Broadleaf weed control is proving decidedly more challenging this spring, so extra care will be needed when applying post-emergence herbicides to thin or stressed crops, leading agronomy firm Hutchinsons advises.
While the recent spell of dry weather has allowed drills to catch up after the wet winter, northern regional technical manager Cam Murray says the winter legacy on soil condition is still being felt in many situations, especially late-sown winter crops that struggled to establish, while spring-sown crops also now need rain to improve establishment.
Spring-germinating weeds continue to emerge though, and will pose a significant challenge to yield potential if not controlled effectively, he says.
Sulfonylurea chemistry is a key option for post-emergence broadleaf weed control, but Mr Murray warns that in thin, patchy crops, there is a risk more active will hit the soil rather than the leaves of the crop being treated, leaving residues that could cause issues in following crops, notably oilseed rape.
The risk is greater when applying actives with longer half-lives (the time taken for 50% of the herbicide to break down into secondary compounds in the soil), such as mesosulfuron, or metsulfuron, he continues.
“Growers with a thin crop where there is more risk of products landing on the soil should therefore consider using a mix based on actives with shorter half-life’s, such as thifensulfuron, or tribenuron.”
Mr Murray also advises that dry, cool weather and weeds under stress can reduce the efficacy of ALS herbicides, which includes sulfonylureas. So, if it is not feasible to wait for warmth and moisture before application, then growers should consider other options with proven performance in cool, dry conditions, such as products based on fluroxypyr/ florasulam and halauxifen-methyl.
“We know that halauxifen has the ability to sit in the plant under cold and dry conditions so it makes it a smarter choice in these situations.”
Spring oats can be more susceptible to herbicide damage, so a “softer” option may be required, such as florasulam and tribenuron-methyl, he adds.
Using hormones, such as MCPA. mecoprop-P and 2,4-D offers a different mode of action, however they require active growth, so may also be reliant on a return to warm, moist weather, and most have an application cut-off around growth stage 31, although this varies, so check product labels carefully, he notes.
Key broadleaf weed problems
- Poppies – can be a particular challenge where ALS herbicide resistance is present. In these situations, the main alternative is to use products based on halauxifen-methyl, but they must be applied at the full label rate when targeting poppies.
- Bur Chervil, Wild Carrot, Shepherd’s Needle – becoming an increasing problem in some areas and can be particularly difficult to control. Growers must control this weed quickly and while it is still small. Sulfonylureas are effective, however mixing phenoxy chemistry such as dicamba + mecoprop-p will boost control.
- Cleavers – particular issue earlier in the season, but can be controlled effectively with florasulam in the early part of the season. As it warms up, then switch to fluroxypyr-based options.
- Chickweed – often a problem in northern Britain and Scotland where resistance to sulfonylurea chemistry is present. Fluroxypyr is the “joker in the pack” for control.
- Mayweed – similar issues with resistance and a very visual weed. SU products are effective providing no resistance is present. If resistance is suspected, then florasulam-based chemistry will control resistant mayweed, however keep rates up and ensure good coverage; a half dead weed is not much good to anyone.
- Volunteer potatoes – best controlled with products based on clopyralid, florasulam + fluroxypyr, as the clopyralid has the ability to stop daughter tubers from developing. However, if straw is being removed from the field, growers must switch to fluroxypyr-based products, and if using a straight fluroxypyr then adding an SU partner will aid control.
- Fumitory – halauxifen-methyl is outstanding on this weed, so if fumitory needs controlling then halauxifen-based products give the best level of control available.
- Groundsel is another widespread weed that autumn residuals do not control; not normally the most competitive weed, however it can appear in very high numbers and will need controlling. Many of the SU containing products will control it – note if the weed is flowering, it is harder to kill and you get a bonsai effect, but the crop will soon out-grow it.