Battling Broadleaf Weeds in Cereals

We often focus extensively on grassweed control in cereals, but on many farms – particularly where cereals have been grown year on year – many broadleaved weeds are becoming challenging and robbing us of yield and quality. Thankfully there are many options available so rotating chemistry, if not rotating crops, can be part of the solution to managing the challenge. We review some of the options available…

Fluroxypyr + MCPA + clopyralid

Very good all-round product with few weaknesses. Strong on difficult populations of Amaranthus up to 1-2 leaf stage. The one downside is Cleavers (‘sticky weed’); fluroxypyr is actually strong on Cleavers but not in cool conditions. And when you consider that Cleavers tend to be found in cooler areas, it often needs help. Adding a product containing an active ingredient that is effective in cool conditions such as florasulam or carfentrazone helps. Where brassica weeds are a challenge, the product also needs help from something like tribenuron, amidosulfuron or florasulam.

MCPA + bromoxynil

Weaker on Amaranthus now that bromoxynil resistance is very widespread across Kenya, the strength of this product is the speed with which it kills competitive weeds such as brassicas (volunteer canola, Wild Radish). It is very fast at killing Fat Hen / Lamb’s Quarter and Gallant Soldier too, but it has no activity on Cleavers.

Pyrasulfatole + bromoxynil

Very fast at killing Brassicas, Mexican Marigold and Blackjack. It does have limited residual activity too, and is useful on Cleavers provided that they are 1-2 whorl stage (3-4 cm max.). I find that this product struggles on Fat Hen (Lamb’s Quarter) and Amaranthus, and can be quite harsh on the crop when experiencing wide diurnal temperature fluctuations. It also has very useful activity on two common grasses; Eleusine multiflora and one species of Eragrostis known as E. racemose. Very useful where you are planning to apply a fop or dim herbicide which otherwise need 7 days before or 21 days after, for applying a hormone herbicide.

CMPP + carfentrazone

Very good on Cleavers, Gallant Soldier, Blackjack and Geranium type weeds. Carfentrazone can be very aggressive on the crop, and the advice from manufacturers the world over is not to mix it with any product containing iodosulfuron + mesosulfuron as it causes very obvious necrotic spotting. I also avoid mixing it with tebuconazole and epoxiconazole containing fungicides for the same reason. Useful for knocking down volunteer potatoes.

2,4-D

The old classic, very cheap and broad-spectrum but limited to between 1st tiller and growth stage 32. No control of cleavers and not effective enough on Wild Radish these days, and it does tend to struggle increasingly on Amaranthus.

Amidosulfuron + iodosulfuron

More of a tank mixing partner as it is not strong enough alone on Amaranthus, but adds useful control of Commelina and is very effective in cool conditions on Cleavers and brassica weeds.

Florasulam (+ flumetsulam)

Again, a mixing partner more than a stand-alone herbicide, that adds useful cool weather Cleaver control and persistence against later germinating weeds. Very useful to ‘hot-up’ mixes against Conyza / Fleabane.

Halosulfuron

We always think of this product as specifically for Watergrass or Sedges, which is largely true, but it adds useful control of Mexican Marigold and Gallant Soldier. Remember that in many countries it has a 13-month plantback period before sunflowers can be planted (in practice this is often a lot less on acidic soils, but be sure to take heed of this warning). Tank mixes well with other products except copper, with which the performance is drastically reduced.

Aminopyralid + halauxifen

An interesting product that will no doubt be exceptional on Cleavers, very useful on volunteer potatoes and Gallant Soldier but has some very lengthy plantback considerations for pulses and potatoes. No control of any brassicas, so be sure to add in a suitable partner.

Fluroxypyyr + halauxifen

An out-and-out Cleaver product, the absolute number 1 option especially in cool conditions, and with a wide application window. Interestingly it has limited effectiveness on volunteer potatoes, does almost nothing on Amaranthus, nothing on brassica weeds (easy to solve by adding some tribenuron or florasulam) but is useful on Conyza / Fleabane. Very little residual so very low risk of plantback issues.

Tribenuron

A useful tank mix partner for controlling brassica weeds and Thistles. Limited control of other weeds.
Remember that hitting weed early is key to achieving good control. Do not fall into the temptation of waiting until absolutely all of the weeds have germinated – for one, they will have already caused permanent yield loss if they get too large.

Secondly larger weeds are stronger and less well controlled, which means that thirdly, survivors will be bigger and shed proportionally more seeds – more problems for future years and a greater chance of herbicide resistance developing.

Spraying early might allow the odd later germinating weed to escape, but these guys are trapped under a dark, thick crop canopy so will struggle to produce viable seed, and the residual component of most herbicides will be fighting them as they try to emerge.

Aphids in Barley

I am seeing quite a few barley crops with Russian Wheat Aphids this season, which is not entirely surprising given the relatively dry October in many parts of the country.

