Archive Article: 1997/10/04

4 October 1997

IS it time to cast pride aside? Not yet say the experts. There is certainly a case for allowing low populations of weeds such as speedwell, which die by early June and wont affect the harvest, says ADAS cereal specialist, Jim Orson. But, he warns, many weeds, even those ranked low in the competitiveness league, can have a knock on effect on the ease and timing of harvest, and on crop quality.

"Even something as innocuous as field pansy, which we know the crop can withstand in high numbers, can grow to canopy height in a wet summer and be green at harvest. So, if growers want to define their particular weed thresholds, they should accept that it isnt just a balance between yield penalty and the cost of herbicide," he suggests. "They cant optimise one input – herbicides – in isolation."

He says that delays to combining, for instance, can be difficult to cost. Current weed control thresholds (see table) dont take factors like that into account. Nor do they account for the benefit of tackling broad-leaved weeds in cereals, where control is invariably cheaper than in broad-leaved crops, he adds.

Weed control

"A lot of broad-leaved weed control is more to do with easing harvest, preventing quality loss and safeguarding future crops from undue weed pressure than protecting yield." In the absence of cleavers, yield response to control of broad-leaved weeds is typically less than 0.2t/ha, points out Mr Orson.

As for cleavers, they rank highest in the competitiveness stakes. They are also a "very variable feast". Cleavers seed stocks from different fields can have very different germination characteristics, he says, some chitting very rapidly in autumn, others not until December.

"But, if you were using a residual autumn herbicide such as diflufenican, you wouldnt want to hang around waiting for them to come through. Many growers have adopted the simple approach of optimising timing for blackgrass control, using rates sufficient to control all weeds bar cleavers. Then in the spring they have the flexibility of a wide application window in which to hit cleavers, and can adjust rates of fluroxypyr knowing theyve had a go at them in the autumn."

Cleavers are by far the main broad-leaved weed concern for Simon Francis, technical manager for the North of England Arable Centre. In the absence to date of a period of cool weather to break their dormancy, many will be late germinating this season, he believes. This could be particularly significant for the increased number of early drillings which have punctuated a stop-start harvest in the region.

Mr Francis subscribes to the view that closer attention to the weed species present can reveal areas of potential saving. Field pansies and speedwells, he suggests, might be tolerable in quite high numbers. Total elimination should not necessarily be the aim.

The difficulty, he adds, is that many growers have been able to afford the luxury of fairly comfortable levels of weed control for several years, and this may have clouded their knowledge about the actual underlying spectrum.

So they could be in for some nasty surprises if they let go of the weed control reins too suddenly. But theres nothing to stop them experimenting on a part field basis to establish for themselves just where the limits lie, he says. "Theres always room for tweaking rates and mixes, and possibly well see growers reducing the margin of dose they allow for insurance purposes," he suggests.

Without the need to prioritise blackgrass in weed control strategies, the main options for the north east of the country would be a DFF/IPU mix, or straight IPU with CMPP and perhaps an HBN herbicide – ioxynil or bromoxynil, says Mr Francis. "Generally its as well to achieve as much control as possible in the autumn when time and the weather are likely to be on your side. This does lay crops open to the risk of late germinating crops, but the chances are that a spring re-visit may be necessary anyway."

From &#42 L Hutchinsons Wisbech office, company agronomist Andrew McShane doesnt anticipate a sea change in weed control within the new economic climate. His advice, as ever, is made on the basis of weed competition ratings. "Cleavers, chickweed, thistles, mayweed and volunteer rape, beans and potatoes are all major competitors which you cant afford to leave unattended, especially in a cereal/root crop rotation," he stresses.

Again he picks out speedwell and field pansy as two less sensitive weeds, but also fools parsley, groundsel and fumitory. "These can have quite a visual impact, especially at flowering. But beyond that, their impact is minimal, unless numbers are quite significant." These are often the species which can also be awkward or expensive to control. But dismissing them does mean making a judgement on the level of populations, he adds.

Deciding which weed species you can afford to ignore can be hypothetical, however, since most will be covered by any reasonable attempt at autumn grass weed control. One exception, suggests Mr McShane, is field pansy, which might have become more significant under regimes of IPU. "But then I would argue strongly that this isnt a major weed," he adds.


"We tend to underestimate the level of control of broad-leaved weeds, especially mayweed and chickweed, offered by straight IPU. Where people are moving towards the contact option, they might find these older forgotten weeds resurfacing."

IPU forms the basis of his recommendations for broad-leaved weed control, being cost effective except for speedwell, cleavers or volunteer oilseed rape. Having established the relative rankings of other broad-leaved weeds or volunteers, he will bolt on pendamethalin or DFF based materials, or CMPP for volunteer beans. "Id also expect tri-allate plus IPU or tri-allate plus flupyrsulfuron-methyl, the new chemical from DuPont to cover virtually everything bar cleavers and thistles," he states.

"Since cleavers dont start to compete aggressively until stem extension, making judgements on their severity is academic until late February or early March. Tri-allate sequences and DFF or pendimethalin materials at quite low rates in the autumn will suppress them, leading into spring control with fluroxypyr or amidosulfuron."

Misjudging broad-leaved weed control in cereals can mean red fields and red faces. Just how far can control be relaxed without embarrassment, asks Tia Rund.

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