Poor weed control has adverse impact on vine yield and nutrition

Written by Victoria Rose.

Speaking to WineGB members on 3rd June, NIAB EMR’s senior research scientist Dr Julien Lecourt revealed that inflorescences were down by up to 40 per cent on vines in rows that had not received any weed control since being planted at the research centre in 2018.

“While there is no difference between vines in rows treated by different weed control methods, the control vines, where we have only used a strimmer, have 30 to 40 per cent fewer inflorescences,” said Julien, who oversees the UK’s only research vineyard at NIAB EMR in Kent. “That data is consistent with other results that we obtained last year on vine growth and nutrition and shows how important it is to keep the area around and under the vines well weeded.”

These results form part of an extensive EU-funded study called IWMPRAISE, for which NIAB EMR planted a separate block of vines in 2018. Ten rows of Chardonnay – clone 96 grafted on 3309C – are now being used to compare the effect of different weed control methods, including the use of herbicides; two different mechanical methods – a Rollhacke and a cultivation blade; and strimming. Canopy establishment and leaf wall area; nutrition; and, when the first crop is taken this year, yield and fruit quality, are all being assessed.

The initial IWMPRAISE project results were due to be discussed in the research vineyard as part of a WineGB viticulture working group meeting. Due to the coronavirus pandemic, the viticulture industry’s trade body instead hosted a webinar entitled “The wonderful world of weeds: and how to control them” via the Zoom video conferencing app.

Sponsored by WineGB patron NP Seymour Ltd, webinar speakers, alongside Dr Julien Lecourt, included Stephen Skelton MW, consultant viticulturist and chair of the WineGB viticulture working group; Sam Barnes, consultant and viticulture sales representative from NP Seymour Ltd; and Chris Cooper of Hutchinsons, who is also a WineGB technical advisor.

At the beginning of the IWMPRAISE project, growers surveyed said that weed control was vital for improving the microclimate; reducing frost risk; keeping the vineyard, which is often used as a communication aid and ‘shop window’ for the wines, looking tidy; and reducing the competition for water and nutrients. “This final point was challenged by a few growers and advisors who didn’t think competition was an issue with the amount of water we receive in the UK, and with the rich soils that most UK vineyards are planted on. Yet the data proves that competition for nutrients and water had a tremendous effect on vine growth and nutrition,” said Julien.

Whilst there was no noticeable difference in the vines at flowering time, in line with other studies that have proven that peak nutrient uptake occurs between bloom and véraison, the results began to shift as the season went on. “Where we strimmed the weeds were still kept under control, but by véraison the leaf wall area was down 20 per cent more than in the other rows,” said Julien. “Looking at the overall nutritional status of the vines, there was a more than 20 per cent reduction in the nitrogen balance index (NBI), which reduced with time and became very low towards the end of the season.”

Early data also shows that mechanical weeders are effective at maintaining low weed pressure and keeping the vines vigorous and healthy, with the most inflorescences being found on vines in rows that had been treated with the cultivator blade.

After a reminder that the vines being monitored are on rootstock 3309C, which is renowned for its poor potassium uptake, Julien discussed the link between potassium in grape must and acidity levels. “When we use mechanical weeders, we are not only disturbing the root of the weeds, we are disturbing the roots of the vines too,” he said. “We want to see if we can use mechanical weeders, not only to control weeds, but to modify the pH of the must and the quality of the wine. While the tidiness of the vineyard is important, I do think we need to be more precise about when we use mechanical weeders, so we will be looking at timing and how deeply we should be working.”

After Julien closed on the key message that weed control is a vital part of vineyard management in the UK, Sam Barnes was invited to talk through several machines that can be employed to control weeds. “Timing is important, but you’ve got to have the right tools to deal with weeds effectively,” said Sam, who has worked for specialist machinery dealership NP Seymour Ltd for the last four years and, having supplied machinery to growers across the UK, has experience of varying soil-types, different alleyways and growing styles.