Aphids of almost any species will always prefer dry and settled weather so will multiply much faster. There is no doubt an element of ‘green bridge’ carry over this year too, from all of the green growth on fallow fields with the August and September rain.

I am a big fan of keeping a living root in the soil, but be mindful that whilst there are benefits, it also brings some detractions to deal with. Aphids are undoubtedly one of them. There was certainly more evidence of aphid-transmitted Barley Yellow Dwarf Virus earlier this year where fallows were not kept clean or where cover crops were grown.

This is not a reason not to cover crop; but be practical and realistic about the likely outcomes. Most insecticide seed dressings are very good at controlling Grain Aphids, Bird-Cherry Oat aphids and Russian Wheat Aphids. Thiamethoxam probably has the edge, but clothianidin is also extremely good and arguably has a slight advantage on Cutworm.

With Russian wheat aphid, depending on how early you find them in the crop, the threshold for treatment is normally around 1-2% of tillers infested. There is an excellent GRDC resource online for looking at the likely return and economic treatment threshold (https://grdc.com.au/resources-and-publications/resources/russian-wheat-aphid).

I tend to find that pirimicarb and the pyrethroids (lambda-cyhalothrin etc) are only moderately effective on Russian Wheat Aphids. Acetamiprid, imidacloprid and thiamethoxam are all much more successful.

Later on in the crop, Grain Aphids and Bird Cherry Oat aphids can also build up and cause direct feeding damage. This is always in the thicker areas of crops first, and is a good reminder of the value of using variable rate fertiliser applications – allowing you to save money on inputs on the strongest areas of soil AND avoiding building up pests and diseases. You will likely also reduce lodging too, which means much higher yields.

The threshold for aphids post ear emergence is when 50% of tillers infested at flowering and 66% of tillers after that. What the threshold doesn’t tell us, slightly unhelpfully, is how many aphids constitute an ‘infestation’. Is it one or is it 20 aphids per tiller? I normally think that five aphids on a tiller definitely counts, so pull a whole plant up and count carefully in several areas throughout the crop.

Be aware of beneficial insects such as Hover Flies and Ladybirds – these do a huge amount of predation and natural control on our behalf. And remember that it is more about where the population is heading than absolute numbers – is it building rapidly, or unchanged over the course of a week? If you set out armed with reasons NOT to spray, with good, objective scouting you can sometimes be pleasantly surprised at your conclusions!

Are We Going Deep Enough With Canola?

I am seeing more and more work coming out of Australia about deep sowing canola. I have often talked about the need to sow canola early so that it flowers and begins pod-filling by the time the rain stops. There is no point having a crop that germinates perfectly and looks good but runs out of water and disappoints because it flowers in January or June.

This means a crop needs to be sown before the onset of the rains – when the soil will be dry on top. Conventional wisdom says that this is not possible because of the small size of the canola seed and the need to plant it shallow, but practice on farms is increasingly challenging this.

A tine seeder is ideal because it clears a furrow above the seed, meaning that even if the seed is 7cm deep there might only be 3-4cm of soil above it. The furrow also helps collect moisture from light rains, but it can fill up in a heavy rainstorm if soil is washed on top.

Disc seeders can also work however and often have the advantage of less disturbance which means less moisture loss around the seed.

Either way, large seed size is required to give the seedling the energy it needs to push out of the ground. As a general rule, 5.5 gram Thousand Seed Weight and above is a good measure of a seed’s ability to achieve this. I do not believe that hybrids necessarily help this, but screening seed with a 2mm sieve certainly does ensure better vigour and chance of emergence from depth.

Reduce press wheel pressure – the firmer the soil the more energy is required for the seed to make it out of the ground. So leave a lighter, less firm layer of soil on top to insulate the damp soil around the seed. Some new planters like the Horizon and Novag do not press directly above the slot, which undoubtedly helps. Emergence from 6-7cm is perfectly achievable with good seed, and leaves some margin for error should the planter go too deep (8cm is about the limit!)

Keep checking soil moisture across the paddock, and adjust the planter as necessary. Planting early means that you have more time to do the job too, so slow down. This will help reduce disturbance and thus conserve precious moisture.

It goes without saying that you should experiment gradually with any new idea. Do not attempt to plant your entire crop 8cm deep at the first attempt, but try different settings and remember to record every change to depth settings and press wheel pressure so that you are able to go back and clearly measure the success of the changes.

Till next time,

David Jones,

Independent Agronomist

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David is an independent agronomist in Kenya and a member of the Association of Independent Crop Consultants. David gives independent advice based on scientific trials and experience. Currently works with the Centre of Excellence for Crop Rotation.

Think Agronomy is brought to you by Cropnuts and the Centre of Excellence for Crop Rotation. We share the same vision for sustainable, dryland farming across Africa, and Think Agronomy is our independent voice to promote profitable, climate-resilient farming through better management of soil health, systems-based agronomy, crop diversification, and farm mechanization.