How to tackle different grass weed control scenarios in the Midlands

Crop Progress

Most cereal crops, in terms of crop vigour, look in good condition. The warm spell around New Year helped race things on, and I don’t think I’ve ever seen our winter wheat trial at Hinton Waldrist, near Oxford, which was drilled on 16 October, look quite so good at this time of year.

Oilseed rape is a bit iffy in places. In places some looks good, but it’s quite difficult to judge generally with it having taken a pasting from the frosts in December and January. It’s when it starts growing away, we will really know.

I’m sure some more oilseed rape will be written off as we approach spring. There’s been some larval attacks in side-shoots – warm weather might be good for growth but it’s also good for bugs, and damage was evident before Christmas.

Ben’s agronomy tips for February

 1. Assess where you are with weed control

Grassweed control is a game of three halves in my area in wheat. You’ve got early drilled crops that look good. The pre-emergence herbicides were applied, but it was too dry and too warm for too long after application so by the time the black-grass had emerged all the activity had disappeared and there’s often a mess.

The second situation is the stuff that probably looks the best, which is perhaps ‘slightly earlier drilled than you might like’ through to mid-October drilled. The pre-ems went on – it was still just ahead of the rain coming, so it did sit there for a week in the dry, but when it did rain the residual herbicides still had enough activity to do a reasonable job.

Half number three are later drilled, almost what you should done culturally in terms of drilling, but people just drilled and drilled and forgot about the sprayer sitting in the yard. It then rained and was too windy, so no pre-ems were applied on some of these crops. The crops came through in record time as it was still warm and then everything was peri-emergence and we were playing catch up. Weed control in these crops wasn’t looking too clever going into winter. However, these are the ones with slightly smaller black-grass and some residual activity still there, where a good dose of cold weather can sometimes help things look better coming into the spring than it did earlier.

So what can you do in those situations? In scenario one, the crop looks quite good so do you gamble on the crop being competitive enough to give a reasonable yield, albeit you’re unlikely to be able to control any of those bigger black-grass plants with anything you can do now with herbicides. You’re really mostly suppressing the weeds to help the wheat get away. The alternative is deciding it’s too bad and using the big R can and going down the spring cereal route.

The scenario two growers are sitting more pretty – the crop has had one, if not two residuals. They got lucky with the first one and topped up, and now just need to watch for weeds that escaped or emerging in the spring. They are probably the most likely candidates for spring mesosulfuron-containing products, if they’re needed.

In the late drilled / no pre-em scenario it is a little too early to say, but already there are some crops that look like it would be crazy to take through to harvest, they are that bad, even having a couple of peri-em or post-em applications. Those will need the reset button.

If mesosulfuron products have not already been used, however, it might be the situation that gets the best results from those products if there is a weather window in February – given the smaller size of weeds being treated. If you’re very lucky the grassweeds might only just be tillering.

Don’t wait if you get an opportunity to apply a mesosulfuron product in February. A couple of days of bright, sunny weather does wonders for making it work, even if it is quite cold.

A new option in mesosulfuron-based products for this spring is Atlantis Star (mesosulfuron + iodosulfuron + thiencarbazone). The big benefit with it is you can apply a full 15g/ha dose of mesosulfuron from 1 February, unlike Pacifica Plus (mesosulfuron + iodosulfuron + amidosulfuron) where that dose is only available from 1 March and only if brome is the target.

2. Continue monitoring for light leaf spot

There’s not been much light leaf spot around in my area so far. It’s a watching brief at the moment – our Spotcheck service with ADAS is there to help identify oilseed rape diseases, if you want to check.

But unless there is rampant light leaf spot – so relatively easily found in crops – in February you may well be able to wait until stem extension to apply a fungicide. But if you are finding those pepper pot type symptoms on the underside of leaves then it will be worth getting on top of it early, particularly if you’re going through the crop with something else.

3. Finish off cover crop destruction

Some growers with cover crops will have knocked back top growth through either rolling on a frost, grazing with sheep or purely the cold weather taking out some species. Where the rolling or sheep techniques might have struggled is to also knock off the below ground part of the cover crop, although it should have made the target a bit easier to hit a second time around.

So if the top growth starts regrowing again before you’re ready to drill the spring crop, then an application of Roundup (glyphosate) will likely be required – I think most growers will put something on.

Think carefully about what you’re trying to achieve with the application. If it is just to control that top regrowth then you can probably bring down the dose a bit, although I would always err to 1000 g/ha for most species.

But if you’re doing it to be translocated down to the roots and get some effect below ground, then bigger doses will be required as they have been built on a much greater amount of top canopy than might be there at the point of spraying.