Article taken from The Vegetable Grower
By Richard Crowhurst
In the final article looking at crop protection The Vegetable Farmer asks four agronomists for their views on the biggest challenges in weed control this season. As with other areas of crop protection, the mild winter has increased weed pressure in many areas, although the actual effects obviously vary according to location and the weed species present. In addition wet weather in the autumn may have prevented some growers from spraying, making it harder to clean up ground prior to spring cropping. “There has been groundsel and chickweed seeding all winter,” says Chris Wallwork of Agrii. “A number of weeds have carried on seeding through the winter and in many areas.”
Irrespective of any seasonal effects, all of the agronomists we spoke to agreed that weed control now represents one of the biggest challenges in terms of crop protection. “One of the main areas of concern is weed control,” says carrot and parsnip specialist Howard Hinds of Root Crop Consultancy Ltd. “We’re losing more and more products, and while we have linuron this season there’s some doubt about how long we can keep it for. Of particular concern for carrot growers are volunteer potatoes, mayweed and groundsel.”
He also points out that there are new restrictions on the amount of metribuzin we can use compared to last year. “We have a reduced growth stage restriction on the EAMU for the use of Defy (prosulfocarb) which will make the control of potatoes trickier and reduce timing flexibility,” he adds.
“Weed control is the biggest single issue we are facing as agronomists,” agrees David Norman of Precision Agronomy. “We’ve got a reasonable range of fungicides on most crops and new insecticides coming through but we are really struggling with weed control across salads and we need some more chemistry.” Specialising in allium and salad crops, David is one of many people coming to terms with how the loss of established herbicide products will affect weed control strategies. “In leeks and onions this is the last year for using Totril (ioxynil) and Aramo (teprasloxydim),” he points out. “We’ve got some replacements for Totril, possibly in the form of Butrylflow (bromoxynil), but it doesn’t look to be quite as crop safe and may cause a few issues for growers. The loss of Aramo is going to cause some big issues as blackgrass and annual meadow grass are big problems and becoming an increasing issue in vegetable crops as well as for arable farmers.”
“Growers need to understand how to use products that they weren’t using in the past,” comments Chris Wallwork. “Growers must use up existing stocks of Totril by the end of August this year. Even if they have enough product to see them through this season they will have to learn how to use the alternatives. Bromoxynil is from the same family as ioxynil and is found in both Buctril and Butryflow, which have different authorisations. We’ve been doing trials since 2009 and have a lot of useful information, but as agronomists we only really know how to use it to best effect once it is used a field scale in different regions and weather conditions, etc.”
He points out that this is situation is not unique to allium crops either. “The same issues apply to Lentagran (pyridate) and chlorphropham,” he says. “There are several different examples out there where people are going to have to learn how to manage with a different set of tools in the toolkit. There are a lot of changes in product availability coming through.”
The loss of herbicides could also lead to an increase in the number of resistant weeds as resistant management becomes harder with a decreasing choice of active ingredients. It is also important to distinguish between weeds which are not, and never have been, controlled by a particular active, and those which have developed resistance over time and repeated exposure. Glyphosate may be under the regulatory microscope due to political and NGO pressures and concerns about possible carcinogenicity, but it is the development of resistance in weed populations, particularly in the Americas, which has been attracting headlines in the farming press for the last few years. “There are one or two weeds that glyphosate really just doesn’t control, but that’s always been the case,” points out Chris. “Then there are weeds that used to be controlled but aren’t now where they’ve developed resistance. Glyphosate is a good example of this and AHDB has published guidance from the Weed Resistance Action Group (WRAG) in response to the risks of glyphosate resistance developing in the UK. This is because in parts of the world where it’s been widely used on GM crops they have seen a number of weeds becoming resistant to it. I don’t think we’ve had any yet in the UK but it makes sense to take care.”
However, a shortage of ‘total’ weed control products means that the industry is already over reliant on using glyphosate in some situations. “We’ve not got the options with glufosinate-ammonium that we had a few years ago,” he adds. “Paraquat lost its approval a few years ago and at European level there are suggestions that diquat may also lose its approval. In terms of burn-off type total herbicides for vegetable growers those three actives are basically it. There are mixtures available, such as Kyleo which contains glyphosate plus 2,4-D. While it can certainly help in terms of minimising future resistance and controlling weeds like small nettle and American willow herb which are not controlled by glyphosate, it is less suited to vegetable situations because there is some residual activity from the 2,4-D, so you have a very long following crop interval. The same thing is likely to be the case with other alternatives for overall weed control. For example, carry over residues from diflufenican use on cereals can be an issue as they are quite persistent and some alternatives are less suited for vegetables than they may be for the amenity or top fruit sectors, for example.”
Agronomists like Chris also warn that reliance on certain actives to control certain weeds is increasing the likelihood of new resistances developing in the future. “I have a big concern about groundsel (Senecio vulgari) which has already shown itself to be very able to develop resistance to a number of other herbicides in the past, for example with simazine resistance in the top fruit sector and resistance to atrazine which was used in sweetcorn,” says Chris. He points out that the first cases of metribuzin resistant groundsel were reported in the west of England a few years ago, causing problems for asparagus growers in particular. “I’ve no doubt that we will see that problem spreading,” he warns.
Chris is also concerned about groundsel’s potential to develop resistance to dimethenamid-p which is found in both Wing-P and Springbok. “In Springbok it is in a mixture with metazachlor, but in Wing-P you have a combination of dimethenamid-p, which is very good on groundsel and pendimethalin which has never had any effect on it. Groundsel is a great concern for salad growers due to its potential toxicity, particularly with the shift to mechanically harvested baby leaf products.”
Dimethenamid-p, and Wing-P in particular is used in many crops, including lettuce, maize, onions and leeks. There is also an EAMU for transplanted brassicas, meaning there are many situations where groundsel is potentially being exposed to selection pressure by dimethenamid-p. “For example, Wing-P is is the standard for pre-planting herbicide for transplanted lettuce crops. Most lettuce growers are also using the Garford Robocrop or a similar guided hoe system,” comments Chris. “In practice if you are a lettuce grower you will tend to build up groundsel on your land as it is impossible to get 100 per cent control. If growers of different crops are mechanically weeding or following up with something like metazachlor, clomazone, clopyralid or pyridate, then you’ve got some resistance management there. However, if only one of two resistant plants survive the application of Wing-P, then growers are unlikely to use a follow-up product. Therefore you won’t necessarily see resistance until it is more widespread.”
Even with vigilance there could still be problems. “The reduction in the number of different herbicides available means it is more likely to be an unexpected resistance that appears rather than the one like this that everyone is looking out for,” concludes Chris.
The ongoing challenge of weed control and the lack of new products from manufacturers means growers are increasing using systems like the Garford Robocrop and other mechanical forms of control. “The issues with weed control have certainly revived interest in what can be done in terms of cultural and mechanical controls,” says John Allan of Agrovista. “It has to be said that precise mechanical control is developing quickly and it may be that a few years in the future there is a solution in terms of robotic weeding machines that just work autonomously and unmanned. That’s an interesting development in precision agriculture and we are doing a lot of work in this area. More simple technology is also being used, such as spot application Precise, accurate spot praying could well have a place, particularly for things like volunteer potatoes which can be a challenge to control in veg crops. The next generation of machinery is already be developed by companies like Garford and the necessary technology is out there and ready to go.”