Advice on difficult decisions growers face for grass-weed control this autumn in the Midlands

Crop Progress

My area of the Midlands is not far off being drilled up with cereals – we were about 75-80% complete a week from the end of October. Unfortunately, we’re closer to being drilled up than sprayed up, and the heavy rain for many over the weekend of 22/23 October has made it the wettest month of the year so far.Hopefully growers do get caught up with residual herbicide applications, although crops are emerging very quickly on the back of very warm soils for the time of year. The rain could bring a monumental flush of grass-weeds now, which will make those pre-emergence herbicides work hard especially on September drilled crops.Oilseed rape is all over the show – some is looking pretty good, some has been written off and some still looking a bit borderline. Attacks have been mainly from cabbage stem flea beetle, but there have also been some pretty active slugs despite how dry it has been. We’ve also seen some turnip sawfly damage.

Ben’s agronomy tips for November

1. Opportunity to lower pre-em spend in November drilled wheat crops

If you have difficult grass-weeds and you’re drilling in November it is usually for good reason. But grass-weed pressure should be less, especially if you’ve maximised out of crop kill, while the residual activity from some of the chemistry should last longer in lower temperatures than it would have done in October.That should open the potential to bring pre-emergence mixes back a bit without compromising control – a Liberator (flufenacet + diflufenican) plus a little bit of pendimethalin or prosulfocarb might be all the field needs. I would try to do the minimum I think I could get away with for November drilled crops as it will reduce the stress on the crop and allow it to get to a reasonable size before winter, which is what you need from a November drilled cereal.

2. Options for emerged wheat crops with grass-weeds

If you’ve drilled earlier and are facing what doesn’t look very clever in terms of high populations of grass-weeds, option one is always: start again.That’s obviously a difficult choice at quite high wheat prices when you have potentially already spent a considerable amount on seed and one or two residual herbicide sprays, but that early decision before any more is spent on post-emergence or growth regulators and fungicides is the cheapest option to make if the pressure is that bad.If you do want to persevere with the crop, the next question is have all the grass-weeds emerged? Even into November the answer is probably not as we know what a protracted germination these grass-weeds have. Even a small amount of protracted emergence as a percentage of the seed bank could still be a lot of plants coming up.In that situation, it probably depends on how much grass-weed has already emerged. If not much, I’d apply a second residual herbicide application and base it around a product containing at least 120g/ha of flufenacet mixed with other actives as you shouldn’t be using flufenacet alone.If I’d used pendimethalin at pre-emergence I’d use prosulfocarb in the peri- / early post-emergence spray or vice-versa. There’s obviously the option to change the base product now with cinmethylin in the market. If I’d used that at pre-em, then flufenacet is the obvious base to a follow up. If flufenacet was used at pre-em, then it can still provide the backbone of the follow up spray, as to use cinmethylin as peri-em may be too expensive an option for many.As soil temperatures hopefully fall, and seedbeds are likely to be moist, actives such as metribuzin also come into their own as partners at this timing. They have short half-lives so when the soil is cooler, they will last a bit longer. Metribuzin is also relatively mobile and can get down to weed roots quicker than other actives. It’s available in combination with flufenacet and diflufenican in Alternator Met or Octavian Met.But if black-grass or Italian ryegrass is already at or past the one true leaf stage you’re probably beyond these options and you will need to turn to a mesosulfuron-based option, such as Atlantis OD, Hatra or Horus (mesosulfuron + iodosulfuron).You won’t apply these on their own so what do you apply with them?Technically the most efficacious mixer is a full 0.6 L/ha dose of Liberator but you must consider what (if anything) has already been applied to the crop and stay within label guidelines on doses and timings. Remember there needs to be a six-week gap between applications of Liberator if used at pre-emergence and then as a follow up and there is a 0.9 L/ha maximum total dose.Alternatively 0.5 L/ha of Octavian Met or Alternator Met, up to 3 L/ha of prosulfocarb or up to 1000 g/ha of pendimethalin are good options.Some might argue against using a mesosulfuron-based product in the autumn, and it is a difficult decision. At this point you have a chance of reducing black-grass numbers, and it is those plants that are there now that will be the ones that impact yield the most and produce the most seed return. If you can reduce them by 40-50% you maybe be giving the crop a fighting chance.What you may be missing by going in the autumn is any spring germinating black-grass, Italian ryegrass, and of course wild oats and bromes, as well as shooting the bolt on using any grass-weed-active sulfonylurea herbicide in the spring.The trouble with March / April applications is for some growers then the chances of getting any meaningful control of black-grass plants is more limited – you are looking at a growth regulatory stunting / suppression type effect. Work has shown that this effect should reduce seed return as the heads will be smaller and have fewer viable seeds, but you are highly unlikely to get the same level of kill that you will in the autumn.It comes down to a choice between a high level of suppression in the spring versus 40-50% control in the autumn, and the jury is probably out on which gives the least seed return. But I would argue that getting some proper control in the autumn is doing a better job for black-grass than just knocking it back in the spring, albeit at the cost of having to use alternative modes of action in the spring for other weeds.The one time I would consider waiting until the spring is if you have or are expecting a rise in rye or meadow brome species – the herbicide options are much more limited so you almost certainly are better waiting until the spring in that case.

3. Monitor for oilseed rape disease

I find it extremely difficult to get excited about oilseed rape disease control in the autumn, especially with the good varietal resistances we now have available to both Phoma and  light leaf spot.There are very few crops currently that I am worried about for disease.With the crops that are getting away even if they were infected with Phoma early on, now with the size of the leaves you would question whether you need an application.If you are concerned that disease might be coming in, then our free Spotcheck service, in conjunction with ADAS is available, which will help confirm whether crops are infected. This is useful into November and through the winter to help justify whether you might need to spray. If you would like a sample kit then please get in touch with your local Bayer technical contact.If you do want to start a disease control programme in the autumn – for example in mix with the Kerb (propyzamide) if you are going through the crop, the obvious thing to add is something that is prothioconazole-based to cover both diseases.Aviator (bixafen + prothioconazole) could be a fit in the autumn, especially if you are worried about using too much straight azole fungicide, with its mixed modes of action. However I am much more of a fan of spring use of this product at the mid-flowering timing.

4. Terminate cover crops later in the year

I’m coming around to thinking that earlier termination of overwintered cover crops is better to make sure they are rotting down well in advance of spring cereal drilling, especially if direct drilling strategies are being employed to prevent stimulating weed emergence. For some who are aiming to drill spring barley in February that will mean spraying off with glyphosate around Christmas.Don’t be stingy with the rate of glyphosate. A lot of cover crop species are quite resilient to glyphosate – for example certain species of vetches and phacelia could well require over 1000g/ha of glyphosate, particularly if there is a lot of biomass. Don’t make the spring drilling harder by not killing it off properly.    

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