf then a stronger contact-acting product will be required. It’s not an easy decision whether to use Atlantis OD (mesosulfuron + iodosulfuron) before Christmas rather than spring, as it does limit your options in the spring. But control now is likely to be better than in the spring, especially while soils remain warm, although you risk spring germinators being left uncontrolled. Adding in a residual partner will help mitigate as much as you can, although if you are expecting bromes then probably wait until spring.

3. Monitor for oilseed rape diseases

With two very different oilseed rape crops around, decisions on whether to consider applying an autumn fungicide will start with whether you think it will make it through winter.Assuming you do, then the next step is to monitor for the key diseases Phoma and light leaf spot. Watch out for the AHDB forecasts for these diseases to help judge risk, and use our Spotcheck service, in conjunction with ADAS, to assess actual risk from leaves sampled in fields. If you would like a sample kit, please get in touch with me.Where a spray is required, the choice is between Proline (prothioconazole) and Aviator (bixafen + prothioconazole). Aviator offers stronger disease control and some physiological benefits to rooting.

4. Consider when to terminate over wintered cover crops

Looking beyond November, there will be the annual debate around when to terminate cover crops. Obviously the longer they can be left in the ground, the better for soil health and carbon cycling.But, particularly before spring cereals, terminating them enough in advance has been shown to important to minimise establishment and yield of the following crop – spring barley seems particularly sensitive.Glyphosate remains the most reliable method of terminating, and probably should be done 4-6 weeks in advance of intended drilling date.Focus on dose and application technique. Dose should be tailored to the most difficult to kill cover crop species in the mix – vetches and other legumes are quite often hard to kill, while mustards and borage easier. You might need over 1000g/ha for legumes, for example.    

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