Weed and disease control advice for growers in the south

1) Grassweed pressure unsettlingly low but keep monitoring

The combination of later drilling plus decent moisture levels has led to extremely low grassweed pressure as we go into March. Feedback from agronomists is that it is difficult to find grassweeds, which is quite unsettling for some, but the later drilling has clearly had a beneficial impact.

Still, field histories suggest there is grassweed pressure, and while we know spring germination is lower than that in the autumn, it remains key to continue to monitor fields because after a good autumn, the last thing you want now is to be caught out.

Where there hasn’t been much germination, but still a risk, or it’s only just starting to come through and black-grass or Italian ryegrass is no bigger than one leaf, then laying down more residual in early March is an option.

The additional metribuzin in Octavian® Met, Alternator® Met or Cadou® Met helps lift their performance over those that only contain flufenacet and diflufenican. These can be applied up to GS25 of the crop at a rate of 0.5-1.0 L/ha. Other actives such as pendimethalin or chlortoluron can be added to boost broadleaf weed control.

Switch into contact herbicides, such as mesosulfuron-containing products, when grassweeds are at or beyond the two-leaf stage. Both Pacifica® Plus, if targeting bromes, and Atlantis® Star can deliver 15g/ha of mesosulfuron and are best applied while the weeds are still small.

2) Watch out for brown rust in wheats

The last couple of seasons it’s been difficult to apply T0 fungicides in wheat because of challenging weather conditions, and certainly last year that was noticeable with the extent of brown rust infections.

Cooler temperatures hopefully have reduced the risk from brown rust this season, although inoculum carryover will be that much greater after last season and there are only two varieties on the Recommended List with resistance rating of more than six. I’ve also already had three reports of brown rust in Crusoe and it’s not even the end of February yet.

That means it would be amiss of us not to plan for another round of infection, and as we learned last year, once brown rust is in the plant, it’s very difficult to eradicate. Getting on top of it at T0 is therefore important.

Options from Bayer include a prothioconazole plus fluoxastrobin approach in Firefly® or Sublime®, or alternatively a good level of azoxystrobin – around 0.75 L/ha – partnered with another mode of action, such as tebuconazole, that targets the same disease.

Yellow rust is the other key disease to watch out for in wheat around this timing. Again, slightly puzzlingly we haven’t yet had the normal round of What’s App messages, highlighting its presence in a susceptible variety, but there’s still plenty of time for that to change as we go through March.

You can spray for Septoria at this timing, and might get some incidental control if you use prothioconazole-containing products for rusts, but generally I don’t think with the options available to us now, that it is worthwhile specifically targeting it at this timing.

3) Retain winter barley tillers at GS30/31

Disease control in winter barley around GS30/31 is crucial as it’s around those growth stages where the crop builds up its yield potential sink. Any disease infections can cause the plant to abort tillers and therefore reduce yield potential – and it’s not yield you will get back later in the season by protecting green leaf area on upper leaves.

As the weather warms up and assuming continued showers during March look out for Rhynchosporium and net blotch, along with brown rust, particularly in hybrid barley.

Products like Ascra® Xpro® and Siltra® Xpro® provide proven all-round disease control in barley, with the bixafen element brings increased nitrate reductase activity and chlorophyll benefits, as well as improved rooting and stress reduction within the crop.

4) Resistance risk brings extra scrutiny on glyphosate applications

With three confirmed cases of resistance to glyphosate in Italian ryegrass populations, including one in Kent, following Weed Resistance Action Group guidelines is ever more important when using Roundup® either to spray off stubbles or cover crops ahead of spring drilling.

These include using no more than two applications of glyphosate on stubbles or cover crops, and making sure any weeds surviving the first applications are controlled using a different method, such as cultivation.

Most of the time poor control won’t be due to resistance. Last year we found that in some cases there was difficulty in getting sufficient control with glyphosate, which was mainly due to spraying when plants were not actively growing in saturated soils coming off the back of a very wet winter. There were also occasions when incorrect doses were applied, when applications had been delayed and the rate recommended was not fitting the size of weed at application.

Typically for 1-3 leaf black-grass and Italian ryegrass rates should be a minimum of 720g a.s/ha, while for tillering grassweeds that should be increased to 1080g a.s/ha.

Be careful if you’re planning to use a post-planting application of Roundup® with some residual chemistry in spring cereals. The maximum label rate for this application is 540g a.s/ha, which is why this is intended to target new emerging grassweeds, and is therefore inappropriate to use if insufficient stale seedbed grassweed control has been achieved. Grassweeds bigger than one leaf might not be controlled and will represent more of a resistance risk.

Most cover crops should have been sprayed off in February or before but if weather has prevented this or its still to do ahead of later-drilled spring crops, carefully assess what the hardest remaining species is to control and tailor rate to this. That’s perhaps most likely to be vetch species, which might need 1080g a.s/ha, and could take up to six weeks to get a full kill, depending on environmental conditions.

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Alternator® Met, Cadou®Met and Octavian® Met contain diflufenican, flufenacet and metribuzin. Ascra® Xpro® contains bixafen, fluopyram and prothioconazole. Atlantis® Star contains iodosulfuron-methyl-sodium, mesosulfuron-methyl and thiencarbazone-methyl. Firefly® and Sublime® contain fluoxastrobin and prothioconazole. Pacifica® Plus contains amidosulfuron, iodosulfuron-methyl-sodium and mesosulfuron-methyl. Siltra® Xpro® contains bixafen and prothioconazole. Roundup® contains glyphosate.

Alternator, Ascra, Atlantis, Cadou, Firefly, Octavian, Pacifica, Roundup, Siltra, Sublime and Xpro are registered Trademarks of Bayer. All other brand names used are Trademarks of other manufacturers in which proprietary rights may exist. Use plant protection products safely. Always read the label and product information before use. Pay attention to the risk indications and follow the safety precautions on the label. For further information, including contact details, visit www.cropscience.bayer.co.uk or call 0808 1969522. © Bayer CropScience Limited 2025