Sprayers, which give a “clean result for minimal effort”, were tackled first, with Sam pointing out that UK growers didn’t realise how “lucky we are to still have them” as an option. While NIAB EMR’s research has proven strimming to be ineffective, the Clemens Multi-Clean was reviewed for its dual-purpose ability to significantly reduce the cost of bud rubbing. Deemed to be a “get out of jail free card”, it would also help growers to take weeds back to ground level before moving to other mechanical weeders.

From the tools being used in the IWMPRAISE project, Sam talked about Rollhackes and the Clemens Radius SL weeder. These cultivator blades undercut the topsoil and cut the root structure of the weeds, while an optional rotary tiller incorporates the topsoil, acting like a small power-harrow. “If you’ve got hardy weeds the Clemens will deal with them in a fairly clean sweep,” said Sam. “They’re by far the most effective mechanical tool that we have available to us, but there’s a there’s a price to match and you will only be able to work at speeds of 2.5 to 5.0 km/h.” For those with fewer hydraulic functions on their tractors, Sam described the Bahr system which features a double-sided, double-disc Rollhacke on the front and two finger weeders behind, to give a “pretty solid result in a single pass”.

Finally, for those who want to reduce the amount of herbicide used, but don’t want to work under the vines with mechanical tools, a swing-wing mower from Fischer was suggested.  “As Julien pointed out, it is not the best method, but it’s a good compromise. You can get very close to posts and around vines at good forward speeds, and it keeps things tidy when you don’t want weeds pushing up into the canopy or fruit zone,” said Sam.

Stephen Skelton reiterated that it was important to have a balance between herbicide and other methods in the vineyard, referencing a report from New Zealand where mechanical weed control “took three times as long” and “cost three times as much”.

Chris Cooper of Hutchinsons focused on the costs and efficacy of the chemicals currently available to UK vine growers. Starting with Finalsan, Chris pointed out that not only does it cost £1,105/ha applied, it does not kill down to the roots. Fusilade Max, is specific to grasses, can only be used once per year and “won’t do a great deal” to certain species at the rate at vwhich it is approved. Kerb Flo must be applied by 31 January, which is problematic as winter weather is “rarely conducive for field work” and many are also still pruning. In addition, it only gives 10-12 weeks of control, although Chris has tried mixing it with an adjuvant to increase the efficacy. Shark was deemed to be very good, especially at dealing with water shoots, but the 90-day harvest interval makes it difficult to use post-flowering.

“When I started over 30 years ago, it was very straightforward; you put on a residual in-season and contact herbicide, and repeated if needed,” said Chris. “These days, those in-season residuals no longer exist and we’re down to a few core contact herbicides. My biggest fear is that glyphosate will disappear or be restricted in its use. Looking forward, growers will have to mix and match herbicides and mechanical tools with other methods to try and get the best results.”

Finally, Stephen Skelton gave an overview of alternative methods. With mulching, PAS100 compost was deemed to be very effective for getting young vines established, but useless as a weed control solution, as it provides a “nice growing environment” for them too. It was also considered to be a costly endeavour, at £1 or more per vine. Chipped material can act as a weed suppressant, but specialist application equipment would be needed.

On the first 5½ acre vineyard that Stephen planted he used a black polythene mulch. Woven Mypex is also an option, but growers will face an “edge problem” as it’s not possible to mow up right to the plastic and a strategy would be required to keep the weeds from creeping over the plastic. Looking at animals, Stephen discussed the traditional horse and plough, geese and pigs. Sheep, which Stephen has used himself, were regarded as being practical up to a point. Hot foam, steam or water were deemed to be not yet cost effective. Lastly, Stephen discussed robots. Smaller options were deemed to be “eminently stealable”, while the larger options were considered “horrendously expensive” and would probably struggle in our wet soils.

Questions were then taken from participants about the use of cover crops, ongoing research at NIAB EMR, the sensitivity of the sensor arms on mechanical weeders, and their carbon footprint, and the environmental impact of strimmers, which leave microscopic pieces of plastic in the vineyard.

The webinar can be viewed in full on WineGB’s YouTube channel.

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