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Early weed control is vital for protecting maize yields – Farmers Guide – Charles Wright, Jim Clark

As maize drilling gets underway, growers are reminded about the importance of effective early weed control for maximising yield potential ...

“Maize is very susceptible to weed competition while establishing,” says Lincolnshire-based Farmacy agronomist, Charles Wright. “Yield losses can be catastrophic before the four-leaf stage of the crop, and the longer weeds go beyond this, the greater the impact.”

Pre-drilling glyphosate followed by a post-emergence spray has been a traditional approach for many growers, although he believes an additional pre-emergence spray can bring significant benefits.

Charles Wright

A pendimethalin-based pre-em minimises pressure from weeds emerging with crops and “buys time” for the subsequent post-emergence spray, should conditions prevent timely application. Good seedbeds are vital for maximising pre-em efficacy though, he notes.

Make the most of post-ems

Earlier is also better for post-em herbicide applications as smaller weeds are generally easier to control and competition with growing crops will be less.

Mr Wright says even with last season’s withdrawal of the key post-em herbicide Calaris (mesotrione +terbuthylazine), there are still plenty of effective products available, although choice should be tailored to weed spectrum. Growers should be mindful of any following crop restrictions, particularly where crops like leeks, onions or potatoes are in the rotation.

Clopyralid residues for example, can pass through the anaerobic digestion process and may affect potato crops grown on land where digestate has been applied.

If necessary, a second post-em can be applied to tidy-up remaining weeds, although growers need to be aware of growth stage cut-offs for individual products.

To minimise the risks of damaging maize with post-em herbicides, Mr Wright advises against using complex tank mixes and to avoid spraying in bright sunlight or when there are large diurnal temperature variations.

“Maize can get stressed very quickly, so if there are likely to be wide temperature swings, it is better to spray in early morning, when post-em efficacy is likely to be better too.”

Applying separate foliar nutrition at the 2-4 true leaf stage before the post-em is a useful way of optimising crop health, making maize more resilient to any potential stress. Phosphite plus zinc can be particularly beneficial, he adds.

Maize under film

Growers of maize under film face slightly different weed control challenges to open ground crops, Cumbria-based Hutchinsons agronomist Jim Clark points out.

Many crops typically receive a pendimethalin-based pre-em herbicide as post-em chemistry cannot be applied until film has degraded, exposing weeds growing beneath.” We normally have to wait until the 4-6 leaf stage, which means weeds are usually bigger, and potentially trickier to control.”

Jim Clark

His favoured approach is to apply mesotrione alone or with nicosulfuron or prosulfuron depending on weed pressure.” Redshank, bindweed and mayweed are the big yield-sapping weeds we’re usually targeting in this region. Generally we’re not chasing black-grass, but where other grass weeds are an issue then nicosulfuron is an option.”

Florasulam + fluroxypyr can be useful for controlling volunteer oilseed rape, although care is needed to avoid any crop effects from fluroxypyr, he adds.

Investigating residual effects

Undersowing maize with grass is increasingly popular for reducing erosion and building soil health, but could residues from sulfonylurea herbicides adversely affect the success of such techniques?

Mr Clark hopes to answer this question with a new trial at the Hutchinsons Regional Technology Centre near Carlisle.

Maize plots will be sown as normal this spring and treated with two herbicide mixes, containing mesotrione and either nicosulfuron or prosulfuron, both of which are SU chemistry. A Zocon Greenseeder drill will then undersow grass six weeks after drilling.

“SU herbicides can sometimes have an effect on following crops, but we need to know whether there is any impact on grass when it is used for undersowing in maize.”

An informal trial last year that involved broadcasting ryegrass seed onto a treated area did not show any significant adverse effects, so Mr Clark is keen to see whether this result is repeated.

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