Review herbicide options for effective weed control

Weed control is critical to the success of all crops and in most situations, herbicides remain the most efficient and cost-effective means of managing weeds. The loss of several broad-spectrum products, however, has created gaps in control that has led to an increase in production costs as more active substances are applied to achieve control.

All sectors have been affected to some extent, but horticulture, which is often reliant on off-label approvals through the granting of extension of authorisation for minor use (EAMU) permissions, has felt the impact perhaps more than most.

A report by Andersons, a consulting firm, in 2014 estimated that the opportunity cost of not controlling broad-leaved and grass weeds to be £110 million a year through reduced yields and lower crop quality. The exact impact varies depending on the crop being grown, but in outdoor lettuce, for example, the AHDB estimates financial impact to growers to be about £70m a year with a 50% loss in marketable yield.

In some crops there is little choice. Emerger (aclonifen) is one of only a few post-planting herbicides authorised for use in stem vegetables such as celery, leek and salad onion. Fortunately, and as is highlighted in the table below, it offers a broad-spectrum with high efficacy against some of the harder to manage weeds.

It is also worth noting that where the weed burden is known to be high and if time allows, a pre-emergence application of Roundup (glyphosate) may be appropriate and could contribute to lower weed pressure later. 

Emerger gives good to excellent control on a range of weeds

Broad-leaved pre-emergence weed control

**Indicates the data contained for this weed is from a dose rate of 1.5 L/ha Emerger, due to reduced data being available at 1.75 L/ha. Increased control may be seen at 1.75 L/ha.

Grass Weed Control

Emerger Weed Spectrum – Post Emergence, Average Control

Average control taken from 2/3 trials, post emergence application only.

The situation is often better in other crops, but options for post-emergence control in carrots, parsnips and alliums remains limited. Depending on the weed species to be controlled, Emerger may need to be applied in a mix with other products.

In carrots, parsnips, onion, garlic and shallots, Emerger can be applied as a single pre-emergence spray up to a maximum 1.75 L/ha and 0.65L/ha as a post-emergence spray.  In leek, celery, and salad onion, Emerger can only be applied up to a maximum dose of 0.65 L/ha. 

Bayer anticipates that in carrots and parsnips a pre-emergence application of 1 L/ha followed by two sprays at 0.3 L/ha at one true leaf stage and again at two true leaves. As practices may differ between crops, growers should discuss how best to utilise Emerger with their adviser. 

CROP SPECIFIC CONSIDERATIONS

Drilled onions

The use of Emerger on drilled onions, even at low application rates requires extreme caution:

• Especially cool, wet conditions after drilling – long period of exposure due to slow emergence can cause large crop losses
• Pre-emergence applications can visibly slow and thin the crop

Onion Sets

Pre-emergence use of Emerger on sets is likely to be safer than on drilled onions though the application should be limited to 1 L/ha.

A tank-mix containing the following active substances has performed well in trials:

• dimethenamid-p + pendimethalin
• pendimethalin

Carrots

Prior to the loss of linuron, the standard pre-emergence mix for carrots was pendimethalin + clomazone + linuron. In trials similar weed control has been seen where Emerger was used in the place of linuron.

Points to consider:

• Irrigation around emergence or early afterwards can cause crop losses, typically when mixed with other herbicides.
• Trials indicate that much of the risk at emergence is associated with the inclusion of DFF. This resulted in some crop thinning, especially when using high doses of Emerger, and 100ml diflufenican (DFF).
• There is greater potential for negative crop effects when tank-mixing with pendimethalin and clomazone +/- DFF.