Weed control advice for wheat crops in November

Crop Progress

Let’s start with the positive – more oilseed rape seems to have survived than I can remember up to this point (20 October) in the past few years, and there are some good-looking crops around with the caveat that I haven’t assessed any for larvae yet.

Most winter barley was drilled and is now up, but whether it had a pre-emergence herbicide is another matter, or what it has done to the crop where it has been sprayed. It’s probably realistic to expect some degree of whitening or yellowing, given the amount of rain we’ve had, although it should grow away from it in around three weeks.

Progress with winter wheat is anything from completely drilled up to those who haven’t turned a wheel. I reckon about 60% maximum had been drilled as we approach the last 10 days of October, with most of that drilled in a window around 3-10 October.

After that the rain started. We had 80mm at Hinton Waldrist near Oxford, where our Field Days site is, in 10 days from 11 October. We now need a good week of dry weather with wind to dry land to allow fieldwork to resume. Hopefully by the time you read this, maybe that will have happened although the current forecast is far from promising.

Ben’s agronomy tips for November and beyond

1. Weed control options for November drilled wheat crops

The good thing about drilling wheat in November, if there is such a thing, is that your grassweed pressure should be greatly reduced, helped, hopefully, by a dose of glyphosate to take out either the first or a subsequent flush that’s come up.

That should mean more grassweeds have emerged before drilling than in earlier drilled crops, and with the amount of soil moisture we now have, that you shouldn’t need to spend quite as much on residual herbicides. Reducing soil temperatures should also help the actives last longer, and mean you only need one autumn residual herbicide spray, which is good because it’s unlikely you would be able to apply two anyway.

So when thinking through the pre-emergence herbicide options available, I think that steers you ones that will hang around for longest, such as aclonifen and diflufenican, which are in a Liberator (flufenacet + diflufenican) + Proclus (aclonifen) mix.

Pendimethalin is also fairly persistent, but clearly on wet seedbeds you’ve got to be quite careful with it, and with others too.

2. Weed control options for October drilled wheat crops

For those who managed to apply a residual already, the main consideration, if you’re thinking of going again, is to make sure the crop has recovered from whatever you did first time around.

The trickier scenario is where you were able to drill in that early October window before the rain came, but haven’t managed to apply anything. The crop by now will probably be well past peri-emergence so the dilemma is what to do now?

In my opinion and the one farmers hate hearing is if you have a carpet of black-grass or Italian ryegrass, the first option is Roundup (glyphosate).

If you do need to or can persevere, then unfortunately it means aclonifen is no longer an option as it is pre-emergence only.

Where it is a true post-emergence spray, then the best option is probably a Hatra / Horus / Atlantis OD (mesosulfuron + iodosulfuron) plus partners and going for a one-hit wonder. In our trials consistently the best partners have been either Octavian Met or Alternator Met (both flufenacet + diflufenican + metribuzin), which you can now use at the full 1.0 L/ha dose. Other options could be prosulfocarb or pendimethalin.

If you do have crops that have only just reached one leaf then the pre-emergence options, except aclonifen, you were thinking of using are probably still in play, although everything will be less effective than it would have been at pre-emergence. But again at that timing our trials have shown Alternator Met and Octavian Met probably fall off the least of any option.

3. Check for oilseed rape diseases

Over the years I’ve become less enthused about the need for oilseed rape fungicides in the autumn. Definitely go and check – as high levels of Phoma, especially on small crops, will need treating – but it’s quite difficult to prove the worth of autumn applied oilseed rape fungicides these days.

If you’ve got a lot of light leaf spot that’s the most obvious reason to spray, and 0.5 L/ha of Aviator (bixafen + prothioconazole) is one of the strongest options according to AHDB Appropriate Fungicide Dose trials. It gives you two modes of action against light leaf spot, but it is a more expensive choice.

But SpotCheck over the past few years has shown that the levels of light leaf spot are relatively low in the autumn, and if it does come in then a February application as soon as you can travel is probably just as effective.

4. Spray off cover crops well in advance of spring drilling

While I recognise the jury is out on this, I’m in the camp of spraying cover crops off early around Christmas. If you’ve had enough of turkey, get on the sprayer and start spraying off cover crops with glyphosate, particularly if you’re looking at a February to early March drilling programme. I think spring crops, especially spring barley, needs a good eight weeks following termination.

Remember dose is important, particularly if you have cover crops with a sizeable burden of grassweeds. Don’t skimp if you have well-tillered grassweeds to spray off, especially Italian ryegrass, and you’ll also need a good dose for some cover crop species that are trickier to kill off. At a minimum you’re looking at 1080g a.s./ha, and some will need more.